An investigation into the parameters that contribute to the gap between the designed and as-built thermal performance of British housing

The UK Government has placed the need to reduce national energy demands and carbon emissions at the forefront of the political agenda, with a commitment made to meet EU targets of 20% reductions in greenhouse gas emissions and primary energy consumption, alongside a 20% improvement in overall energy efficiency, across all EU Member States, by 2020. Building performance has been identified as a key area where significant progress towards meeting these ambitions can be made. It is fundamental to ensure that the building fabric of a property functions correctly in order to achieve high levels of thermal effectiveness, which should result in lower energy demands and carbon emissions. However, research to date shows that a gap exists between predicted and actual performance levels. This research utilises the dwelling Heat Loss Coefficient (HLC) as a common output in design stage and post-construction evaluation techniques, that can be used to compare predicted and measured fabric performance. The Standard Assessment Procedure (SAP), coheating tests, air pressure tests and thermal imaging are used to evaluate in-situ buildings. Sensitivity analysis and controlled conditions experiments are utilised in order to investigate the reliability of the assessment techniques used. The key findings from the study include the demonstration, through novel coheating test, that post-installation mechanically ventilated heat recovery (MVHR) system efficiency levels can have a pronounced effect on the measured HLC, and, in conjunction with use of assumed theoretical efficiency levels, can cause divergence in theoretical and measured data of 10-15%. This can largely be resolved through correct design, installation and commissioning. Environmental conditions, both notional and site-specific, can also cause divergence in the HLC data, including wind speed (15%) and solar gains (10-26%). In addition, it has been shown that, when considering thermal bridging values, inaccurate calculation at the design-stage and poor attention to detail during construction could cause underperformance in this element by up to 50%. This is of significance as there are currently no mandatory procedures to assess post-construction compliance with thermal bridging levels specified within the UK Building Regulations