Zero Carbon Homes and the Performance Gap

Tom Dollard | Pollard Thomas Edwards

The Zero Carbon Hub’s recent work with industry has found extensive evidence of a considerable ‘Performance Gap’ between the energy use of new homes as-designed and the actual use of the completed building. This gap occurs when a constructed home requires more energy than was predicted during its design, before taking into account the behaviour of occupants.

This gap represents a significant risk to the UK’s carbon reduction commitments. It has the potential to result in higher than expected household energy bills, undermining buyer confidence in new (low carbon) homes. As we approach the zero carbon homes target of 2016, housebuilders are producing higher performing homes and need to be confident that they truly perform as intended.

Evidence of the Performance Gap

Since January 2013, Zero Carbon Hub has carried out an extensive study that involved over 160 experts and 21 live housebuilding sites across the UK. This project forms the initial phase of industry activity to address the Performance Gap, the ultimate aim being that by 2020 a minimum of 90% of all new homes meet or perform better than their design.

At the start of the research there was a misconception that the Performance Gap was simply caused by inaccuracies within energy modelling software and poor construction practice on site. The Evidence Review Report showed that a gap can arise due to issues at various stages of the housebuilding process. Throughout this practice, a number of cross-cutting themes keep recurring: unclear allocation of responsibility; poor communication of information; and a lack of understanding, knowledge and skills.

As part of the wider research, a housebuilding process review was developed and undertaken throughout 2013 -2014, which included detailed analysis of over 200 plots across 21 sites of different sizes throughout the UK. Two multi-disciplinary teams led by industry experts spent a week analysing each site, comprising: project team interviews, a construction site walkthrough, a design review and SAP audits. This process was then supplemented on a number of sites with on-site testing using thermography and air-testing across a selection of the plots. The site inspections analysed several hundred elements of the building at all stages of the build. Any deviations from the design were fully documented with good and poor practice noted, and graded in terms of impact on the as-built performance. The energy performance was then analysed using SAP and site findings to ascertain the gap in terms of KgCO2/m2 (% improvement over Dwelling Emission Rate). This could be compared to the original SAP and EPC.

The review has identified 15 priority issues that require action to begin to address the Performance Gap. These range from: planners failing to understand the energy challenges, current material and product testing protocols not reflecting 'real world' dynamic conditions when calculating thermal performance, procurement teams failing to consider energy related site skills when reviewing tenders; to site managers considering energy related issues as a comparatively low priority during their quality checks

Recommendations for industry

For the housebuilding industry, research and development is needed to create innovative and commercially viable methods to test and measure the energy use of completed homes, so that industry can understand their true performance. The industry also needs to embed energy literacy across the sector, with all professionals and operatives undertaking energy training and up-skilling. As a result of the project, government has indicated it is keen to work with industry to develop a new Construction Details scheme which will be set up to provide assured, as-built energy performance for the most common major building fabric elements and junctions.

Recommendations for government

In place of immediate additional regulation, government must clearly indicate that it expects the construction industry to act now to ensure that the Performance Gap is being addressed; industry must then commit to demonstrating that this has been achieved by 2020. The compliance regime needs to be strengthened, with a number of refinements to energy modelling and verification procedures. Lastly, government should stipulate that only energy-certified operatives and professionals be employed on public land developments from 2017, which would accelerate demand for industry developed qualification schemes.

Passivhaus and the Performance Gap

Whilst the project did not specifically look at the benefits of the Passivhaus methodology, the principals can play a part in reducing the Performance Gap. The site inspections and interviews have witnessed good practice stemming from the Passivhaus standard, and some of this has fed into the recommendations for industry and government. However at a larger scale, Passivhaus projects still suffer the same issues and demonstrate a significant Performance Gap. Common problems are found at ground floor and foundation junctions, where insulation is inconsistent or missing and significant thermal bridging occurs. Other areas causing deviation from the design are: poor installation of insulation, incorrect window performance, optimistic U values/psi values and unaccounted thermal bridging. Site inspections may not pick up on optimistic product performance, site installation factors such as rain soaked insulation, hidden thermal bridges, site tolerances, level thresholds and window performance and installation.

The Passivhaus methodology can assist in helping reduce the Performance Gap with early stage consideration of energy performance and robust compliance and commissioning procedures. Site inspections are also crucial to delivery on site, but this is difficult to ensure on a larger scale without significant extra resources. It has proven successful on small sites of less than 20-30 units, but gets harder on larger sites with hundreds of workers and more units to be monitored. In Belgium and other parts of Europe, Passivhaus has been delivered on a large scale with the help of offsite construction, but the UK industry currently has very different priorities and procurement methods.

With the completion of the End of Term Report, the proposals are now moving forwards, to 2020 and beyond. If you would like more information on Zero Carbon Hub’s current projects, or if you have further evidence or projects that could be included in the research on the Performance Gap, please get in touch and sign up to the newsletter here: www.zerocarbonhub.org

Tom Dollard is Head of Sustainable Design at Pollard Thomas Edwards. He leads a research team which inspected 21 sites for the Zero Carbon Hub Performance Gap report throughout 2013-2014.

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Reports available from: www.zerocarbonhub.org/current-projects/performance-gap

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