Datasheet. Thermal Benefits of Solid Construction

Transcription

1 > Datasheet > Thermal Benefits of Solid Construction Introduction Recently, attention has been focussed on environmental issues, and, in particular, energyefficient construction. Building designers are now taking into consideration the combined use of orientation, mass, insulation and glass, all of which can contribute to the comfort of the occupants, increased energy efficiency and reduced costs to maintain a comfortable living environment. The contribution of thermal mass to the overall performance and energy efficiency of a building has always been recognised, but quantifying the extent, or knowing how various types of solid construction compare has been difficult. A recent research project by the Cement and Concrete Association of Australia (C&CAA), and Concrete Masonry Association of Australia (CMAA), and carried out by the CSIRO, assesses the energy efficiency of various combinations of wall and floor construction, to determine the contributions that thermal mass and insulation can make. This Data Sheet presents a summary of the research findings, which include: nov 22 For cooling the building n Buildings using uninsulated solid walling options such as concrete panels and concrete masonry have similar performance to insulated brick veneer construction. When R1. insulation is used with solid construction, the solid walling options outperformed the brick veneer wall system. n Insulated cavity brick construction shared the best performance. n Insulation has a relatively minor effect on the energy efficiency of the walling systems tested. n Internal solid partition walls give the best performance, and these should not be insulated so that the thermal mass is available. > Thermal mass is why buildings having solid construction typically feel cooler during the summer and warmer during the winter

2 For heating the building (without insulation in the walls) n Solid cavity construction outperforms all other wall types tested regardless of climate. n Solid single leaf concrete and concrete masonry walling systems require only plasterboard on battens to give similar performance to that of cavity construction. For heating the building (with insulation in the walls) n gives similar performance to other wall types tested. n Concrete and concrete masonry walls require only foil-backed board on battens to provide effective insulation similar to other walling systems tested. By using the information gained through this research work, it may be possible to save on insulation and walling system costs, increase energy efficiency and reduce ongoing heating and cooling costs. Find out how the benefits of solid construction can work for you in your next project. Definitions Thermal mass This refers to a material s ability to store up thermal energy (via its mass), rather than simply allowing heat to flow through the building element, whether a wall or a floor. Thermal mass is related to the material's thermal capacitance. Thermal capacitance or the C-value The amount of heat required to raise the temperature of a unit area of a material of a particular thickness by 1 C. It is calculated as the product of the material s density, thickness, and specific heat and expressed in J/m 2 K or kj/m 2 K. Specific heat An intrinsic property of a material, the specific heat is the amount of heat required to raise one kilogram of a material by 1 C, expressed in J/kgK. Thermal resistance or the R-value A measure of the product s insulating ability, expressed in m 2 K/W. The higher the R-value of a building product, the more resistance it has to heat loss in winter, and heat gain in summer. Codes and Standards All structures built in Australia have to comply with the Building Code of Australia (BCA). The BCA contains special provisions for the various States, especially with regard to thermal insulation. This is understandable, as States such as Victoria would have more concern about heating losses and the energy required to heat buildings, whereas Queensland would have more concern about the heat gain or energy required to cool buildings. For this reason the BCA has mandatory provisions for minimum R-values of building elements in various States and Territories. The Australian Standard for thermal insulation of dwellings AS gives R-values for various building materials in standard building systems (eg concrete masonry block walls), and hence ways of complying with the BCA requirements. At present, the building and construction industry because of the various Codes and Standards, tends to focus on the R-value of building products, rather than the combined effect of both the R-value and the thermal mass or C-value, when assessing a building s energy efficiency. Some acknowledgement of the contribution of thermal mass is given, however, through provisions that allow for no insulation in the walls if the thickness of solid construction is greater than 18 mm, and the building is constructed on a slab-on-ground. The research work referred to earlier is intended to quantify the contribution, and allow a move away from the typical R-value based assessment system, where compliance is based on achieving either a minimum R-value for the wall, or providing a minimum thickness. HEAT FLOW W/m Heat flow disregarding thermal capacitance Actual heat flow (Including thermal mass effect) Mean heat flow 6-hour time lag midnight noon TIME OF DAY Figure 1 Heat flow through a concrete roof (2 mm thick) from Addleson, L Materials for Building Vol 4, Newnes-Butterworth, 1976 How does Thermal Mass Work? Figure 1 shows how the mass of a concrete roof, which gives it the capacity to store thermal energy, lowers the heat flowing through it. Solid or high-mass walling systems act the same way. This ability to store thermal energy causes the peak temperature to be offset by approximately 6 hours. The offsetting of the peak temperature is known as the thermal lag. With a thermal lag of say 6 hours, the maximum indoor temperature will not occur until 6 hours after the maximum outdoor temperature has been reached (usually between noon and 2 pm). This normally represents a cooler part of the day or evening, with the stored thermal energy then keeping the inside warmer during low night-time temperatures. Thus thermal mass tends to 'iron out' the outdoor variations in temperature, Page 2 of 6 > Thermal Benefits of Solid Construction

3 Table 1 R-values and C-values for building materials c-value Wall system or thermal thickness Density R-value capacitance Wall system and R-value Material mm kg/m 3 m 2 K/W kj/m 2 K indoor finish of system Concrete solid wall, paint.26 Hollow core concrete panel single skin, render.35 Concrete solid wall, paint.23 Concrete masonry single block, render.39 Clay masonry brick veneer.54 Concrete masonry single block, render.32 Glass curtain wall.16 Timber cladding clad frame.47 thereby reducing the maximum and minimum temperatures inside, providing a more uniform and pleasant indoor living environment. Thermal mass is why buildings having solid construction typically feel cooler during the summer and warmer during the winter. R-value v C-value The two main criteria that designers have used when evaluating the thermal efficiency of building products, are the R-value, and to a lesser extent the thermal mass. Table 1 lists R-values, as well as the C-values (thermal capacitance), for various building materials and thicknesses. It can be seen that high-density walling materials such as concrete do not fare well when assessed purely on the criteria of R-value. However, when assessed on thermal capacitance, concrete far outperforms its lightweight alternatives. It is for this reason that concrete is thermally more effective than the R-value alone would indicate. Evidence of this can be felt when one walks into a concrete, concrete masonry, or other solid type building with concrete slabs above and below, on a hot day. The temperature inside is noticeably cooler, because the mass of the walls is storing some of the heat, rather than allowing it to pass directly through. Clearly the two work in combination, but what effect does each have? Research Work The research project funded by the C&CAA and CMAA, and carried out by the CSIRO was a systematic study that compares the thermal performance of a wide variety of materials currently in use, and includes the interaction of insulation and thermal mass. The study evaluates the thermal performance of a number of wall systems for three building types. n A detached house identical to that used to develop the VicHERS scheme, n A three-storey apartment building for which units on each level were evaluated in both corner and central positions, and n A low-rise commercial building incorporating a mixture of retail, specialty shops and office space. Each of the buildings was evaluated for a full year of weather conditions using meteorological data for Melbourne, Sydney and Brisbane. This covers a range of climates from heating or 'winter dominated' through 'balanced' to cooling or 'summer dominated'. Thus the results represent most climatic areas within Australia. The computer simulation used the simulation engine from the most recent version of the NatHERS software for the residential buildings and CSIRO s BUNYIP software for the commercial building. Modifications were made as required to cater for wall types and finishes not available in the standard versions. To support the simulations, the thermal resistances were measured for those materials where such data was lacking. These simulation programs were used to determine heating and cooling requirements for a large number of combinations of wall type, interior finishes and insulation alternatives for each of the three building types: detached house, apartment building and commercial building. Page 3 of 6 > Thermal Benefits of Solid Construction

4 Details of Buildings External wall types investigated 2 mm () 14 mm and 19 mm concrete masonry block (MB14/MB19) 15 mm solid concrete (C15) Cavity brick construction () Brick veneer () Sandwich panel (concrete + polystyrene) ( apartments/commercial) Hollow core concrete panels (HC2 apartments/commercial) Interior finishes to walls Paint 1 mm render Plasterboard direct fixed to wall Plasterboard on battens Plasterboard on studs ( only) Insulation alternatives wall Foil backed board (for MB and C15) Cavity insulation of 1. m 2 K/W (for only) Reflective foil over studs (for only) Other general details Apart from each wall type being checked with combinations of the internal finishes and insulation options typically used for that wall type, the effect of using solid and lightweight internal partitions was also checked. For detached houses, a 1-mm concrete slab and a tiled roof with bulk insulation of 3. m 2 K/W (R3) in the ceiling were used. Apartments also incorporated concrete slabs except in the case of brick veneer construction, which necessitated the use of a light weight timber floor. The roof was of pitched, steel-sheet construction with insulation as for the house. For the commercial building, the roof was pitched steel-sheet with 2. m 2 K/W (R2) insulation on the ceiling, and internal partitions were lightweight stud walls in all cases except for where internal walls were 1 mm. Research Findings Figures 2 and 3 are typical of the results that were obtained for an apartment building in a cooler climate (Melbourne), and Figures 4 and 5 for a warmer climate (Brisbane). Despite the range of answers that analysing such a wide combination of walls, finishes, insulation and floors for each of the various climates could give, the main points were few and similar for all building types. These are listed below. Some specific comments relating to apartments and commercial buildings are also given. For cooling n The cooling energy required is essentially independent of the R-value of the wall system. Specifying a minimum R-value for the walls for climates that require cooling thus has little impact on the energy efficiency of the building. Refer Figures 2 and 4 and Table 1. n Solid partition walls internally give the best perform ance, and these should not be insulated so that the thermal mass is available. For heating n In cooler climates where heating is the predominant requirement, once the walls are insulated, the heating energy differences between the wall types is relatively small and the location of the insulation (inside face, outside face, central or both faces), has little impact. Refer Figure 3 and the list of insulation alternatives. n Insulated concrete panel walling systems are similar to all other wall types tested regardless of climate or insulation being present. Note that for commercial buildings (refer specific comments), the performance of all wall types is about the same. n External concrete and concrete masonry walls need to be finished with plasterboard on battens internally to approximate the performance of uninsulated cavity construction. Apartments specific comments n Corner apartments have greater heating energy requirements than centre apartments (about twice as high), but only slightly higher cooling energy require ments. The higher heating energy is a consequence of the greater area of exposed walls, while the thermal mass reduces the cooling requirements. n For cooling of the building, brick veneer or lightweight construction performs poorly compared with all other wall types. This is because the suspended floors must also be of lightweight construction, thereby further reducing the thermal mass available internally to assist in reducing the cooling requirements. Commercial buildings specific comments n Cooling energy consumption is about the same for all wall types regardless of climate or insulation as the cooling loads tend to be dominated by internal and solar gains rather than building fabric gains or heat flow through the walls. Page 4 of 6 > Thermal Benefits of Solid Construction

5 5 15 Cooling Energy (MJ/m 2 ) Cooling Energy (MJ/m 2 ) MB19 MB14 C15 Figure 2 Annual cooling energy requirement, centre apartment, Melbourne MB19 MB14 C15 Figure 4 Annual cooling energy requirement, centre apartment, Brisbane 35 3 Heating Energy (MJ/m 2 ) Heating Energy (MJ/m 2 ) MB19 MB14 C15 Figure 3 Annual heating energy requirement, centre apartment, Melbourne MB19 MB14 C15 Figure 5 Annual heating energy requirement, centre apartment, Brisbane Render/paint Plasterboard Plasterboard on battens Insulated/foil-backed board Conclusions The research undertaken by the C&CAA and CMAA, and carried out by the CSIRO clearly confirms the contribution that thermal mass can make to the energy efficiency of buildings. The research also demonstrates that the use of R-values alone in assessing the thermal performance of a building can be misleading. Under cooling conditions the thermal mass offered by solid construction makes a significant contribution. Under heating conditions, the use of insulation does improve the energy efficiency and there is virtually no difference between the walling systems tested once insulation is added. The use of a solid concrete or concrete masonry wall with foil-backed board on battens gives similar performance to other walling systems such as and insulated cavity construction (all with higher R-values). The Building Code of Australia contains some exemptions for solid walling systems, and also allows the use of walling systems that have an equivalent perform ance to those satisfying current requirements for minimum R-values. Based on the research, it is now possible to use the Nationwide House Energy Rating Scheme software (NatHERS) to examine and assess alternative residential thermal mass walling systems. The performance of these walling systems (which may also be more economical) can be equivalent or similar to walling systems that satisfy the current requirements for minimum R-values. Summary The mass provided by solid walling construction, results in a more comfortable living and work environment within the building as maximum temperatures are not reached and the moderated peak temperature corresponds to a cooler part of the evening due to the thermal lag. Other factors such as orientation and glass area will combine to maximise the benefits of solid construction. Page 5 of 6 > Thermal Benefits of Solid Construction

6 nov 22 The use of concrete or other solid construction systems to either offset thermal gain (usually in summer), or store up and release thermal warmth via passive solar principles (usually in winter), is highly beneficial in building design to ensure the optimum use of finite energy resources. Less energy is required to maintain a comfortable internal temperature when thermal mass is incorporated into the building or structure. Thermal mass will not only reduce the running costs, but also provide opportunities for cost savings on the walling type used. For climates requiring the building to be cooled, the omission of insulation may also be possible, offering further savings. As thermal performance is only one of the many benefits offered by solid construction, consider the advantages of solid construction for your next project. CCAA OFFICES SYDNEY OFFICE: Level 6, 54 Pacific Highway St Leonards NSW Australia 265 POSTAL ADDRESS: Locked Bag 21 St Leonards NSW 159 TELEPHONE: (61 2) FACSIMILE: (61 2) BRISBANE OFFICE: Suite 2, Level 2, 485 Ipswich Road Annerley QLD 413 TELEPHONE: (61 7) FACSIMILE: (61 7) MELBOURNE OFFICE: 2nd Floor, 1 Hobson Street South Yarra VIC 3141 TELEPHONE: (61 3) FACSIMILE: (61 3) PERTH OFFICE: 45 Ventnor Avenue West Perth WA 65 TELEPHONE: (61 8) FACSIMILE: (61 8) ADELAIDE OFFICE: PO Box 229 Fullarton SA 563 TELEPHONE: (61 8) premixed concrete and EXTRACTIVE INDUSTRIES OFFICE PO Box 243 Henley Beach SA 522 TELEPHONE: (61 8) FACSIMILE: (61 8) TASMANIAN OFFICE: PO Box 246 Sheffield TAS 736 TELEPHONE: (61 3) FACSIMILE: (61 3) WEBSITE: info@ccaa.com.au Layout: Helen Rix Design Disclaimer: Cement Concrete & Aggregates Australia is a not for profit organisation sponsored by the cement, concrete and aggregate industries in Australia to provide information on the many uses of cement, concrete and aggregates. This publication is produced by CCAA for that purpose. Since the information provided is intended for general guidance only and in no way replaces the services of professional consultants on particular projects, no legal liability can be accepted by CCAA for its use. CCAA respects your privacy. Your details have been collected to provide you with information on our activities, publications and services. From time to time your details may be made available to third party organisations who comply with the Privacy Act such as affiliated associations, sponsors of events and other reputable organisations whose services we think you may find of interest. If you do not wish to receive information from CCAA or wish to be taken off the database please write to the Privacy Officer, CCAA, Locked Bag 21, St Leonards, NSW, 159 Page 6 of 6 > Thermal Benefits of Solid Construction