A Comparative Study of the effect of Thermal Bridging and Moisture on the Thermal Performance of Straw Bale and Cavity Wall Construction Final Year

Transcription

1 A Comparative Study of the effect of Thermal Bridging and Moisture on the Thermal Performance of Straw Bale and Cavity Wall Construction Final Year Dissertation

2 Abstract This research dissertation analyses and compares the thermal performance of straw bale construction and cavity wall construction with consideration for additional heat loss due to thermal bridging and decreased thermal resistance as a result of the presence of moisture. Straw bale dwellings constructed in North Kesteven, England, are used as a built example for the investigation while the Acceptable Construction Details produced by the Department of the Environment, Heritage and Local Government are used as a basis for the analysis of cavity wall construction. In the investigation of the thermal performance of both wall types, a three dimensional heat transfer software, AnTherm, and a one dimensional moisture transfer software, WUFI, are used as tools to analyse the potential heat loss of both forms of construction. The research has shown that when the effects of thermal bridging and moisture are considered, straw bale wall construction produces an average u-value of W/m 2 K, while cavity produces a u-value of W/m 2 K. The increased u-values differ by W/m 2 K, which is in stark contrast to the original dry u-values of the wall types which differ by W/m 2 K in favour of straw bale. This demonstrates the dramatic effect that thermal bridging and, in particular, the presence of moisture can have on the thermal performance of a straw bale wall, while the effect on cavity wall construction is minimal by comparison. These results show that straw bale construction will offer a superior thermal envelope to cavity wall construction. However the susceptibility of straw bale construction to moisture ingress requires adequate consideration due to its dramatic effect on thermal conductivity. i

3 Declaration I hereby declare that the work described in this dissertation is, except where otherwise stated, entirely my own work and has not been submitted as an exercise for a degree at this or any other university ii

4 Acknowledgements [Hidden] iii

5 Table of Contents Abstract... i Acknowledgements... iii 1.0 Introduction The Wider Context Background Scope of Research The Effects of Thermal Bridging on Thermal Performance The Nature of Thermal Bridges The Characterisation of Thermal Bridges Linear and Point Thermal Transmittance An Evaluation of the Conventions for the Analysis of Thermal Bridges Method for the Evaluation of Thermal Bridging An Investigation into the Nature of Psi-Values Are Psi-Values Necessary? An Analysis of the effects of Thermal Bridges in Straw Bale Construction Ventilated Eaves Junction First Floor Junction Ground Floor Junction Conclusion An Analysis of the effects of Thermal Bridges in Cavity Wall Construction Cavity Wall Detailing Conclusion Comparison and Conclusion...32 iv

6 3.0 The Hygrothermal Performance of Materials The Effect of Moisture on Thermal Conductivity The Moisture-Dependent Thermal Conductivity of Straw The Moisture-Dependent Thermal Conductivity of Polyurethane Simulation Method of the Hygric Processes of Materials Simulation Variables and Time Period Simulated Moisture Content of Straw Simulated Moisture Content of Polyurethane The Effect of Moisture Content on Thermal Performance The Moisture-Dependent U-Value of Straw Bale Wall The Moisture-Dependent U-Value of Cavity Wall Analysis and Conclusion Conclusion...48 References...50 Bibliography...51 Relevant Standards...52 Appendix A Case Study: North Kesteven & ACDs...53 Appendix B Evaluating the Accuracy of AnTherm...62 Appendix C Wall Section U-Value Calculations...67 Appendix D Thermal Bridging Assessment...72 Appendix E WUFI Simulation Data...80 v

7 Table of Tables Table 1 Thermal Conductivity (W/mK) of Materials at 25% increment increases Table 2 Psi-Values of Thermal Bridges with altering U w values Table 3 Heat Flow through Area of Materials at Eaves Table 4 Heat Flow through Area of Materials at 1F Table 5 Heat Flow through Area of Materials at GF Table 6 Total Heat Flow and Average Heat Flow Table 7 U-Value Calculation for Cavity Wall Table 8 Heat Loss Totals for both Construction Types Table 9 Material Properties of Straw Table 10 Material Properties of Polyurethane Table 11 WUFI Simulation Variables Table 12 U-Value Calculation: Straw Bale Table 13 Moisture-dependent U-Value of Straw Bale Wall Table 14 U-Value Calculation: Cavity Wall Table 15 Moisture-dependent U-Value of Cavity Wall vi

8 Table of Figures Figure 1 2D Intermediate Floor Model created using AnTherm Figure 2 Intermediate Floor Modeling Conventions Figure 3 Detail of the Intermediate Floor Junction designed for psi-value calculation Figure 4 Scope of Straw Bale Wall Section Figure 5 Eaves Detail Figure 6 Intermediate Floor Detail Figure 7 Ground Floor Detail Figure 8 Eaves Detail Figure 9 Intermediate Floor Detail Figure 10 Ground Floor Detail Figure 11 Scope of Cavity Wall Section Figure 12 The effect of Moisture on the Thermal Conductivity of Materials Figure 13 Moisture-dependent Thermal Conductivity of Straw Figure 14 Moisture-dependent Thermal Conductivity of Polyurethane Figure 15 Animated Film of Straw bale WUFI Simulation Figure 16 Moisture Content in Straw Bale Layer of Wall Construction Figure 17 Animated Film of Cavity Wall WUFI Simulation Figure 18 Moisture Content in Polyurethane Layer of Wall Construction vii

9 Table of Abbreviations & Definitions Thermal Bridging Part of a building envelope where an otherwise uniform thermal resistance is significantly changed by full or partial penetration of the building envelope by materials with a different thermal conductivity. Linear Thermal Transmittance Heat flow rate in the steady state through a linear metre of a material. Measured in W/mK, referred to as a psi-value Point Thermal Transmittance Heat flow rate in the steady state through a specific area of material. Measured in W/K, referred to as a chi-value U-Value Measure of a planar elements thermal transmittance, measured in W/m 2 K. Applied U-Value Measure of planar elements thermal transmittance over its surface area, measured in W/K. Thermal Conductivity Measure of a materials ability to conduct heat. Unit is W/mK Thermal Resistance Measure of a materials ability to resist heat transfer through it. Unit is m 2 K/W Psi Value A measure of the thermal transmittance of a junction between two planar elements, also known as a Ψ-value or linear thermal transmittance. Unit is W/mK. Chi - Value The point thermal transmittance of a point thermal bridge separating two environments being considered. Unit is W/K. Thermal Coupling Coefficient Heat Flow Rate per temperature difference between two environments which are thermally connected by construction. Hygrothermal Performance The effect of moisture on the properties of viii

10 materials. Moisture Content The quantity of water contained in a material. Units are kg/m 3 or %. Moisture-Dependent Thermal Conductivity The thermal conductivity of a material when exposed to a specific quantity of moisture. Moisture-Dependent U-Value The u-value of a construction with moisturedependent thermal conductivity of its materials considered. AnTherm A three dimensional thermal modelling software developed to the conventions set out in BS EN ISO WUFI 1D A one dimensional software for the simulation and calculation of the coupled heat and moisture transfer in building components. ACDs Acceptable Construction Details ix

11 1.0 Introduction 1.1 The Wider Context Over the course of the 21 st century, energy use has become an issue of great importance to many countries, particularly those of the developed western hemisphere (United Nations, 1998). This can most likely be attributed to the increase in awareness of global climate change and the role of greenhouse gases and fossil fuels as catalysts (United Nations, 1998). The Intergovernmental Panel on Climate Change has estimated that the temperature of the Earth will increase by 0.2 C per decade or from 1.1 C, as presently, to 6.4 C by the end of the 21 st century (Intergovernmental Panel on Climate Change, 2007). This increase could potentially have a major effect on the state of the Earth and all life present, effecting animal migration patterns, crop cycles and regional climates (United Nations, 1998). In response to these potential occurrences, a initiative has been undertaken to reduce climate change, by targeting the influencing factors such as greenhouse gas emissions and fossil fuel use, in the form of the Kyoto Protocol document, which is signed by 37 industrialised nations across the globe (United Nations, 1998). The construction industry in the United Kingdom and Ireland is responsible for the production of 47% of the dominant greenhouse gas, carbon dioxide (Department for Business, Innovation and Skills, 2010). In total, 27% of all carbon dioxide produced by the construction industry originates from the domestic sector, through the construction process and daily use of buildings (Atkinson, 2008). Due to the huge contribution of construction, and residential housing in particular, to global warming, nations have begun to produce legislation for efficient use of energy within buildings, therefore reducing carbon emissions (Department of the Environment, Community and Local Government, 2011). Within a residential unit, energy is lost through the fabric of the building (i.e. walls, floor, roof, windows, doors) in the form of heat. With space heating making up 57% of energy used in a residential unit, it is vital that the energy loss through the building fabric is minimised and the need for excess space heating minimised (Atkinson, 10

12 2008). As building standards increase further, some experts believe traditional building techniques, such as cavity wall construction, are being pushed to their limit and new alternative methods of achieving energy requirements are being sought (Little, 2005). Straw bale construction is one of various eco-friendly methods that are mentioned in the pursuit of alternative construction techniques. 1.2 Background Straw bale construction was first developed in Nebraska, USA in the late 1800s, corresponding with the development of baling machines (Jones, 2009). This unique building technique flourished in the region until the outset of war in the 1940s and the development of cement which resulted in its virtual extinction (Atkinson, 2008). In the 1970s, committed environmentalists Judy Knox and Matts Myhrman spearheaded the attempt to revive the construction method (Jones, 2009). With the rise of the ecomovement, straw bale construction spread beyond the USA and the first known UK straw build was completed in 1994 and 1996 in Ireland (Jones, 2009). By contrast, cavity wall construction is a relatively modern invention employed to provide efficient fabric performance (Little, 2005). The cavity wall design has evolved steadily since its inception in the 19 th century as method of preventing moisture ingress. During the 1970 s, as heat within a building became an issue of importance due to the oil crisis, insulation was incorporated into the cavity wall design and became legislation in 1990 under the building regulations (Little, 2005). Modern cavity wall design is the pinnacle of domestic brick construction, providing effective moisture and thermal resistance. The technique has gained rapid popularity among designers since its creation and is now a dominant feature of the domestic market in Ireland and the United Kingdom (Little, 2005). In 2009, North Kesteven Council, Lincolnshire constructed two 3 bedroom semidetached straw bale houses within a scheme dominated by traditional cavity wall dwellings. Straw bale enthusiasts, including North Kesteven Council, regularly use these developments as an example of the advantages of straw bale construction over cavity wall design, citing the thermal resistance as one of the major improvements (North Kesteven District Council, 2011). Due to the importance of this scheme to the arguments for straw bale construction, it shall be used as a base case study for this research. 11

13 1.3 Scope of Research Before an alternative technique becomes a viable replacement for an established construction method, thorough research should be conducted into its performance in comparison to the established technique. The aim of this research is to examine the thermal performance of both cavity wall and straw bale construction and the real world factors which may impact on the heat loss through these forms of construction. Within the construction industry, a measurement known as a u-value is used in the assessment of the thermal performance of building envelopes. A u-value essentially measures the linear heat flow through a metre squared of building fabric. However, these linear calculations assume the wall fabric to be uniform throughout which, in generally, is not the case within construction. Penetrations of the insulation layer of the building fabric by materials which conduct heat at an increased rate, or a higher thermal conductivity as it is known, create pathways by which heat can exit the building fabric at increased speed. These occurrences are known as thermal bridges. Thermal bridging within construction are well known and documented within the industry, with the building regulations establishing criteria which thermal bridging must meet in Technical Guidance Document Part L, which is highlight in Appendix D. However, there are numerous other factors which can influence the thermal performance of building fabrics which are not heavily researched or widely known. One of these factors is the hygrothermal properties of a fabric. This essentially deals with the effect of moisture on thermal heat transfer through materials (Hygrothermal Modelling, 2007). The presence of moisture in a wall construction, particularly the insulation layer, can result in increased heat flow, or thermal conductivity, through that material, allowing heat to move through the fabric at an increased pace (Introduction to WUFI, 2011). It is the aim of this research to investigate the potential effect of thermal bridging and moisture on the thermal transfer and performance of straw bale and cavity wall construction. As mentioned previously, the built straw bale homes in North Kesteven will provide the basis for the straw bale detailing, while the Acceptable Construction Details, as published by the Department of the Environment, Heritage and Local 12

14 Government, will be the basis of the cavity wall details. Within the cavity wall details, the altering effect of stainless steel and galvanised steel ties will also be evaluated to determine the most thermally efficient form of this technique. Heat loss through thermal bridging and the effect of moisture on material conductivity can have a major effect on performance of a wall construction, particularly in the case of straw bale construction where numerous bridges occur and potential for moisture ingress can be high. With this in mind, a question may be asked: how does the thermal performance of straw bale construction and cavity wall construction compare, factoring in the effect of thermal bridging and the presence of moisture? 13

15 2.0 The Effects of Thermal Bridging on Thermal Performance 2.1 The Nature of Thermal Bridges The purpose of all building envelopes, from the walls of a cave to the modern façade, has been to create an environment for its occupants protected from the elements and where heat can be retained. Building envelopes has evolved significantly since the days of the cavemen and now require comprehensive consideration in order to provide an environment which effectively retains heat. The most common way of retaining heat within an environment is the use of a thermal barrier around its perimeter to prevent excessive heat transfer from the interior environment to the exterior. This barrier is usually constructed using a material which provides the necessary resistance to heat flow through it, commonly referred to as insulation. The continuity of this insulation layer is essential to the retention of heat within a building environment. However due to building shape, structural requirements or aesthetic design, continuity of the insulation layer is not always possible. Where the insulation is broken by a material of higher thermal conductivity, a pathway is formed by which heat can bypass the insulation layer at increased speed and create what is known as a thermal bridge. A thermal bridge is defined in BS EN ISO as: part of the building envelope where the otherwise uniform thermal resistance is significantly changed by full or partial penetration of the building envelope by materials with a different thermal conductivity, and/or a change in thickness of the fabric, and/or a difference between internal and external areas, such as occur at wall/floor/ceiling junctions. (The British Standard, 2007) It is estimated that thermal bridging in a dwelling can account for approximately 15% of heat loss through the building fabric (Department of the Environment, Heritage and Local Government, 2008). Heat loss aside, thermal bridging can result in increased surface and interstitial condensation, which is where moisture forms within the construction, which can seriously affect the performance of the envelope (Department of the Environment, Heritage and Local Government, 2008). Thermal bridging generally occurs at the various material junctions in a building and these points must be comprehensively considered. 14

16 As knowledge of the effects of thermal bridging has grown, measures have been incorporated into the building regulations in order to measure and limit the effect of these bridges. It is proposed that, within this research, the conventions by which thermal bridges, as stated in Technical Guidance Document Part L (TGD), are assessed will be evaluated for the purposes of the comparison of thermal bridging in straw bale and cavity wall construction. If the assessment methods, as highlighted in the TGDs, are found to unsuitable in a comparative context, it is proposed that the thermal bridging found in both straw bale and cavity wall construction will be assessed and compared using an alternate methodology which will be based on the Y-Value method which will be explained at a later stage Characterisation of Thermal Bridges The accepted standard for assessing thermal bridging, BRE IP 1/06, separates thermal bridges into two distinct categories; repeating and non-repeating. Repeating thermal bridges are defined as regular, recurring breaks of the insulation layer or generic wall build-up such as timber joists, mortar joints, mullions in curtain walls and timber studs in timber frame wall construction. These types of thermal bridges should be considered within the determination of the wall U-value (Ward & Sanders, 2007). Non-repeating thermal bridges are defined as irregular breaks in the insulation or wall build-up such as at external wall junctions or around window and door openings. These bridges occur due to the geometric shape of the building and are generally twodimensional (linear thermal transmittance) or three-dimensional (point thermal transmittance) and require separate calculation. It is these forms of thermal bridges that will be assessed in relation to straw bale and cavity wall construction Linear and Point Thermal Transmittance Linear thermal transmission is defined in BS EN ISO as the heat flow rate divided by the external and internal temperature difference and the length of the bridge. These types of bridges are quantified by psi-values (Ψ) and measured in W/mK. An example of a linear thermal bridge would be a wall plate at an eaves junction which breaks the insulation layer. The default equation for the calculation of psi-values, which will alter depending on the junction in question, is as follows: 15

17 Ψ = L 2D (U w x L w ) Where L 2D = the thermal coupling coefficient of the two-dimensional model U w = the U-value in W/m 2 K of the plane flanking elements L w = the length over which U w applies Point thermal transmission is defined in BS EN ISO as the heat flow divide by the external and internal temperature difference, and generally applies to singular thermal bridges such as steel brackets supporting a bris soleil system. These bridges are quantified by chi-values (x), measured in W/K and can be incorporated into the u- value determination of walls. The equation for the calculation of x-values is as follows: x = L 3D Σ (U w x A) Σ (Ψ x l) Where L 3D = the thermal coupling coefficient of the three-dimensional model U w = the U-value in W/m 2 K of the plane flanking element A = area in m 2 over which U w applies Ψ = linear thermal transmittance of the linear thermal bridge L w = the length over which Ψ applies In the next section of this research, an assessment of the psi and x-value calculation conventions will take place with regard to suitability for the comparison of two different wall types; straw bale and cavity wall construction. 2.2 An Evaluation of the Conventions for the Analysis of Thermal Bridging In 1997, the Department of the Environment, Community and Local Government published guidance documents highlighting how construction projects may comply with the required building regulations. It was in part L of these documents that measures were put forward for the prevention of thermal bridging. These methods have continually evolved through the various revisions to the guidance documents. The most recent guidance document, published in 2011, states that procedures and 16

18 conventions outlined in BRE IP 1/06 and IS EN ISO should be used in the calculation of thermal bridging values, with specific guidance outlined in BR 497 for use of numerical software calculations. As mentioned in section 2.1, psi and chi values are the conventional methods for assessing thermal bridging. We can see from the equation for calculating psi-values that the u-value of elements flanking the thermal bridge (U w ) and the length (L w ) applied to that u-value are considered within the resulting psi-value. A question can then be asked with regards the use of psi-values; will the value of an unchanging thermal bridge alter with differing values for U w? and if so, is it suitable to compare the psi-value for similar junctions in two different forms of construction? Essentially, an investigation will take place to establish whether differing psi-values will be produced in two differing wall build-ups with identical thermal bridges (with identical width, length, thermal conductivity and hence heat flow). The outcome of this investigation will be to determine whether or not it is suitable to compare psivalues for various junctions in both straw bale and cavity wall construction. The suitability of chi-values will also be demonstrating due to their reliance on psi-values within their calculation Method for the Evaluation of Thermal Bridges In order to accurately and comprehensively investigate the suitability of thermal bridging calculation conventions, the methodology and procedures to be used within the evaluation should be outlined to provide a basis to allow full understanding of the findings. The calculation of thermal bridges requires the use of a two or three dimensional model, developed in line with the modelling conventions set out in BR 497: Conventions for calculating Linear Thermal Transmittance and Temperature Factors (2007). 17

19 For the purpose of clarity, a simplistic model of an intermediate floor junction has been modelled in line with the mentioned conventions as in Figure 1 and 2. This model will be used throughout the investigation with no change to size or symmetry except where explicitly noted. Fig 1. 2D Intermediate Floor Model created using AnTherm Fig 2. Intermediate Floor Modelling Conventions (Ward & Sanders, 2007) Overall, a set of five psi-values will be calculated whereby the thermal conductivity of all materials, except that of the thermal bridge, will increase by a factor of 0.25 or 25% per calculation up to 100% thermal conductivity of the materials thereby editing the U w value of the psi-value calculation. This set of five calculations will seek to demonstrate whether differing wall build-ups (i.e. with differing U w value) with the same thermal bridge will produce different psi-values An Investigation into the Nature of Psi-Values Within this section, the results of the calculations will be compiled, analysed and a conclusion put forward. All calculation models comply with the modelling conventions of BR 497, and will be modelled using the validated thermal modelling software, AnTherm. 18

20 Figure 3 shows the material details of the model to be used throughout the various calculations. The information shown will remain constant throughout the investigation except where explicitly noted. It should be noted that the model shown in Figure 3 was designed with the intent of simplicity and does not accurately represent a construction detail in reality. Having outlined the basics of the proposed investigation, the resulting calculations will now be compiled and analysed. The results of the calculations to determine the effects of the U w value on the thermal bridge are shown in Table 2 and the edited thermal conductivity values used in those calculations are shown in Table 1. The figures shown were produced by the thermal modelling software, AnTherm, which has been validated to BS EN ISO A separate calculation, testing the accuracy of the AnTherm program can be found in Appendix B. Fig. 3 Detail of the Intermediate Floor Junction designed for psi-value calculation Layer +0% T.C 1 +25% T.C +50% T.C +75% T.C +100% T.C Lime Render Straw Bale Lime Plaster Table 1 Thermal Conductivities (W/mK) of Materials at 25% increment increases 1 T.C. refers to Thermal Conductivity, measured in W/mK 19

21 Calculation Thermal Coupling Wall U-Value Length Psi-Value (Ψ) No. Coefficient (L 2D ) (U w ) (L w ) +0% TC W/mK W/m 2 K 3.21m W/mK +25% TC W/mK W/m 2 K 3.21m W/mK +50% TC W/mK W/m 2 K 3.21m W/mK +75% TC W/mK W/m 2 K 3.21m W/mK +100% TC W/mK W/m 2 K 3.21m W/mK Table 2 Psi-Values of Thermal Bridge with altering U w values As Table 2 demonstrates, the psi-value of the thermally bridged junction decreases steadily as the thermal conductivity of the wall build-up above and below the thermal bridge increases. Throughout each of these psi-values, the magnitude and conductivity of the bridge has remained constant and thus the heat flow through it has not altered, despite the psi-value changes. It can therefore be stated that psi-values of two different wall constructions, even with identical thermal bridges, cannot be accurately compared. It could be argued that the difference in psi-values is due to the accuracy level of the calculations, which can vary from ±5 % to ±20 % depending on the method of calculation. However even if this is the case, the difference in psi-values will still result in inaccurate total heat flow results and therefore would not be suitable for the proposed comparison Are Psi-Values Necessary? As shown in the previous section, questions can be asked about the use of psi-values for the comparison of thermal bridges within construction. However, with the results of the experiment aside, the use of psi-values in generally can be questioned in terms of necessity. In the process of calculating the psi-value of a thermal bridge, the total heat flow through the bridge and non-bridging wall area above and below need to be calculated. The non-bridged wall heat flow is then subtracted from the overall heat flow, leaving only the heat flow through the bridge. This is the psi-value of the bridge. It can be asked whether the calculation of the heat flow through the non-bridged wall area, only 20

22 to subtract it later, is totally necessary and represents a round-a-bout and convoluted method of calculating the heat flow through the bridge. As highlighted previously, u-values are an industry known and accepted method of calculating heat flow through a metre squared area of a wall. It is possible to determine the heat flow through a thermal bridge by use of a u-value which is applied across the area of the bridge, and will produce the same results as that produced using the psi-value calculation method. This u-value calculation eliminates the need to calculate the heat flow through the un-bridged wall areas which are required by psivalues, which speeds up the calculation process. As well as provided a faster method of heat flow calculation, u-values and their calculation process are well established within the construction industry with most Architectural Technologists and Architects able to perform these calculations. Psivalues on the other hand are not widely known and their calculation method is even less known or understood. Members of the construction industry who may need to calculate psi-values for the first time are required to comprehend and use two or three dimensional thermal modelling software or complex building physics equations. Neither of these processes are widely known and would require further training to carry out. In comparison, u-value calculations are widely used throughout the industry and only need be applied over an area to be converted into heat flow. It is this u-value methodology that will be used in the assessment of thermal bridging within this research, the accuracy and simplicity of which is shown in Appendix D. With a simple, straightforward and well known method of thermal bridging assessment, in the form of applied u-values, the purpose and necessity of complex, specialist psi-value calculations can be brought into question. 21

23 2.3 An Analysis of the effect of Thermal Bridging in Straw Bale Construction There are numerous methods of straw bale construction circulating around Europe and North America from loadbearing and compressive straw walls to non-loadbearing straw infill walls. In 2009, North Kesteven District Council constructed several twostorey straw bale dwellings. These straw bale dwellings were constructed using the loadbearing construction method, meaning that the straw bales act as both the insulation and structural layers with a finish applied to either side. This method provides the most thermally efficient straw bale envelope, with minimal thermal bridging along the wall. The details of these dwellings in North Kesteven, as a built example, will be used as a template for the investigation of thermal bridging within straw bale construction to follow. Within this method of straw bale construction, thermal bridging occurs at the various junctions of the external wall such as the eaves, first floor and ground floor connections. The thermal bridging at these junctions will be analysed and assessed over the course of a five metre long external wall elevation and from finished ground floor level to wall plate level (5392mm with a total wall area of 29.96m 2 ) as shown in Figure 4. As proposed in section 2.2, the standard convention for the assessment of thermal bridging, psi-values, will not be appropriate for the purposes Fig 4. Scope of Straw Section of comparing the thermal bridges of straw bale and cavity wall construction. Therefore an alternative method of quantifying and comparing the thermal bridges must be used. It is proposed that applied u-values be used to quantify the effects of the various thermal bridges. In simple terms, a u-value measures the heat flow through one metre squared of a material (W/m 2 K). U-values will be taken through the various heat flow paths found throughout the wall, and the 22

24 result will be multiplied by the area over which that u-value applies, which will produce total heat flow through that specific area of wall (W/K). This will allow effective comparison of the numerous thermal bridging found throughout straw bale and cavity wall construction Ventilated Eaves Junction The thermal performance of the eaves junction of straw bale construction, like the numerous other junctions, is affected by the method used to connect the meeting planes; the timber box wall plate. This wall plate is used to adequately fix the timber elements of the roof construction to the straw bale wall. It consists Fig 5. Eaves Detail of timber beams, braced together and encased in plywood sheeting. These plywood sheets span the width of the straw bale layer, as do the timber bracing members at regular intervals, creating continuous and point thermal bridges respectively. The various thermal bridges are highlighted in Figure 5. The results shown in table 3 have been manually calculated and are found in Appendix C. No. Element U-Value (U) Area (A) Heat Flow (UxA) Notes 1. Plywood Sheet W/K Bridge 2. Box Insulation W/K - 3a. Bracing Insul W/K - 3b. Timber Bracing W/K Bridge 4a. Straw Bale Wall W/K - Total Heat Flow (W/K): W/K Total Heat Flow (W/K) through Bridges: W/K Table 3 Heat Flow through Area of Materials 23

25 The total heat flow shown in Table 3 represents all heat exchange through the eaves and also a large section of straw bale wall below. Looking exclusively at heat flow through the timber box wall plate, it can be seen that total heat flow through the box over a five metre length is W/K. Of this total heat flow, the flow through the thermal bridges, the plywood sheets and timber bracing, is W/K which is 43% of total heat flow through the box. These thermal bridges only comprise 20% of the total area of the wall plate across the five metre wall elevation. This shows that approximately half of the heat passing through this junction occurs within one-fifth of the materials and highlights the increase in heat transfer at thermal bridges First Floor Junction Similar to at the eaves junction, the first floor junction s thermal continuity is broken by the presence of a timber box wall plate which is used to support the intermediate floor construction. As well as this box wall plate, a timber beam wall plate sits above it with bracing at intervals which cause identical thermal bridges to the bracing found within the timber box wall plate. The timber joists used in the construction of the first floor also span the depth of the box wall plate, breaking the horizontal continuity of the insulation within the Fig 6 Intermediate Floor Detail wall plate. The various bridges are highlighted in Figure 6. Due to its inclusion within the eaves calculation, thermal bridge 4a will be excluded from these calculations. Temperature within the first floor construction is assumed to be the same as within the room spaces above and below due to heat circulation. The results in Table 4 have been manually calculated and are found in Appendix C. 24

26 No. Element U-Value (U) Area (A) Applied U-Value (UxA) Notes 1. Plywood Sheet W/K Bridge 4b. Straw Bale W/K - 5a. Bracing Insul W/K - 5b. Timber Brace W/K Bridge 6a. Box Insulation W/K - 6b. Timber Joist W/K Bridge 7a. Bracing Insul W/K - 7b. Timber Brace W/K Bridge Total Heat Flow (W/K): W/K Total Heat Flow (W/K) through Bridges: W/K Table 4 Heat Flow through Area of Materials The total heat flow through the intermediate floor junction and straw bale wall below is W/K over the five metre length of elevation. The thermal bridges all occur around the junction between first floor and external wall and looking exclusively at this region, the heat flow between the top thermal bridge and the lowest thermal bridge is W/K (the figure includes the non-thermal bridging elements). Of this figure W/K heat loss is as a result of the thermal bridges such as the plywood sheets, timber joists and bracing within the wall plates. This is represents 47% of total heat flow through the junction despite the thermal bridges accounting for just 21% of the total surface area along the five metre long elevation. This shows, similarly to at the eaves, that half of the heat escaping from the building occurs at the thermal bridges, which only account for a small area, approximately one-fifth, of the junctions Ground Floor Junction The ground floor junction within most dwellings is usually a cause for concern in terms of thermal bridging and improper detailing can lead to high condensation and mould growth as well as thermal loss. The junction of the North Kesteven dwellings has been designed to effectively combat potential heat loss by way of lengthy overlapping insulation layers of the floor and wall. In terms of heat loss through the 25

27 external wall at this junction, a thermal bridge can be found where the masonry plinth (used to provide water resistance for the bales) meets the straw bales, usually 350mm to 450mm above finished ground level. Where these materials meet, a timber base plate, similar to at first Fig 7 Ground Floor Detail floor level, is used to fix the straw bale wall to the masonry plinth below. The bracing members of this wall plate bridge the width of the straw bale wall, allowing heat to escape at an increased rate. Heat may also be lost by transfer downward through the internal masonry leaf, however this thermal bridge does not fall within the scope of this research. The applied u-values of the elements within the scope of the external wall are shown in Table 5 with their location noted in Figure 7. Heat Flow path 4b has already been considered in table 4 and therefore will not be considered within the scope of the ground floor junction. No. Element U-Value (U) Area (A) Applied U-Value (UxA) Notes 8a. Plate Insul W/m 2 K m W/K - 8b. Plate Brace W/m 2 K m W/K Bridge 9. Plinth W/m 2 K m W/K - Total Heat Flow (W/K): W/K Total Heat Flow (W/K) through Bridges: W/K Table 5 Heat Flow through Area of Materials The total heat flow shown in Table 5 represents all heat transfer through the ground floor junction within the scope outlined in figure 4. As can be seen, total heat flow through this region is W/K. Of this total, the heat flow through the thermal bridge is W/K which is 25% of total heat flow through the junction and comprises 11% of total surface area of the wall elevation. When contrasted with percentage heat flow caused by bridges of the other junctions, it can be seen that the 26

28 ground floor detail is far more efficient in terms of heat retention through the external wall Conclusion Having assessed and analysed the heat loss through each of the junctions of the straw bale wall sections, it is important to analyse the wall section as a single entity. Table 6 shows the total heat loss through the entire five metre long straw bale wall elevation. Selected Element Heat Flow per Element (W/K) Eaves Junction First Floor Wall Intermediate Floor Junction Ground Floor Wall Ground Floor Junction Total Heat Flow (W/K): W/K Average Heat Flow (W/m 2 K): W/m 2 K Total Heat Flow Through Bridges (W/K): W/K Table 6 Total Heat Flow & Average Heat Flow As Table 6 shows, total heat flow over the area of the elevation, 26.96m 2, is W/K with thermal bridges accounting for W/K or 6.9% of that. This works out at W/K per metre squared area (W/m 2 K) when applied across the entire surface of the elevation. This figure essentially is the u-value of the wall construction, when heat flow through the thermal bridges is incorporated. This represents an increase of W/m 2 K on the original u-value of W/m 2 K. Although this may seem minor in terms of comparing u-values, over the course of an entire wall elevation, it could result in excessive heat loss. The total heat loss results shown in Table 6 are validated by use of a three dimensional model of the wall section, with the heat loss results found in Appendix D. 27

29 2.4 An Analysis of the effect of Thermal Bridging in Cavity Wall Construction Cavity Wall Detailing Modern insulated cavity wall systems developed as a result of the oil crisis of the 1970 s and has since evolved into the most common method of dwelling construction in Ireland and Britain (Little, 2005). Due to its two leaf system, cavity wall construction allows the continuity of the insulation layer across the dwelling wall and various junctions, making it thermally efficient in terms of continuous resistance and minimising thermal bridging. Along with the technical guidance documents, the Department of Local Government, Community and the Environment produced a set of construction details, known as the Acceptable Construction Details (ACDs), which will comply with the requirements of technical guidance documents Part L if copied. The cavity wall details for ground floor, intermediate floor and eaves (Fig 8, 9, 10) will be used in the assessment of heat flow through the external wall fabric and any thermal bridges within that fabric. The Acceptable Construction Details have been produced specifically to eliminate unnecessary heat exchange through thermal bridging hence there are very few thermal bridge variations (Department of the Environment, Heritage and Local Government, 2008). Despite this effort Fig. 8 Eaves Detail Fig. 9 Intermediate Floor Detail Fig 10 Ground Floor Detail 28

30 to eliminate thermal bridging within these details, the methodology of cavity wall construction will always produce a particularly repeating thermal bridge; cavity wall ties. Cavity wall ties are used in order to tie the two masonry leaves of the wall together for structural stability and as a method of securing the insulation to the internal leaf. Due to this, the wall ties will generally penetrate the depth of the insulation layer when connected the two leaves. These wall ties are generally made of galvanised, carbon or stainless steel 2, all of which are highly conductive materials. In this section, the quantity of heat flow through the entire five-metre long wall elevation (see Fig. 11) will be calculated, including the extra heat flow through the cavity wall ties. The method and convention for the calculation of heat flow through cavity wall ties is outlined in BS EN ISO 6946: Building Components and Building Elements Thermal Resistance and Thermal Transmittance Calculation Method. Before calculating the effect of the wall ties on the thermal performance of cavity wall construction, the heat flow through the non-bridging wall area must be calculated. Table 7 shows the u-value calculation through a cavity wall construction. Element Thickness (m) Thermal Cond. (W/mK) Thermal Res. (m 2 K/W) External Surf Brick Leaf Cavity Insulation Block Leaf Plasterboard Internal Surf Total Resistance (R): U Value Calculation (1/R): W/m 2 K Table 7. U-Value Calculation for Cavity Wall 2 Plastic Wall Ties with nominal thermal conductivity have recently been developed, offering far superior thermal resistance than steel ties. These were not used in the North Kesteven scheme. 29

31 It can be seen that this u-value will achieve the required standard of 0.21 W/m 2 K set out in technical guidance document Part L. When this u-value is applied over the area of wall outlined in Figure 10, as used in the straw bale calculations (26.96m 2 ), it can be seen that total heat flow through the wall area in question is W/K excluding additional heat flow through wall ties. The method for calculation of additional heat flow through cavity wall ties is outlined in BS EN ISO The formula used in this calculation is as follows: ΔU f = α (λ f.a f.n f / d 0 )(R 1 / R 2 ) 2 Where α = 0.8, from BS EN ISO 6946 λ f = the thermal conductivity of the tie, W/mK n f = no. of wall ties per m 2 A f = cross sectional area of 1 wall tie, m 2 d 0 = thickness of the insulation layer, m R 1 = thermal resistance of the insulation layer, m 2 K/W R 2 = total thermal resistance of component ignoring any bridging, m 2 K/W Fig. 11 Scope of Wall Section Materials used for cavity wall ties can vary from galvanised steel to stainless steel to plastic. For the purposes of this research, the effect of the galvanised and stainless steel ties on thermal performance will be assessed. Plastic wall tie use does not fall within the scope of this research and is generally considered to be nominal (Department of the Environment, Community and Local Government, 2011). The first calculation will investigate the effect of stainless steel wall ties on the thermal performance of cavity wall construction. The stainless steel wall ties to be used have a thermal conductivity (λ f ) of 17 W/mK, a cross-sectional area (A f ) of 30

32 19.2mm 2 or m 2 and, with 900mm horizontal spacing and 450mm spacing, the number of wall ties per metre squared (n f ) is As shown in Table 7, the thickness of the insulation layer (d 0 ) is 0.1m with a thermal resistance (R 1 ) of m 2 K/W while the overall resistance of the wall build-up (R 2 ) is seen to be m 2 K/W. With all variables accounted for, the calculation for the additional heat flow through the wall ties can now be completed, as seen below. ΔU f = α (λ f.a f.n f / d 0 ) (R 1 / R 2 ) 2 = 0.8 (17 x x 2.47 / 0.1) (4.348/4.955) 2 = x 0.77 ΔU f = or W/m 2 K This figure represents the additional heat flow through the cavity wall ties of a metre squared of wall area and therefore can be simply added to the u-value of the wall build-up for the same area, which is W/m 2 K. With the addition of the heat flow through the wall ties, it can be seen that the revised u-value for the wall is W/m 2 K. This represents a 2.4% increase in heat flow through the building envelope. The technical guidance document Part L stipulates that heat flow through the wall ties below 3% of total wall u-value can be discarded, however for the sake of accuracy, the additional heat flow will be included. Over the area of the wall elevation in question (26.96m 2 ), total heat flow would amount to W/K. The second calculation will investigate the effect of galvanised steel wall ties on the thermal performance of cavity wall construction. The galvanised steel wall ties to be used have a thermal conductivity (λ f ) of 50 W/mK, a cross-sectional area (A f ) of 19.2mm 2 or m 2 and, with 900mm horizontal spacing and 450mm spacing, the number of wall ties per metre squared (n f ) is As shown in Table 7, the thickness of the insulation layer (d 0 ) is 0.1m with a thermal resistance (R 1 ) of m 2 K/W while the overall resistance of the wall build-up (R 2 ) is seen to be m 2 K/W. With all variables accounted for, the calculation for the additional heat flow through the wall ties can now be completed, as seen below. ΔU f = α (λ f.a f.n f / d 0 ) (R 1 / R 2 ) 2 31

33 = 0.8 (50 x x 2.47 / 0.1) (4.348/4.955) 2 = x 0.77 ΔU f = W/m 2 K As shown, the additional heat flow caused by the penetration of the galvanised cavity wall ties is W/m 2 K which can be added to the overall u-value of the wall build-up, W/m 2 K. The resulting u-value, taking account of the heat flow through the wall ties, is W/m 2 K. This represents a 7% increase in heat flow through the wall fabric and would need to be included in the u-value calculation under the building regulations. Total heat flow through the wall area, with the revised wall u-value, would be 5.86 W/K Conclusion It can be seen from the two revised u-values of cavity wall construction, that cavity wall ties, and the type of wall tie, can play a significant role in the thermal performance of a cavity wall envelope. The choice of galvanised steel will increase the heat flow through the wall build-up by 7% per metre squared compared with 2.4% per metre squared for a stainless steel tie. It is therefore important that the specification of wall ties is adequately considered by designers with regard to their effect on thermal performance of a cavity wall. It should be noted that from this point forward, results from the cavity wall with stainless steel wall ties will be used as they represent the most thermally efficient method of cavity wall construction and it would be unfair and biased to compare a less effective method of cavity wall construction to straw bale construction in terms of thermal performance. 2.5 Comparison and Conclusion In order to determine the effect of thermal bridging on the thermal performance of both straw bale and cavity wall construction, it is proposed that they be compared under the following results: 1. Total Heat Flow through 26.96m 2 of Construction 2. Total Heat Flow through the Thermal Bridges within the 26.96m 2 Wall 3. Average U-Value of the Construction, incorporating heat loss through bridges 32

34 Construction Total Heat Flow (W/K) Total Heat Flow through Bridges (W/K) Average Wall U- Value (W/m 2 K) Straw Bale Construction Cavity Wall w/h SS Ties Cavity Wall w/h Galvanised Ties Table 8 Heat Loss Totals for both construction types In terms of total heat flow through the 26.96m 2 of wall elevation, it can instantly be seen that, as expected, straw bale construction offers far greater resistance to heat transfer through an external wall. Specifically, the heat loss through the cavity wall construction is 35% higher, with stainless steel wall ties, than straw bale construction. Heat flow per metre squared through the straw bale construction rose from W/m 2 K to W/m 2 K, an increase of W/m 2 K. Comparatively, heat flow through a metre squared of cavity wall construction increased from W/m 2 K to W/m 2 K, a rise of W/m 2 K when stainless steel ties are used. From these figures it can be seen that heat flow through the cavity wall construction increases by 2.5 times that of the straw bale construction when thermal bridges are incorporated. Although the thermal bridges through straw bale construction are evident and have a much larger surface area than that of cavity wall construction, the depth of the construction allows for greater resistance through each thermal bridge, while the performance of the envelope above and below the bridge also decrease the effect of these bridges over a metre squared area. In cavity walls, the quantity and increased thermal conductivity, as well as the smaller insulation layer, allow excessive amounts of heat to exchange between the external and internal environment. From the figures shown, it can be concluded that over the course of a metre squared area of wall, a straw bale wall will perform far better than the same area of cavity wall construction in terms of thermal transfer, despite its evident thermal bridges. 33

35 3.0 The Hygrothermal Performance of Materials An often unconsidered and unknown property of construction, the hygrothermal performance of materials can have a significant impact on the thermal performance of most building fabrics. The hygrothermal performance of materials pertains to the movement of both heat and moisture through a building and the effect of this movement on the properties of the material it moves through (Hygrothermal Modelling, 2007). The transport processes of heat and moisture throughout the building are not independent but are strongly coupled and the effect of one on the other requires adequate consideration, often in terms of material stability, degradation and thermal conductivity (Introduction to WUFI, 2011). Research into the coupled effect of heat and moisture on building fabric is known as hygrothermics. Within this research, the likelihood, quantity and effects of moisture on the straw bale and cavity wall designs will be investigated and analysed, ultimately demonstrating the impact on the thermal performance of both construction techniques. 3.1 The Effects of Moisture on Thermal Conductivity As mentioned, the presence of moisture within a material is strongly linked to the rate of heat flow through that material, otherwise known as the thermal conductivity (Introduction to WUFI, 2011). The potential effect of moisture on the thermal conductivity of a material, also known as the moisture-dependent thermal conductivity, is not generic across all materials and depends entirely on the material in Fig. 12 The effect of moisture on the thermal conductivity of materials (extract from WUFI 1D) question and the percentage moisture content within that material. For instance, the conductivity of mineral wool and other insulations can rise dramatically depending on moisture level while the conductivity property of stone materials, such as sandstone, will not drastically rise even at 100% moisture content. Figure 12 demonstrates a number of different reactions in thermal conductivity of 34

36 materials as moisture content increases. It can be seen that polystyrene foam will progressively increase while aerated concrete will increase linearly with moisture content. The mineral wool will dramatically increase at the first presence of moisture before becoming more linear after a certain point. This is as a result of the moisture distribution by vapour diffusion when a temperature gradient is applied. The effect of moisture on the thermal conductivity of straw and a generic cavity wall insulation, polyurethane, will be assessed by software simulation using the program WUFI 1D which complies with the required minimum criteria for software simulation of one-dimensional transient heat and moisture transfer set out in the European standard; EN The Moisture-Dependent Thermal Conductivity of Straw As stated in section 2.1, heat flow through a material, or its thermal conductivity, is strongly linked to the percentage moisture content in that material. This is known as the moisture-dependent thermal conductivity. Generally, the presence of moisture has been found to increase the thermal conductivity of most materials to varying degrees depending on the individual material properties. Using the simulation software WUFI 1D, a moisture-dependent thermal conductivity graph can be generated showing the thermal conductivity fluctuation as moisture content increases. The graph produced by WUFI for the moisture-dependent thermal conductivity of straw is shown in Figure 13. It can be seen that the conductivity rises from 0.06 W/mK at zero percentage moisture content to 2.9 W/mK at 100% moisture content. This represents a conductivity increase factor of 48 times. Even at minimal moisture content, which is expected when dealing with straw bales, this graph suggests that the conductivity of straw would increase to a point where heat flow through the wall will far exceed the requirements of Fig 13. Moisture-Dependent Thermal Conductivity of Straw Graph, as extracted from WUFI 35

37 the building regulations. For example, at 10% moisture content, which is below the level expected by straw bale experts, the thermal conductivity of the straw would be approximately 0.3 W/mK. This would result in a u-value calculation of W/m 2 K which is over double the allowed wall u-value of 0.21 W/m 2 K required within the building regulations. The increased thermal performance of straw bale walls in built case studies is well documented (Jones, 2009) and it may be asked if this WUFI graph represents accurate results. Therefore, it is proposed that these results for the moisture-dependent thermal conductivity of straw be manually assessed for accuracy using the following equation (Kunzel, 1995): (w)λ = λ 0 (1 + b.w/ρ s ) Where λ 0 = Thermal Conductivity of Dry Material b = Moisture-dependent Thermal Conductivity Supplement w = Moisture content within the material ρ s = Bulk Density of the Material The material properties for used in this equation are shown in Table 9 while a moisture content of 10%, as previously used, will be used to assess the accuracy of the WUFI graph for straw bale and for all future calculations and simulations using WUFI. The calculation is shown below. (w)λ = λ 0 (1 + b.w/ρ s ) = (1 + 4(10)/100) = (1.4) (w)λ = W/mK As can be seen the moisture dependent conductivity using the equation is far less than that stated by the WUFI graph at 3.0 W/mK and represents a more realistic representation of the effect of moisture on the thermal conductivity of straw. 36

38 Species of Straw: Rye/Wheat Bulk Density: 100 kg/m 3 Porosity: 0.9 Heat Capacity: 1800 J/kgK Thermal Conductivity: W/mK Diffusion Resistance Factor: 2.0 Moisture-Dep. Thermal Cond. Supplement: 4 Typical Built-in Moisture: kg/m 3 Layer Thickness: 0.45m Table 9 Properties of Straw The reasons for the potential inaccuracy of the WUFI graph are varied. The file and material properties of the straw bale are not part of the initial WUFI package but instead are the result of external research which is available to download and within the research report, there is no indication of results pertaining to moisture-dependent thermal conductivity and therefore the origin of this graph may be questioned. Considering this, it is proposed that determination of the moisture-dependent thermal conductivity of materials within this research is based on the equation used previously. A graph representing the moisture-dependent thermal conductivity using the equation mentioned can be found in Appendix E. In section 3.2.2, the moisture content within straw bale over a period of time will be simulated using the WUFI 1D software, after which the moisture-dependent thermal conductivity may be calculated. It should be noted that varieties of straw will dramatically impact on the results of this research and therefore the material properties of the straw used in this research should be adequately assessed The Moisture-Dependent Thermal Conductivity of Polyurethane Similarly to straw, the moisture-dependent thermal conductivity of polyurethane depends heavily on the individual properties of the material. It is therefore vital that before any simulations take place, the material properties of a selected manufacturer s product are attained to produce accurate results. The material properties of the polyurethane used for all WUFI simulations are shown below. 37

39 Bulk Density: 40 kg/m 3 Porosity: 0.95 Heat Capacity: Thermal Conductivity: 1500 J/kgK W/mK Diffusion Resistance Factor: 50.0 Moisture-Dep. Thermal Cond. Supplement: 0.4 Typical Built-in Moisture: 0 kg/m 3 Table 10 Properties of Polyurethane This data has been used to simulate the moisture-dependent thermal conductivity graph for polyurethane. It should be noted that variations of the outlined properties Fig 14. Moisture-Dependent Thermal Conductivity of Polyurethane Graph, as extracted from WUFI may result in small differences in results produced. Unlike with straw, the material data for polyurethane was researched and included with the WUFI package and therefore its accuracy has been evaluated by the program creators. However in the interest of consistent results and methodologies, the equation for the moisture dependent thermal conductivity of polyurethane will be used as was used in section As seen in Figure 14, the thermal conductivity of polyurethane acts differently to that of straw when exposed to moisture. The conductivity of the straw progressively increases as moisture content increase, from a dry conductivity of W/mK up to a maximum of 0.62 W/mK representing a conductivity increase of 26.9 times from dry to 100% saturation. With the completion of a WUFI simulation showing mean moisture content of the polyurethane, a moisture-dependent thermal conductivity will be assessed using this graph. 38

40 3.2 Simulation Method of the Hygric Processes in Materials Given the numerous variables and outputs of the program WUFI, it may be important to detail and outline the simulation method by which all results were attained which place any output data in context and allow future research to be undertaken using similar parameters. This section will outline the details of the straw and cavity wall simulations and assess the results of those simulations Simulation Variables and Time Period Material properties aside, a WUFI simulation requires the user to input various data based on the environment surrounding the wall in question. Criteria such as location, wall inclination and orientation among others need to be adequately considered as they may have a significant impact on the results of the simulation. Below the numerous variable inputs used for both the straw bale and cavity wall simulations are listed which place the simulation within an environmental context. Orientation: South-West Inclination: 90⁰ Building Height: < 10m Initial Relative Humidity: 80% Initial Temperature in Component: 20⁰C Calculation Period: 01/01/12 01/01/17 Internal Moisture Load: High Climate File: Dublin, Ireland Table 11 Wufi Calculation Variables Possibly the most important variable within any WUFI calculation is the climate file selected. This file will have an impact on the relative humidity, temperature and moisture content within the component. With this importance in mind, the properties of the Dublin, Ireland climate file are outlined in Section 1.0 of Appendix E Simulated Moisture Content of Straw During the simulation of the straw bale wall, WUFI produces an animated film (Fig. 15.) which demonstrates the fluctuating temperature, relative humidity and moisture content throughout the various components as time passes. This film demonstrates the 39

41 connection between relative humidity, temperature and moisture content. It can be seen that relative humidity may increase the moisture content within the wall while temperature may impact on the drying of the components. Fig 15. Animated Film of Straw bale WUFI Simulation Upon completion of the film, WUFI will produce a detailed report (Section 2.0 of Appendix E) consisted of in-depth information with regard to the performance of the wall build-up. The following results focus entirely on the straw layer of the wall construction due to the nominal effect of the moisture on the finish layers with regard to thermal conductivity. It is recommended that a typical built-in moisture content of 12-16% should be considered for a basic construction grade straw bale (Sutton, Black, & Walker, 2011). For this simulation, a build-in moisture content of 12.75% has been used as recommended by WUFI. The results report, produced by WUFI, states that there was a minimum of 12.06% moisture content over the 5 year period and a maximum moisture content of 15.42% with a mean moisture content of 13.8% (Fig 16). 40

42 Fig 16. Moisture Content Graph in Straw Bale Layer of Wall Construction Having established the mean moisture content within the straw at 13.8% it is now possible to calculate the moisture-dependent thermal conductivity of straw at this point. The equation used previously, and shown below, will be used in this calculation. The material properties which are used in the equation variables can be found in Table 9. (w)λ = λ 0 (1 + b.w/ρ s ) = (1 + 4(13.8)/100) = (1.552) (w)λ = W/mK This represents an approximate conductivity increase of 50%, which shows the dramatic effect of moisture on heat transfer within a straw bale, highlighting the need for continuous monitoring of all straw bale used in construction. This new thermal conductivity will be used to determine the thermal performance of straw bale wall construction, accounting for moisture, in section Simulated Moisture Content of Polyurethane Using the same parameters as used in the straw bale simulation, a WUFI calculation was completed for a cavity wall system with partial fill polyurethane insulation. A film (Fig. 17) was produced showing the fluctuation of water content throughout the various layers including the polyurethane. 41

43 With the completion of the animated film, an in-depth report is produced by WUFI, detailing the various performance aspects of the cavity wall construction including the moisture content of the various layers. Within the polyurethane layer, with a build-in moisture content of zero, the minimum moisture present was 6.8% up to a maximum of approximately 11%, with a mean moisture content of 8.45% (Fig 18). Fig. 17. Animated Film of Cavity Wall WUFI Simulation Fig. 18 Moisture Content Graph in Polyurethane Layer of Wall Construction Having established the mean moisture content of 8.45% within the polyurethane layer of the cavity, the moisture-dependent thermal conductivity of the polyurethane at this level of saturation can now be calculated using the following equation. (w)λ = λ 0 (1 + b.w/ρ s ) 42

44 = ( (8.45)/40) = (1.036) (w)λ = W/mK This represents a conductivity increase of 4% on the dry thermal conductivity of polyurethane which is W/mK. The moisture-dependent thermal conductivity graph (Fig.14), produced by WUFI, produces an approximate conductivity of W/mK at 8.45% moisture content however the graphs produced by WUFI are only approximations and the equation calculation method represents a more accurate value for the increased thermal conductivity. This moisture-dependent thermal conductivity will be used to assess and calculate the thermal performance of cavity wall construction, with the effects of moisture, in section The Effect of Moisture Content on Thermal Performance It is widely known within the construction industry that the performance of insulation is greatly affected by the presence of moisture, and a built-in moisture content is often specified prior to construction. As mentioned previously, this impact on performance is mainly due to the effect of moisture on the thermal conductivity of the insulation. Using the simulation software WUFI, an investigation has been undertaken to establish the levels of moisture within the insulation layers of a straw bale wall and cavity wall and the effects of that moisture on the thermal conductivity of the material, otherwise known as the moisture-dependent thermal conductivity. Having established the moisture-dependent thermal conductivity of both straw and polyurethane, it is now possible to determine the impact of this increased conductivity on the thermal performance of both wall constructions. This will be established by way of a comparison of wall u-values, which is the approved method of establishing the thermal efficiency and performance of building fabrics, quantifying the movement of heat through a metre squared of wall area, per temperature difference. A base wall u-value of each construction type will be calculated, using dry thermal conductivity values for all layers which will then be compared to a u-value calculation incorporating the new moisture-dependent thermal conductivity. 43

45 3.3.1 The Moisture-Dependent U-Value of a Straw Bale Wall In order to quantify the effect of moisture on the thermal performance of a wall buildup, a base value which it can be judged against must be established. U-Value calculation is the chosen method for the quantification of the thermal performance. This method was chosen because its use is a requirement of the building regulations and is therefore an industry standard for analysis of the thermal performance of building fabrics. Below is a u-value calculation of the straw bale wall construction used throughout this research. All materials within the calculation are taken to be in a dry state and therefore the effect of moisture on the performance is not included. Material Layer Thickness (m) Thermal Conductivity Thermal Resistance (W/mK) (m 2 K/W) External Surface Lime Render Straw Bale Lime Plaster Internal Surface Total Resistance (R): U-Value (1/R): W/m 2 K Table 12 U-Value Calculation: Straw Bale The calculation shows that the straw bale wall in a dry state achieves a u-value of W/m 2 K. This u-value surpasses the u-value required for a wall build-up under the building regulations (0.21 W/m 2 K) and that required to achieve passive house standard (0.15 W/m 2 K). The results of this calculation will have no issues with regard achieving planning standards and if maintained in a relatively dry state will provide a thermally efficient fabric. However as shown previously, as a permeable wall, a straw bale wall is liable to contain some moisture throughout the year and its effects on the thermal performance and hence u-value of the wall should be assessed. Below is a second u-value calculation of the same straw bale wall construction however the moisture-dependent thermal conductivity assessed in section is incorporated. 44

46 Material Layer Thickness (m) Thermal Conductivity (W/mK) Thermal Resistance (m 2 K/W) External Surface Lime Render Straw Bale Lime Plaster Internal Surface Total Resistance (R): U-Value (1/R): W/m 2 K Table 13 Moisture-dependent U-Value of Straw Bale Wall As can be seen, when the moisture-dependent thermal conductivity is incorporated into the u-value of the straw bale wall, the result rises from W/m 2 K to W/m 2 K. This represents an increase of W/m 2 K or 52.7% on the original, dry u- value The Moisture-Dependent U-Value of a Cavity Wall Below is a u-value calculation of the cavity wall construction used throughout this research. All materials within the calculation are taken to be in a dry state and therefore the effect of moisture on the performance is not included. Material Layer Thickness (m) Thermal Conductivity (W/mK) Thermal Resistance (m 2 K/W) External Surface Brick Outer Leaf Air Cavity Polyurethane Block Inner Leaf Plasterboard Internal Surface Total Resistance (R): U-Value (1/R): W/m 2 K Table 14 U-Value Calculation: Cavity Wall 45

47 The calculation shows that the cavity wall in a dry state achieves a u-value of W/m 2 K. This u-value surpasses the u-value required for a wall build-up under the building regulations (0.21 W/m 2 K). However as shown previously, due to the nature of a cavity wall in allowing water into its air cavity, some moisture will be absorbed by the insulation layer which will have an effect on the thermal performance and hence u-value of the wall must be assessed. Below is a second u-value calculation of the same cavity wall construction however the moisture-dependent thermal conductivity assessed in section is incorporated. Material Layer Thickness (m) Thermal Conductivity (W/mK) Thermal Resistance (m 2 K/W) External Surface Brick Outer Leaf Air Cavity Polyurethane Block Inner Leaf Plasterboard Internal Surface Total Resistance (R): U-Value (1/R): W/m 2 K Table 15 Moisture-dependent U-Value of Cavity Wall As can be seen, when the moisture-dependent thermal conductivity is incorporated into the u-value of the cavity wall, the result rises from W/m 2 K to W/m 2 K. This represents an increase of W/m 2 K or 3.47% on the original, dry u- value. 3.4 Analysis and Conclusion A comparison of the resulting moisture-dependent u-value calculations will show that both wall constructions are affected by the presence of moisture, however it is not in equal measure. The u-value of a straw bale wall increases by W/m 2 K while the u-value of a cavity wall increases by The increase difference of W/m 2 K between the two wall constructions is substantial, demonstrating the dramatic effect that moisture can on the thermal performance of straw bale construction. When the presence of moisture is considered, it can be seen that the heat flow through a metre 46

48 squared of straw bale construction increases by approximately 50%, while heat flow through a cavity wall increases by just 3.47%. This figure and the linear thermal conductivity increase seen on the moisture-dependent thermal conductivity graph (Appendix E) demonstrates the potential for the thermal performance of a straw bale wall to get increasingly worse. It is therefore vital that adequate monitoring of the moisture levels in straw bale walls is undertaken. 47

49 4.0 Research Conclusion Over the course of this research the effect of both moisture and thermal bridging on the thermal performance of straw bale and cavity wall construction has been completed. In terms of the effect of thermal bridging on thermal performance, it was found that within a straw bale section, W/K were lost from a wall of 26.96m 2, which resulted in an average heat loss of W/m 2 K through the fabric. Thermal bridges account for W/m 2 K of this figure. Within cavity wall construction 5.51 W/K were transferred through the building fabric over the same wall area, if stainless steel wall ties are considered. This resulted in an average heat loss of W/m 2 K. The thermal bridges within the construction account for W/m 2 K of this average. Despite the evident nature of the thermal bridges within the straw bale junctions, the depth of material which heat is moving through is between 450mm to 510mm and this depth provides enough thermal resistance to minimise the potential heat loss through thermal bridges within straw bale construction. In contrast, the thermal bridging within cavity wall construction, in the form of steel wall ties, is well documented but often considered to be unworthy of acknowledgement. However the increased conductivity and the quantity of wall ties used in cavity wall construction adds up to a substantial heat loss and double that of straw bale construction. When the effect of moisture on thermal performance was assessed, it was found that within a metre squared area of straw bale wall, the heat flow increases by W/m 2 K, giving a total heat flow through the wall of W/m 2 K. Within the same area of a cavity wall, the heat flow increases by W/m 2 K, with a total heat flow of W/m 2 K. From these figures, it is clear that moisture can have a dramatic effect on the thermal performance of straw bale construction. If we consider the effect of both thermal bridging and moisture on the thermal performance of a wall at the same time, a total heat flow, or u-value, for the construction types can be calculated. Straw bale construction can be seen to increase by W/m 2 K as a result of thermal bridging and W/m 2 K as a result of the presence of moisture. This gives a total heat flow increase of W/m 2 K which can then be added to the generic u-value of a straw bale wall; W/m 2 K. The resulting revised u-value of a straw bale wall, when thermal bridging and moisture are considered, is W/m 2 K. 48

50 Cavity wall construction can be seen to increase by W/m 2 K as a result of thermal bridging and W/m 2 K as a result of the presence of moisture. This results in a total heat flow increase of W/m 2 K which can then be added to the generic cavity wall u-value of W/m 2 K. The revised u-value of a cavity wall, when the thermal bridging and effect of moisture is considered, is W/m 2 K. The difference in u-value between the two forms of construction, when thermal bridging and moisture are considered, is just W/m 2 K. This is a dramatic drop on the difference between the generic, dry u-values of straw bale and cavity wall construction, which was W/m 2 K. From these figures it can be seen that although straw bale construction fares twice as good as cavity wall construction in terms of retention of heat through thermal bridges, its susceptibility to moisture ingress, and the effect of that moisture on the thermal conductivity of the straw, dramatically decreases the heat retention through the straw bale fabric. It is important to note that if excessive levels of moisture were within the straw bale wall, the u-value would dramatically escalate far beyond the requirements of the building regulations. It is therefore vitally important that the moisture content of straw bale be properly assessed prior to construction and monitored after construction has finished. From these results it can be concluded that straw bale construction does offer a superior thermal barrier than cavity wall construction within a dwelling, however the dramatic superiority as quoted by straw bale enthusiasts such as Barbara Jones, the founder of the largest straw bale consultancy firm in the United Kingdom, is not accurate when considered in a practical, realistic environment. 49

51 References Hygrothermal Modelling. (2007). Retrieved April 18, 2012, from Built Environments: Introduction to WUFI. (2011, March 15). Retrieved April 18, 2012, from WUFI Pro: Atkinson, C. (2008, January). Energy Assessment of a Straw Bale Building. Retrieved from Home Grown Home: Department for Business, Innovation and Skills. (2010). Estimating the amount of CO2 Emissions that the Construction Industry can influence. London: Department for Business, Innovation and Skills. Department of the Environment, Community and Local Government. (2011). Technical Guidance Document L: Conservation of Fuel and Energy Dwellings. Dublin: The Stationary Office. Department of the Environment, Heritage and Local Government. (2008). Limiting Thermal Bridging and Air Infiltration: Acceptable Construction Details. Dublin: The Stationary Office. Intergovernmental Panel on Climate Change. (2007). IPCC Fourth Assessment Report: Climate Change 2007 (AR4). Valencia: Intergovernmental Panel on Climate Change. Jones, B. (2009). Building with Straw Bales: A Practical Guide for the UK and Ireland. Cornwall: Green Books. Kunzel, H. M. (1995). Simultaneous Heat and Moisture Transport in Building Components. Stuttgart: Institute for Building Physics. Little, J. (2005). Partial Fill Cavity Walls: Have We Reached the Limits of the Technology? Construct Ireland, McLeod, R., Mead, K., & Standen, M. (n.d.). Passivhaus Primer: Designers Guide. Retrieved April 4, 2012, from BRE Passivhaus: Sutton, A., Black, D., & Walker, P. (2011). Straw Bale: An Introduction to Low- Impact Building Materials. Watford: IHS BRE Press. The British Standard. (2007). BS EN ISO 10211: Thermal Bridges in Building Construction - Heat Flows and Surface Temperatures Detailed Calculations. Brussels: Management Centre. United Nations. (1998). Kyoto Protocol to the United Nations Framework Convention on Climate Change. Kyoto: United Nations. 50

52 Ward, T., & Sanders, C. (2007). Conventions for calculating Linear Thermal Transmittance and Temperature Factors. Watford: IHS BRE Press. Bibliography Allen, P. K. (2004). Home Grown Houses: The potential for large-scale production of renewable construction materials from crops grown in the UK and possible impacts. London: University of East London. Anderson, B. (2006). Conventions for the U-Value Calculations. Watford: BRE Press. Anderson, C. (2004, June). Southern Comfort in a Straw Bale Home. Mother Earth News, pp Byfield, M. (2000, August 14). Bale comes on Strong. The Report, p. 32. Doran, S., & Kosmina, L. (2000). Examples of U-Value Calculations using BS EN ISO 6946: Glasgow: Building Research Establishment. Hilton, R. (2007). Straw Bale Construction: Is Straw Bale Construction suitable for Self-Builders in Britain? Welsh School of Architecture. Igaz, T., Szirtesi, K., & Lakatos, G. (2011). The Straw Bale House's Environment and Energy Conscious Buildings. Vasile Goldis University Press, pp Lietaert, M. (2008, Summer). A Straw Bale Village in Denmark. Communities, pp Little, J. (2009, July). Breaking the Mould III. Construct Ireland. Little, J., & Arregi, B. (2011, April). Thermal Bridging: Understanding its Critical Role in Energy Efficiency. Construct Ireland. Magwood, C., Mack, P., & Therrien, T. (2007, Summer). Expert Advice on Straw Bale Building. Mother Earth News, pp Pruteanu, M. (2010). Investigations regarding the Thermal Conductivity of Straw. Iasi: Gheorghe Asachi Technical Insitute. Seager, A. (2007, February 28). Construction Sector rises to challenge of building eco-friendly homes of the future. Retrieved from The Guardian: Steen, B., & Steen, A. (2001, April). Beauty of Bales. Mother Earth News, pp , 104. Steen, B., Steen, A. S., & Bingham, W. J. (2006, Summer). Build the Perfect Retreat. The Mother Earth News: Guide to Homes, pp

53 Stone, N. (2003). Thermal Performance of Straw Bale Wall Systems. Ecological Building Network. Wanek, C. (2005, June). Solar in the City. Mother Earth News, pp Wihan, J. (2007). Humidity in Straw Bale Walls and its effect on the decomposition of straw. Dagenham: University of East London School of Computing and Technology. Relevant Standards Anderson, B. (2006). Conventions for the U-Value Calculations. Watford: BRE Press. Ward, T., & Sanders, C. (2007). Conventions for calculating Linear Thermal Transmittance and Temperature Factors. Watford: IHS BRE Press. Department of the Environment, Community and Local Government. (2011). Technical Guidance Document L: Conservation of Fuel and Energy Dwellings. Dublin: The Stationary Office. The British Standard. (2007). BS EN ISO 13370: Thermal Performance of Buildings - Heat Transfer via the Ground calculation methods. Brussels: Management Centre. The British Standard. (2007). BS EN ISO 14683: Thermal Bridges in Building Construction - Linear Thermal Transmittance simplified methods and defauly values. Brussels: Management Centre. The British Standard. (2007). BS EN ISO 6946: Building Components and Building Elements - Thermal Resistance and Thermal Transmittance Calculation Method. Brussels: Management Centre. The British Standard. (2007). BS EN ISO 10211: Thermal Bridges in Building Construction - Heat Flows and Surface Temperatures Detailed Calculations. Brussels: Management Centre. 52

54 Appendix A Case Study: North Kesteven & ACDs 1.0 North Kesteven District Council As a built example of straw bale construction, the use of the North Kesteven dwellings inserts the research into a realistic setting. In order to use the scheme, details of the dwellings were sought from North Kesteven District Council. Below is a list of the various documents acquired from the council which can be found overleaf. 1. Straw Bale Dwelling: Ground Floor Plan; 2. Straw Bale Dwelling: First Floor Plan; 3. Straw Bale Dwelling: Section 4. Straw Bale Dwelling: External Elevations 5. Straw Bale Dwellings: Window Detail 6. Cavity Wall Dwellings: Preliminary Planning Details 2.0 Acceptable Construction Details Due to the lack of development within the cavity wall details provided by North Kesteven District Council, it was decided that the acceptable construction details as produced by the Department of the Environment, Heritage and Local Government would be used as a modern version of cavity wall construction. The three details used from the Acceptable Construction Details are listed below and found overleaf. Detail 1.02a Ventilated Eaves Junction Detail 1.05 Intermediate Floor Junction Detail 1.09 Ground Floor Junction 53

55 Appendix B Evaluating the accuracy of AnTherm Before choosing which thermal modelling software to be used to produce accurate results for thermal bridging, the accuracy and validity of the software must be investigated to ensure any results produced will be within the required accuracy levels set out in BS EN ISO which are highlighted below. Numerical Calculation: ± 5 % Thermal Bridge Catalogues: ± 20 % Manual Calculations: ± 20 % Numerical calculations refer to calculations produced by software in line with the conventions set out in BS EN ISO and BR IP 1/06 while manual calculations refer to any calculation completed manually using the mathematical theorems set out in BS EN ISO Thermal Bridge Catalogues are default psi-values based on the replication of catalogued detail. Thermal Bridge Catalogue method will not be used within this evaluation. In the evaluation of AnTherm, a manual calculation of the thermal coupling coefficient (L 2D ) will take place on simplistic model as used in section 2.2 of this report and shown in fig. 1. These results will then be compared against the results of an AnTherm simulation of the same model. 62

56 1.0 Manual Calculation of L 2D As detailed in BS EN ISO 10211, the thermal coupling coefficient is calculated using the following formula: L 2D = Q / Ti Te Where Q = Total Heat Flow W/K Ti = Internal Temperature (K) Te = External Temperature (K) In line with the conventions for modelling set out in BR IP 1/06, the difference between Ti and Te should always remain 20⁰ C. However the heat flow, Q, must be calculated manually. The formula for the calculation of Q is as follows: Q = λa (Ti Te) / t Where λ = Thermal Conductivity of all elements (W/mK) A = Cross Sectional Area considered (m 2 ) Ti Te = 20⁰ C t = Thickness between two environments (m) In order to establish the total heat flow through the model in fig.1, it must be divided in sections. The wall sections above and below the thermal bridge will be Section A and B (1530mm in height each) while the wall section containing the bridge will be Section C (150mm in height). 1,1 Heat Flow through Section A & B In this calculation, section A will be looked at exclusively with the result simply doubled due to section B being identical in all aspects. The cross sectional area (A) of section A is calculated by multiplying height, 1530mm by length, 1000mm, resulting in 1.53m. The thickness, t, between environments is 510mm or.51m and the 63

57 temperature difference as stated previously is 20. The total thermal conductivity of the section can be calculated by dividing the wall thickness by the total thermal resistance of all the elements within section A. The calculation of the total thermal conductivity of section A is shown below. Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Straw Lime Plaster Int Surface Total Resistance (R): Total Conductivity (t/r): W/mK Now that all the variables have been established, heat flow, Q, can be calculated through section A. The calculation is as follows: Q = λa (Ti Te) / t = (0.066)(1.53)(20) / 0.51 = 3.96 W/K per section Q = 7.92 W/K for Section A and B 1.2 Heat Flow Through Section C The total heat flow, Q, through section C, which contains the thermal bridge, can now be calculated once the variables have been established. The cross section area, A, of the section is 0.15m, the thickness between layers, t, is 0.51m and the temperature difference is 20. The calculation of the total conductivity of the section is shown below. 64

58 Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Timber Floor Int Surface Total Resistance (R): Total Conductivity (t/r): W/mK Having attained the required variables, the total heat flow through section C can now be established. The calculation is shown below. Q = λa (Ti Te) / t = (0.188)(0.15)(20) / 0.51 Q = 1.11 W/K It can therefore be established that total heat flow Q through the entire model is the heat flow through section A and B, 7.92 W/m, added to the heat flow through section C, 1.11 W/K, which results in a total heat flow through the model of 9.03 W/K. 1.3 Calculation of the Thermal Coupling Coefficient (L 2D ) With the total heat flow through the model now established, the calculation of the thermal coupling coefficient (L 2D ) of the model can take place. L 2D = Q / Ti Te = 9.03 / 20 L 2D = W/mK 65

59 2.0 Numerical Calculation of L 2D Following the calculation of the thermal coupling coefficient by the manual method, the resulting numerical calculation using the software an AnTherm can be compared to establish accuracy. Using an identical model to that used in the manual calculation, the thermal coupling coefficient has been calculated using AnTherm. The resulting figure from AnTherm for L 2D is W/mK which can be seen in Section 2.1 of Appendix D. The difference between the results is W/mK, representing a 0.7% accuracy, which achieves the minimum accuracy levels of 5%. It can therefore be established that AnTherm will comply with the conventions for calculation of linear thermal transmittance outlined in BS EN ISO and will produce accurate results for the determination of thermal transmittance. 66

60 Appendix C Wall Section U-Value Calculations This section provides the calculation method and results used for the u-value section within section 2.0 of this report, with particular reference to tables 3, 4 and 5. The location of each calculation path is shown in each section. 1.0 Straw Bale Eaves Detail Pathway 1 Through Plywood Sheet Pathway 2 & 3A Through Box Plate Insulation Pathway 3B Through Wall Plate Bracing Pathway 4A/B Through Straw Bale Wall Path 1 Plywood Sheet Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): 0.27 W/m 2 K Path 2 & 3A Insulated Box Wall Plate Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Beam Sheeps Wool Timber Beam Plywood Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K 67

61 Path 3B Wall Plate Bracing Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Bracing Plywood Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K Path 4A/B Straw Bale Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Straw Bale Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K 68

62 2.0 Straw Bale Intermediate Floor Detail Pathway 5A Through Wall Plate Insulation Pathway 5B Through Wall Plate Bracing Pathway 6A Through Box Wall Plate Insulation Pathway 6B Through 1F Timber Joist Pathway 7A Through Box Wall Plate Insulation Pathway 7B Through Wall Plate Bracing Path 5A Insulated Wall Plate Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Sheeps Wool Timber Plywood Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K Path 5B Wall Plate Bracing Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Plywood Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K 69

63 Path 6A Insulated Box Wall Plate Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Sheeps Wool Timber Plywood Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K Path 6B Floor Joist Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Plywood Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K Path 7A Insulated Wall Plate Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Sheeps Wool Timber Plywood Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K Path 7B Wall Plate Bracing Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Plywood Timber Int Surface Total Resistance (R): U-Value Calculation (1/R): 0.94 W/m 2 K 70

64 3.0 Straw Bale Ground Floor Detail Pathway 8A Through Insulated Section of Wall Plate Pathway 8B Through Timber Brace of Wall Plate Pathway 9 Through Masonry Plinth Path 8A Insulated Wall Plate Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Timber Sheeps Wool Timber Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K Path 8B Wall Plate Bracing Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Lime Render Timber Lime Plaster Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K Path 9 Masonry Plinth Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Brick Leaf Insulation Block Leaf Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K 71

65 Appendix D Thermal Bridging Assessment 1.0 Thermal Bridging Legislation As knowledge and understanding of thermal bridging has increased within the construction industry, provisions for the assessment of thermal bridging have been incorporated into the various documents of legislation in an attempt to combat the effects of these bridges. Within the Irish construction industry, the technical guidance documents set out criteria by which construction projects must comply. Technical Guidance Document: Part L deals with energy use in dwelling and non-dwelling schemes, in which the provisions for the prevention of thermal bridging are set out. Section of Part L states that for new buildings: To avoid excessive heat losses and local condensation problems, reasonable care should be taken to ensure continuity of insulation and to limit local thermal bridging (Department of the Environment, Community and Local Government, 2011) Reasonable care, while a vague and opinionated term, seems to specifically refer to the risk of surface or interstitial condensation, as stated in section , rather than potential heat flow through these bridges. Technical Guidance Document Part L highlights the various methods of assessing heat flow through the thermal bridging, all to be used with the DEAP calculation procedure, which is a program for calculating energy use in a building. This guidance documents provides various approaches to taking reasonable care with regard to limitation of thermal bridging: 1. Use of Acceptable Construction Details, as produced by the government 2. Use of certified details, which have been checked by a third party certification body 72

66 3. Use of alternative details which limit the risk of mould growth and condensation Within the DEAP calculation, various methodologies may be used depending on which of the above approaches is taken. If the first approach, use of the Acceptable Construction Details, is used, a multiplier factor of 0.08 can be used in the calculation of the heat loss through the building fabric, which is deemed to take account of the extra heat loss through the various thermal bridges (Department of the Environment, Community and Local Government, 2011). Where the second approach of certified details is used, the psi-values of the various wall junctions need to be calculated and a heat loss coefficient produced which will be used instead of the multiplier factor mentioned previously within the DEAP calculation software. Conventions for the calculation of psi-values are set out in the Appendix D and in various other legislation documents (Department of the Environment, Community and Local Government, 2011). A similar method should be used if the third approach is used. 2.0 Psi-Values vs. Applied U-Values When it is proposed to determine and analyse thermal bridging in construction, psivalues are generally used in the assessment. As stated in section of this research, psi-values represent a complex and specialist method of thermal bridging assessment that are beyond the skills of many construction industry workers. In 2.2.3, it is proposed that applied u-values be used to assess the heat flow through thermal bridging. In this section, the total heat flow through a simplified model similar to but not identical (length of over which u-value applies is shorter) to that used in section (fig 1) will be calculated using the psi-value and applied u-value methods to demonstrate the accuracy of applied u-values and also its simplicity compared to psivalues. 73

67 2.1 Psi-Value Calculation Below is the calculation method for determination of heat flow through a thermal bridge at an intermediate floor junction using psi-values. The equation for the calculation is shown below. Ψ = L 2D (U w x L w ) Where L 2D = the thermal coupling coefficient of the two-dimensional model U w = the U-value in W/m 2 K of the plane flanking elements L w = the length over which U w applies The Thermal Coupling Coefficient (L 2D ) of the junction is calculated by way of 2D software modelling using AnTherm software. If a model is not used, complex heat flow equations are required which require several lengthy calculations. The thermal coupling coefficient of the model as produced by AnTherm is W/K which represents total heat flow through the entire model. The u-value of the wall above and below the bridge (U w ) is W/m 2 K (which must be calculated beforehand) and the length over which this applies (L w ) is 3.06m. These figures can then be inputted into the equation for determination of psi-values. Ψ = L 2D (U w x L w ) = (0.129 x3.06) = W/mK or W/K This figure represents the total heat flow through the bridge of 1m length. To achieve this, a 2D or 3D model has to be created, a u-value calculation and a psi-value calculation performed. If the bridge were not one metre in length, a further calculation would be necessary. This shows the lengthy process required to establish the heat flow through a basic thermal bridge. 2.2 Applied U-Value Calculation A u-value calculation will now be performed through the thermal bridge and then applied over the area of that thermal bridge. 74

68 Element Thickness Thermal Conductivity Thermal Resistance Ext Surface Timber Bridge Int Surface Total Resistance (R): U-Value Calculation (1/R): W/m 2 K The surface area of the thermal bridge is its height (0.15m) multiplied by its length (1m) which is 0.15m 2. This is multiplied by the u-value of the bridge resulting in a total heat flow of 0.05 W/K. The difference between this calculation, 0.05 W/K, and the psi-value, W/K, is just 0.44% which is negligible. This demonstrates how applied u-values can be used to assess thermal bridging without the need to create a software model of the junction or perform complex equations which require research. The calculation method is also widely known throughout the building industry which will increase efficiency, accuracy and speed in the assessment of thermal bridges. 3.0 Thermal Coupling Coefficient Reports from AnTherm Thermal Coupling Coefficients, or total heat flow W/K, have been produced using software modelling, AnTherm, for the scope of straw bale and cavity wall sections and also the experimental model used in These found overleaf in the order of experimental model, straw bale wall and cavity wall. 75

69 3.1 Experimental Model Report Thermal Coupling Coefficient W/K Space(s) Internal External Internal External

70 2.2 Straw Bale Model Thermal Coupling Coefficient W/K Space(s) Internal External Internal External

71 2.3 Cavity Wall Model Report Thermal Coupling Coefficient W/K Space(s) Internal External Internal External