732: Meeting the 2011 zero carbon buildings target for Wales using the Passivhaus standard

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1 732: Meeting the 2011 zero carbon buildings target for Wales using the Passivhaus standard Christopher Tweed 1 *, Robert McLeod 2 BRE Centre for Sustainable Design of the Built Environment (SuDoBE), Welsh School of Architecture, Cardiff University, Wales, UK 1* TweedAC@cf.ac.uk Building Research Establishment (BRE) Wales, Port Talbot, Wales, UK 2 Abstract The Welsh Assembly Government has stated that from 2011 all new buildings in Wales must be zero carbon, five years ahead of the 2016 deadline set for homes in England. When policy makers and politicians set such goals they are often unaware of the technical, social and economic difficulties in achieving them. If the Welsh construction industry is to deliver on these promises, it urgently requires low or zero carbon standards and solutions that can be scaled up to meet increasing demand for new housing. The Passivhaus standard is enjoying a growing popularity on continental Europe, particularly in Germany and Austria, and although it does not satisfy zero carbon definitions, it could provide a useful starting point from which to develop low or zero carbon buildings. Its main premise that design heat load is limited to the load that can be transported by the minimum required ventilation air captures the essence of the approach in a way that is tangible and easily grasped by the imagination. This paper reports on research on the Passivhaus standard as a basis for new low carbon housing in Wales by examining the standard for its appropriateness in the milder Welsh climate, considering the construction technologies required to satisfy the standard, and considering the implications of applying the standard in Wales. The research concludes that Passivhaus offers a useful starting point but there is uncertainty about its dynamic thermal stability, the demands it places on occupant operation and requirements for high standards of construction skills. Keywords: energy, zero carbon, housing, Passivhaus, construction, Wales 1. Introduction The need to limit CO 2 emissions resulting from the construction and operation of the built environment is well established. Carbon has become a political as well as a chemical element, and the need to reduce the impact of CO 2 on the global climate features prominently in most recent manifestoes. Against this backdrop the Welsh Assembly Government (WAG) set an ambitious target that all new buildings should be zero carbon by 2011 [1]. This is five years ahead of the 2016 deadline set for England by the UK Government [2, 3]. These targets will require huge changes to current design and construction practices, impinging on all parts of the industry from material component manufacture to on-site operations and all aspects of their operations skills, financing and transport [4]. The purpose of this short paper is to consider the impact the 2011 zero carbon emissions target for new buildings will have on the construction industry in Wales. The task of delivering zero carbon buildings is made more difficult because of the absence of a tradition of providing energy efficient buildings in the UK, apart from a few isolated, showcase examples, such as the Vales Autonomous House, Hockerton Housing and BedZED. For various reasons, none of these has served as a template or widespread emulation. Perhaps, the construction industry is unwilling to apply readymade solutions developed by others because they seem too prescriptive. If models of low carbon buildings are not acceptable to the industry, then it may be better to develop performance based standards that allow developers and designers flexibility in how they meet them. The German Passivhaus standard is a candidate for this role and although it still requires some heating and power for ventilation, and so is not zero carbon (unless these can be supplied by a zero carbon sources), it could provide a useful starting point for developing zero carbon buildings. This paper investigates its possible role in the delivery of zero carbon housing to meet the 2011 target for Wales. The paper begins by describing the background to the aspiration set out by WAG. This is followed by a brief description of the Passivhaus standard before describing some calculations we carried out on an innovative house design. The results of the calculations are discussed before widening the scope of the discussion to consider how the Passivhaus standards might be applied in Wales. The paper concludes by identifying the main issues that face the pursuit of zero carbon housing in Wales.

2 2. Background 2.1 Emerging low carbon/energy standards for housing In the UK there are two main definitions of zero carbon dwellings: one defined in the widely accepted Code for Sustainable Homes [5], the other in the UK Government s proposals for exemption from stamp duty, the Stamp Duty Land Tax (Zero-Carbon Homes Relief) Regulations 2007 [6]. Both methods of assessment make use of concepts first introduced in Part L of the Building Regulations 2006, specifically the Dwelling Emission Rate (DER) which is the estimated carbon dioxide emissions per m 2 for the building, as designed, for energy in use for heating, hot water and lighting, and the Heat Loss Parameter (HLP), which is the rate of heat loss through the building fabric and via ventilation, divided by the total floor area and per degree of temperature difference between inside and outside, expressed as W/m 2 K. The Code for Sustainable Homes, which is derived from the BRE s EcoHomes standard, defines six levels of compliance for dwellings, with each level progressively requiring an improvement over the energy regulations defined in Part L In discussions about energy and carbon it is sometimes forgotten that the Code sets minimum standards for materials, surface water run-off and waste at entry level in addition to the requirements for energy efficiency. Level 6 is the most demanding and is called zero carbon. According to the Explanatory Memorandum to the Stamp Duty Land Tax Regulations: A zero-carbon home is one that will have all the energy used for heating, hot water, lighting and appliances on average over a year from renewable sources. [7] Both methods use the current version of the UK Government s Standard Assessment Procedure for energy rating of dwellings (SAP), known as SAP 2005 [8]. In addition to a net zero CO 2 emission, a zero carbon dwelling must also have a Dwelling Emission Rate (DER) less than or equal to zero and a Heat Loss Parameter of less than or equal to 0.8 W/m 2 K. The main difference between the two definitions is that in the Code two credits are awarded for a HLP value of less than 1.10 and one if less than 1.30, whereas the Stamp Duty definition requires a HLP value of less than or equal to The use of credits in the Code, therefore, allows the applicant to trade-off HLP against other creditawarding variables, such as internal lighting or a drying space below Code level 6. In the absence of detailed guidance on how to achieve zero carbon in buildings, the Passivhaus standard provides some concrete values which designers can use as tangible limits and targets. 2.2 The Passivhaus standard With over ten years of research and development in Central Europe the Passivhaus standard represents an established method of delivering very low energy buildings. One of the key tasks of the current project has been to assess the suitability of adopting the Passivhaus standard as a template for achieving low and zero carbon housing in Wales. Since the Welsh climate differs significantly from that of the European continent this study seeks to investigate any important climatic implications for the implementation of the Passivhaus standard in Wales. The hypothesis of this study is that because the climate in Wales is milder than that of central Europe, it may be slightly easier to meet the Passivhaus standard and thereby allows the construction some flexibility in its transition to zero carbon buildings. Dr Wolfgang Feist founded the Passivhaus Institut (PHI) in Germany to oversee the development and certification of this concept and to act as the guarantor for the strict criteria and requirements for Passivhaus certification. From an operational perspective the most important of these requirements is that the maximum space heating load does not exceed 10W/m 2 so that a comfortable indoor climate can be maintained without using a conventional heating system in a central European climate [9]. The second key criterion uses the concept of specific annual heat requirement, which is the useful heat requirement for the building divided by its Treated Floor Area, which is the living area within the thermal envelope [10]. The second criterion is the one most often quoted in relation to the Passivhaus standards, and is that the specific annual heat requirement must not exceed 15kWh/m 2 per annum. Both of these criteria must be met to obtain Passivhaus certification [11]. In practical terms, the ultra low heating demand in a Passivhaus makes it feasible to do away with a conventional heating system completely and use low energy alternatives, such as post-heated supply air to supplement the free heat gains provided by the sun, the building s occupants and appliances. A variety of supplementary heating systems has been successfully used in Passivhaus buildings, including electrical and solar hot water powered post-air heating, compact services units and district heating. As a result of focusing on reducing space heating demand the Passivhaus standard is defined by an energy performance specification rather than a CO 2 emissions rating (as used by the UK Building Regulations). Unfortunately, for the current study, this energy-based approach in Passivhaus creates a risk of there being significant variability in the actual CO 2 emissions depending on the fuel used in the heating system. For this reason the Association for Environment Conscious Building (AECB), one of the early adopters of the Passivhaus standards in the UK, has imposed an additional carbon limit on the Passivhaus projects they certify [12].

3 To cap excessive energy consumption from lighting, appliances and hot water use, the PHI has specified a maximum primary energy demand from all sources of 120kWh/m 2 per annum [11]. This figure is 2-4 times lower than the primary energy consumption levels of most new building standards across Europe [9]. However, as noted above, such an energy-based cap does not directly limit CO 2 emissions. Heat recovery ventilation is widely used in the Passivhaus concept and whilst it can be passive, Mechanical Ventilation with Heat Recovery (MVHR) is predominantly used [11]. Mechanical ventilation in turn necessitates the use of fans powered by electricity, which Passivhaus requirements limit to 0.4W/m 3 h -1 of supply air to limit excessive emissions [11]. It is worth noting that this corresponds to approximately 1.4 W/ls -1 and is considerably lower than the 2W/ls -1 guideline for the Specific Fan Power of MVHR in SAP [8]. Without controlled ventilation systems the ventilation heat loss component typically dominates the net heat loss from a well insulated building. By using tightly controlled supply ventilation rates of less than 0.6 ach 50 Pa in conjunction with heat recovery systems and a very air tight envelope, the Passivhaus concept achieves very low ventilation heat losses. Table 1: A comparison of Passivhaus standards and UK standard practice. Specification Passivhaus UK standard practice U-values < 0.15 W/m 2 /K 0.25 w/m 2 /K Ψ values (linear thermal bridges) Window U- values Air tightness Passive ventilation and preheating Passive solar Lighting Total energy space heating Reduced to 0.0 W/mK or negative in some cases Vary - up to 0.5 W/mK for some UK accredited constructions < 0.80 W/m 2 /K < W/m 2 /K < 1m 3 /hr/m 50Pa Ground ducts and / or heat recovery Key design factor Low energy lighting 7-10 m 3 /hr/m 50Pa None Limited 30% low energy building required < 15 KWh/m 2 /yr 55 KWh/m 2 /yr In summary, the Passivhaus concept is characterised by the use of a highly insulated thermal envelope, incorporating construction detailing that virtually eliminates thermal bridging. This is combined with a highly efficient MVHR system which operates at relatively low flow rates, providing continuous ventilation rates in the region of ach -1 during the heating season. Dwellings are orientated to maximise passive solar gains in winter where possible, however this is not seen as a prerequisite of Passivhaus design and the standard has been demonstrated in a number of northern European urban contexts where south facing solar orientation was not feasible due to site constraints [9, 15]. We get some idea of the step change that Passivhaus will demand from the UK construction industry by comparing specifications for the main aspects of design, as shown in Table Applying Passivhaus standards in Wales The ability to reduce the peak supplementary heating load to 10W/m 2 or less roughly a tenth of the space heating requirement of the existing UK stock average is a key characteristic of the Passivhaus concept; since reducing the peak load to this level allows a small ventilation supply air post heater to meet the entire annual specific heat requirement. In terms of applying the Passivhaus standard to the Welsh context this raises a fundamental question: The Post Heated Supply Air maximum load (P HSA) is a product of the rate of supply air changes (V) the maximum winter design temperature difference (Δθ) and the volumetric specific heat capacity of air (ρ.c p) P HSA = V. Δθ. ρ.c p Inserting typical values for the rate of air change, the central European climate, and the properties of the air results in (after Brans [16]): P HSA = 1m 3 /(hm -2 ). 30K. 0.33Wh/(m 3 K) = 10 W/m 2 The question this peak load sizing calculation raises is how representative a winter peak load design temperature difference (Δθ) of 30K is for Wales, when the figure of 30K is based upon an internal heating set point of 20 C and an external design temperature of -10 C [10]. Whilst external design temperatures of -10 C exist in central and northern continental Europe, winter monthly minimum temperatures for Wales have since 1994 (on average) exceeded 1 C [17]. Based on the use of the formula above the peak post heating load would become 6.27W/m 2 assuming an average winter minimum design temperature of 1 C in Wales. Global air temperature records also illustrate a pronounced upward trend towards warmer than average design years reflecting both increasing minimum design temperature s and a reduction in the absolute number of heating degree days. Despite climate data for Wales suggesting a more appropriate winter design temperature in the region of 1 C as opposed to -10 C it must be acknowledged that these figures do not highlight the most extreme design temperatures possible, nor do they show sub-regional temperature variations, irradiation or wind exposure implications. Even accounting for regional variations in the minimum winter design temperature, it is likely that a relaxing of aspects

4 of the Passivhaus standard as applied in Wales is appropriate where a peak load well within the 10W/m 2 requirement can be demonstrated [18]. 2.4 Satisfying the Passivhaus standard in different climatic zones across Europe Research on UK Passivhaus heating load sizing was carried out by Schneiders under a European funded PEP project in Schneiders research illustrated that with opaque U values of 0.17 W/m 2 K and glazed U values of 0.85 W/m 2 K it is possible to comfortably meet the Passivhaus heating load and peak demand criteria in Manchester provided that a south facing glazed area of between 10 and 30m 2 is used in the reference dwelling. This glazing range can be considered reasonable since the reference dwelling used in the PHI heating load sizing study is based upon an end of terrace dwelling with an exterior façade area of 74m 2, total glazed area of 30.4m 2 and a south facing glazed area of 19.9m 2. The derivation methodology for this heating load sizing assessment is based upon the use of two different extreme weather data sets (Weather 1 and 2) which are likely to occur only once every two years, to which an additional safety margin of 1.5 W/m 2 is added [19]. Using Schneiders Manchester data it is possible to infer the total Specific Heat Loss for the combined opaque and glazed constructions and so derive an upper limit for the glazing U-value assuming that a standard Passivhaus wall U value of 0.15 W/m 2 K was used in place of the higher value of 0.17 W/m 2 K used in this study. Table 2. Total Specific heat loss of glazed and opaque wall elements (Manchester) Area (m 2 ) U value (W/m 2 K) Specific Heat Loss (W/K) Total Using the total specific heat loss from Table 2 as a constant, the peak heating load will remain unchanged and the following upper limit can be derived for the glazed U value (U G) : U G = (74* 0.17)/ 30.4 = 0.9 W/m 2 K Therefore, assuming an external wall U value of 0.15 W/m 2 K, it would be possible to relax the glazed U value slightly to 0.9 W/m 2 K in the Manchester climatic context and still fulfil the PHI peak heating load and specific heat demand criteria. By reducing the south facing glazed area further it can be shown that the peak heating load could be further reduced, potentially allowing for even greater tolerance in the glazed U-value. Schneiders research illustrates that there is a clear correlation between increased glazing area and increased peak heating loads. However in more temperate climates such as Amsterdam, Birr (Ireland) and Manchester an inverse correlation also exists between annual space heating demand and glazing area. This finding was also demonstrated in colder climates where more advanced glazing standards were specified. It is clear from this research that an optimum relationship between heating load, heating demand and glazing area needs to be determined on a location specific and glazing dependent basis for each Passivhaus project. Further research investigating the effect on peak heating loads and space heating demand of slightly relaxing Passivhaus thermal standards have also been carried out by researchers at Lund University, Sweden. Thermal modelling carried out on an apartment building at Värnamo, Sweden demonstrated that the difference between using windows with an installed U-value of 1.0 W/m 2 K instead of the PHI recommended installed value of 0.85 W/m 2 K resulted in only a small increase in the peak heating load from approximately 8.8 W/m 2 to 9.8 W/m 2 while the annual space heating demand remained well within PHI criteria [20]. Considering that the winter design temperatures in this study (Värnamo, Southern Sweden, with monthly low averages in January and February of approximately -6 C) are significantly colder than in Wales, this finding has particular relevance to the adaptation of the Passivhaus thermal performance criteria to the Welsh context. 3. Case Study St Athan, Cardiff, Passivhaus project, Wales The Passivhaus Planning Package (PHPP) was used to investigate the influence of infiltration levels and glazing U-values on peak heating load and the annual heating demand in a a proposed low energy house for a site in St Athan, near Cardiff. Preliminary inspection of the current PHPP07 climate data sets, revealed a lack of critical heat load sizing data (Weather1 and Weather2) for UK locations other than Manchester and a complete absence of climate data for Wales. In the absence of location specific data, Test Reference Year (TRY) data for Plymouth was considered to be a reasonable spatial analogue and this was combined with Weather 1 and 2 data sets for Manchester as means of approximating Cardiff weather. Scenario 1 investigated a range of infiltration levels to determine their influence on peak heating load and annual heating demand and to establish the limits to complying with Passivhaus criteria in the Cardiff context. For the purposes of this investigation, U-values for the opaque constructions were assumed to be constant without thermal bridging to reduce the number of variables. In practice, thermal bridging losses must be included in the PHPP calculation, but with good design these can be minimised. In Scenario 2 a constant air tightness value of 0.6 ach 50 Pa was used and variations in the glazing U-value from 0.8 W/m 2 K to 1.1 W/m 2 K

5 were investigated. Baseline values for opaque thermal transmittance and all other heat loss variables remained constant in this scenario. 3.1 Scenario 1: Results - Infiltration vs. peak load and heating demand Fig 1. Scenario 1: influence of infiltration rates on peak heating loads and annual space heating demand in the St Athan house design Results for Scenario 1 show a clear correlation between increased infiltration rates and increasing peak heating loads and annual heating demand, as might be expected. Notably, the maximum 10W/m 2 peak heating load requirement is not exceeded until 3 ach 50 Pa is reached. 3.2 Scenario 2: Results U-value of glazing vs. peak load and heating demand U values. Significantly, air infiltration rates of over 2 ach 50 Pa and glazing U-values of up to 1.1 W/m 2 K did not (in isolation) prevent the dwelling from meeting either of the Passivhaus key performance criteria. The research findings of Schneiders [19] for Manchester and Ireland suggest similar findings with the Passivhaus standards being met using opaque U-values of 0.17 W/m 2 K and glazing U- values of 0.85 W/m 2 K. Similarly a study carried out on an apartment building in Värnamo, Sweden (Janson and Wall, 2007) indicated that it was possible to meet the Passivhaus peak heating load requirement in Southern Sweden using installed glazing U-values of 1.0 W/m 2 K. Collectively these finding indicate that it may well be possible to achieve the Passivhaus standard in Wales with a significant degree of tolerance in the glazing U-values (in excess of 40%) and an even greater degree of tolerance (+300%) in the air tightness of the building fabric. If confirmed through wider research studies, these findings could have implications for the procurement of locally sourced Passivhaus components as well as the design specification of a Welsh Passivhaus. It must be noted however that the overall rate of heat loss from any dwelling is highly context specific and therefore general design guidance should not be inferred from these preliminary findings. Fig 2. Scenario 2: influence of glazing U-values on peak heating loads and annual space heating demand in the St Athan house design. Despite increasing the glazing U-values from 0.8 to 1.1 W/m 2 K there is only a modest increase in the peak heating load and specific heating demand. Notably, both key criteria for the Passivhaus standard are still comfortably met, even when the glazing U-value is relaxed to 1.1 W/m 2 K. 4. Discussion The findings of this study broadly confirm that the Passivhaus standard can be more easily achieved in a Welsh climatic context than in continental Europe as would be expected from the milder winter design temperatures experienced in Wales. Research using the PHPP model in the context of the St Athan project provides an indication of the likely design tolerances that could be permissible for a Passivhaus located in South Wales in terms of higher rates of air permeability and higher glazed 5. Issues arising from adopting the Passivhaus standard in Wales We have established that in principle the Passivhaus standard is a viable starting point from which to develop zero carbon housing solutions. There are, however, other issues to consider in promoting this standard. These may be revealed using three broad questions. First, can the construction industry in Wales build to this standard? Our initial exploration of appropriate construction technologies suggested the standard might be achieved best through modern methods of construction (MMC), relying primarily on timber framing skills and techniques. Wales is only beginning to develop an MMC capability and so is unlikely to be able to deliver sufficient quantity of buildings for several years. However, the database of Passivhaus projects in Austria [21] shows that a significant number (30%) use masonry construction, suggesting that it should be possible to continue using traditional construction methods in Wales, though it is highly likely that the skills required may be different if only to achieve the air tightness standard. The second question is: can we afford to build to this standard? A cost review carried out by Cyril Sweett [22] suggests there are significant costs associated with building to low carbon standards and these vary according to the dwelling type detached, terraced, or apartments. Tellingly, the review stops short of costing compliance with level 6 of the Code. Cost will obviously be a

6 major factor in delivering a high volume of new zero carbon housing. Finally, it is important to ask if people will be willing and able to adapt to living in Passivhaus dwellings? Thermal conditions in a Passivhaus building are finely balanced because the thermal capacity of the heated air is so low. Higher ventilation rates or higher internal heat gains are likely to destablise the internal environment and so the occupants need to be skilled to operate the building correctly. There are further issues related to building maintenance. Comments from Austrian contributors, who have direct experience of Passivhaus buildings, to the Sullivan Report [23], highlight the need for occupants, for example, to maintain the filters on MVHR systems to ensure adequate ventilation. Studies into the health implications of low ventilation rates are also being conducted. These questions underline the need to conduct more detailed research in future studies to discover the most practicable models for zero carbon housing. 6. Acknowledgements The authors would like to thank the Welsh Assembly Government for supporting the research through Knowledge Exploitation Fund grant HE07 KEF We would also like to thank our colleagues in the Design Research Unit in the Welsh School of Architecture who provided details of their design for the St Athan house. 7. References 1. Jones, C., (2007). Programme of action to tackle climate change. Available: ntpress/2007/ /?lang=en 2. Weaver, W., (2006). Brown pledges to build zero carbon homes. Guardian Unlimited Dec 6. Available: [June 07]. 3. HM Treasury. Pre Budget Report Chapter 7, Section 7.46 p168 Available: 4. NHBC (2008). Zero carbon: what does it mean to homeowners and housebuilders? NF9. Available: Default.aspx [August 2008]. 5. DCLG (2008). The Code for Sustainable Homes: setting the standard in sustainability for new homes, Department of Communities and Local Government. Available: [April 2008]. 6. UK Government, (2007). The Stamp Duty Land Tax (Zero-Carbon Homes Relief) Regulations Statutory Instrument No The Stationery Office Limited, London. 7. UK Government, (2007). Explanatory Memorandum To The Stamp Duty Land Tax (Zero-Carbon Homes Relief) Regulations Available: [June 2008]. 8. BRE (2005). SAP 2005: The Government's Standard Assessment Procedure for Energy Rating of Dwellings. Available: [May 2008]. 9. Schnieders, J. and Hermelink, A., (2006). CEPHEUS results: measurements and occupants satisfaction provide evidence for Passive Houses being an option for sustainable building, Energy Policy, Feist, W., (2007). Passive House Planning Package PHI-2007/1(E). 11. Feist, W., (2004). Passive House Planning Package PHI-2004/1(E). 12. Ref. AECB 2008, CarbonLite Programme, Volume 3: The Energy Standards [available] AECB (2006) Gold and Silver Energy performance Standards for Buildings. Available: [31 Aug 06]. 14. Feist, W., Schneiders, J., Dorer, V. and Haas, A., (2005). Re-inventing air heating: Convenient and comfortable within the frame of the Passive House concept. Energy and Buildings, 37, International Energy Agency. Demonstration Houses in Germany. IEA-SCH Task 28/ ECBCS Annex 38: Sustainable Solar Housing pdf 16. Brans J., (2008). Passiefhuis-Platform vzw. PHPP Course Feb 21, 2008 Milton Keynes, UK. 17. UKCIP. The climate of the UK and recent trends: Maps. 18. Schneiders, J., (2008). In conversation, 11 April. 12th International Conference on Passive Houses, Nuremberg. 19. Schneiders, J., (2003). Climate Data for the determination of Passive House Heat Loads in North Western Europe. PEP Project Information, EIE Janson, U. and Wall, M., (2007). Experiences from apartment buildings as passive houses in Sweden. 11 th International Conference on Passive Houses p118 PHI, Darmstadt. 21. IG Passivhaus Oberösterreich. Available: [June 2008]. 22. Scottish Building Standards (2008). A Low Carbon Building Standards Strategy for Scotland (The Sullivan Report). Available: [June 2008]. 23. Cyril Sweett, (2007). A Cost Review of the Code for Sustainable Homes, February. Available: odecostreview.pdf [June 2008].