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Leeds Sustainability Institute

Improving the energy efficiency of homes

Research on the domestic energy performance gap, resulting in lower fuel bills and fewer carbon emissions in new builds and retrofits.

Improving the energy efficiency of homes

the challenge

This case study is a continuation from work which outlined how previous research undertaken for the Office of the Deputy Prime Minister (ODPM) and the Joseph Rowntree Foundation (JRF) by Leeds Beckett University (LBU) identified that the actual energy used in homes was greater than their designs intended. This suggested that all 27 million homes in the UK had higher fuel bills and greater carbon emissions than was previously thought. Since 2003, LBU have further characterised, measured and investigated the causes and implications of this domestic energy performance gap through various building performance evaluation (BPE) research projects.

Impact previously evidenced, included Bell serving on the CLG's regulatory teams for a review of the Building Regulations as conveyor and technical author to lead industry working parties for the 2010 (CLG, 2009) and 2013 (CLG, 2012) reviews, as well as LBU research recognition in a National Audit Office report (NAO, 2008) and in evidence to the House of Commons Public Accounts Committee (PAC, 2009). Citations of the Zero Carbon Hub work on recognising the performance gap are also found in the Low Carbon Transition Plan a National Strategy for Climate and Energy (HM Government, 2009). Impact has accelerated further, specifically, LBU research’s role in establishing and quantifying the building fabric thermal 'performance gap' and contributing to a U-turn in the approach of housebuilders to this phenomenon (Sunday Times, 2015), and the adoption of the party wall bypass into the Building Regulations for England and Wales (HM Government, 2016), and applying these findings to the approach to retrofitting existing dwellings.

Our approach

  1. Discovering the performance gap in homes

    This case study is a continuation from the previous REF submission which outlined how research undertaken for the Office of the Deputy Prime Minister (ODPM) and the Joseph Rowntree Foundation (JRF) by Leeds Beckett University (LBU) identified that the actual energy used in homes was greater than their designs intended. This suggested that all 27 million homes in the UK had higher fuel bills and greater carbon emissions than was previously thought. Since 2003, LBU have characterised, measured and investigated the causes and implications of this domestic energy performance gap through various building performance evaluation (BPE) research projects.

  2. Establishing heat loss through party walls in new build homes

    Research was carried out by LBU during the Stamford Brook project, a development of around 700 masonry dwellings, between 2004 and 2010, in partnership with the Department of Communities and Local Government (CLG), National Trust and housebuilders Redrow and Bryant Homes. This evaluated the impact of an enhanced energy performance standard designed for possible incorporation into an amendment to the new build element of Part L of the Building Regulations. Using coheating tests, whole house heat transfer coefficients were measured to be up to 100% higher than predicted, a significant finding with large implications for national carbon policy and householder bills. Further investigations revealed, a thermal bypass operating in the party wall cavity. This phenomenon involves cold air entering the edges of an uninsulated and unsealed cavity party wall being heated by the conduction of heat through the party wall. The resultant hot air rises through the cavity and then escapes through the cold loft space, increasing the heat lost from the dwelling thermal envelope. This was a significant finding, as previous design and regulatory practice assumed that the heat loss from party walls was insignificant and hence had been ignored in the calculation of dwelling carbon emissions.

  3. Developing methods for measuring true building performance; the Coheating test

    Between 2007 and 2010, LBU undertook the Elm Tree Mews project, also funded by JRF. LBU again measured significant underperformance in both fabric and services, with dwelling heat loss measured to be 54% higher than designed (3.3). The electric coheating test for measuring the real performance of buildings, called the heat transfer coefficient (HTC), were significantly refined and developed during this project. One meta-study published by the group summarised the data specifically for a range of 25 new dwellings, indicating that homes consume on average over 1.6 times their predicted values.

  4. Reducing heat loss through existing party cavity walls

    The Core Cities Green Deal funded by the Department for Energy and Climate Change (DECC), between 2013 and 2017, LBU further explored the party wall bypass phenomenon, identifying its presence this time in existing dwellings, not just new build dwellings. It was identified that a cavity party wall full-fill retrofit could reduce whole house heat loss by 8% in some instances. Following this discovery, LBU undertook another project in 2017 with the Department for Business Energy and Industrial Strategy (BEIS) to characterise and measure heat loss through a variety of cavity party walls. This work identified that the mean measured party wall U-values varied greatly from 0.21 to 0.81 W/m²K, indicating that party wall insulation retrofits could meaningfully reduce heating consumption and fuel bills.

  5. Identifying the performance gap in retrofits of existing buildings

    These projects also identified more broadly that the domestic performance gap relates not only to new build homes but also retrofits taking place in existing homes. In the Core Cities Green Deal project, the energy efficiency of 65 homes was measured before and after they underwent retrofits. The research measured a performance gap in wall U values of up to 21%, and identified that there were similar causes, and implications, for existing as observed in new build homes.

The impact

The impact from LBU research has benefited homeowners through lower fuel bills, domestic construction-related businesses by creating new markets, policymakers by providing new insights and the environment via lowering carbon emissions.

This represents a huge financial saving of tens of millions of pounds to UK households every year.

Thanks to LBU research, party wall heat loss was been incorporated into the 2010 Part L of the UK Building Regulations. This leads to substantial impact in this REF cycle since all new homes built since 2014 have fuel lower bills. Removing the party wall bypass in existing dwellings may reduce whole house heat loss by up to 8% when a party cavity wall is not capped. This may be used as a proxy to quantify the impact of the regulatory changes for new build homes. The average annual gas bill in England and Wales in 2019 was £588, and of this around 2/3rds may be used for space heating. Thus, removing the party wall heat loss mechanism could equate to a maximum of £31 per home, per year. 53.5% of homes are terraced or semi-detached properties and therefore, contain at least one party wall, and there have been just over 1 million homes built 2013 and 2019. Assuming the same proportion (53.5%) of these have a cavity party wall, this equates to cumulative maximum savings of £66 million during this period alone, assuming these party walls would have otherwise been poorly performing. Even if only a fraction these party walls would have been poorly performing this still equates to many millions of savings.

In addition, data suggests that 1,724 existing homes have had party cavity wall insulation (PCWI) retrofits funded via the Energy Company Obligation (ECO) during this period, resulting in a further maximum saving of £53,000 per annum collectively for these homes. It is important to recognise that these savings will be achieved every year as long as the insulation remains installed in the homes which could be many decades. These savings are based on heat loss reduction measured from the retrofit of a single party wall, however, even were the national average savings to be only half this per house (i.e. some party walls were better performing), this still represents a huge financial saving of many tens of millions of pounds to UK households every year.

Incorporation of the party wall bypass for new build homes has increased the sales of insulation substantially. Cavity wall insulation can cost £10 per m2  and party walls may be 25m2 representing a new build market of up to £250 million for the million new homes built during the 2014 to 2020 REF period. This will grow year on year as new homes are added to the housing stock. The party wall bypass is also now being addressed through retrofitting existing homes. There are estimated to be 7.3 million existing homes in the UK with cavity party walls with potential to be insulated, though the extent of the party wall bypass in these homes is not known. Assuming only half of these homes could have a cavity party wall retrofit, and considering that the average cost of a retrofit is £350 per home  LBU findings have unlocked a market worth up to £1.27 billion. Following LBU research, party cavity wall retrofits were adopted into the Energy Company Obligations (ECO) policy in 2013, a government energy efficiency scheme to help reduce carbon emissions and these now make up 0.7% of all retrofits taking place. 

ECO targets fuel poor households and in the REF period has funding party cavity wall insulation for free in 1,724 homes. People in fuel poverty are more at risk of health problems resulting from cold homes, party wall retrofits can improve thermal comfort and make homes easier to heat. The discovery of the bypass by LBU has therefore contributed to health and wellbeing of occupants in these retrofitted homes.

The impact of party cavity wall insulation in new build and existing homes also means fewer carbon emissions. The maximum estimated national fuel bill savings, previously mentioned of £66 million, means that, assuming gas sells at 4p per kWh, up to 2.6 GWh of gas use could be avoided. At the Government’s conversion rate of 1.83 kgCO2eq per kWh of gas, we can estimate up to 120,000 tonnes of CO2eq for the qualifying period was avoided by insulating party cavity walls, assuming they would have otherwise been poorly performing. The significance of this saving is further enhanced in that these savings will continue to be achieved each year, and every new home built in the UK in the future will also benefit.

Bell was chair and lead author for several Zero Carbon Hub a public private partnership, set up to review industry practice, academic research and state of the art of UK construction and which reported a highly acclaimed end of term route map for achieving zero carbon homes in this REF period.

This led to LBU research being recognized in national press for establishing and quantifying the building fabric thermal “performance gap” and contributing to a U-turn in the acceptance of UK housebuilders to this phenomenon.

Additionally, the LBU coheating protocol has promoted by the IEA, and is in the process of being developed into an international standard by the European Committee for Standardization (CEN) and it has also been adopted by the UK Government in multiple research projects worth over £10 million during the REF period as the ‘best currently available and proven measurement option for calculating the heat transfer coefficient of homes”.

Outputs

  • Bell, M., Smith, M. and Miles-Shenton, D. (2005) Condensation risk — impact of improvements to Part L and Robust Details on Part C. Report Number 7 -Final report on project Field work. IN Oreszczyn, T. Mumovic, D, Davies, Ridley, I. Bell, M., Smith, M., Miles-Shenton, D. (2011) Condensation risk — impact of improvements to Part L and robust details on Part C: Final report: BD2414. Communities and Local Government, HMSO, London. [ISBN: 978 1 4098 2882 2 UK] 
  • Lowe, R.J., Wingfield, J. Bell, M. and Bell, J.M. (2007). Evidence for heat losses via party wall cavities in masonry construction. Building Services Engineering Research and Technology, Vol 28 No. 2 (2007) pp.161-181
  • Bell, M., Black, M., Davies, H., Partington, R., Ross, D., Pannell, R. And Adams, D. (2010) Carbon compliance for tomorrow's new homes: A review of the modelling tool and assumptions. — Topic 4: Closing the Gap Between Designed and Built Performance. Report number ZCHD130210, Zero Carbon Hub, London. 
  • Wingfield, J., Bell, M., Miles-Shenton, D., South, T. and Lowe, R.J. (2011). Evaluating the impact of an enhanced energy performance standard on load-bearing masonry domestic construction: Understanding the gap between designed and real performance: lessons from Stamford Brook. Communities and Local Government, HMSO, London. [ISBN: 978 1 4098 2891 4] 
  • Johnston, D., and Miles-Shenton, D., and Farmer, D., (2015) Quantifying the domestic building fabric 'performance gap'. Building Services Engineering Research and Technology, 36 (5). 614 - 627
  • Gorse C, Glew D, Johnston D, Fylan F, Miles-Shenton D, Smith M, Brooke-Peat M, Farmer D, Stafford A, Fletcher M (2017) Core cities Green Deal monitoring project, Leeds, Department of Energy and Climate Change.  

Contact Dr David Glew

  • Engineering, computing and sustainability
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