Green recovery: why energy efficiency matters
15 Jul 2020
By Professor Rajat Gupta, Low Carbon Building Research Group.
The UK’s legal commitment to a net zero emissions target by 2050 has driven the need for new dwellings to be built with high standards of insulation, with mechanical ventilation, high-efficiency heating systems, and renewables. Given that new-build housing is increasingly expected to meet higher and potentially more complex levels of performance, it is reasonable that the actual performance of such housing is evaluated against expected performance.
Building performance evaluation studies have shown that that low/zero energy dwellings often underperform as compared to the design specifications, due to discrepancy in building fabric thermal performance, the performance of heating and ventilation systems, and resident behaviour. Past studies have demonstrated that in-use energy use can be up to three-five times more than design predictions. However, access to these studies, their findings, and most importantly the knowledge from it, is limited.
Moreover, national policy targets for carbon reduction cannot be met without understanding, quantifying, and minimising this performance gap between design intent and reality.
The inaugural State of the nation study published by Professor Rajat Gupta and Matt Gregg at Oxford Brookes University has revealed for the first time, a detailed national picture of the actual performance of new build homes at scale through a detailed analysis of building performance evaluation projects.
An online and interactive map on housing performance has been developed that spatially maps 91 performance evaluation studies across the UK along with their meta-data. The 91 HPE studies cover 826 dwellings, of which 105 are Passivhaus and 721 Non-Passivhaus dwellings (low energy).
Analysis of data on actual energy use, thermal performance, building fabric performance, environmental performance, and feedback from occupants has provided insight into housing performance at scale.
New-build dwellings were found to often underperform compared to actual design specifications. Mean energy consumption in Non-Passivhaus dwellings was found to be 117 kWh/m2/year while Passivhaus dwellings use 38% less energy; 73 kWh/m2/year. Energy used for space heating was 2.4 times higher in Non-Passivhaus dwellings. The energy performance gap (as compared to SAP) in Non-Passivhaus dwellings was nearly twice that of Passivhaus dwellings. Heat loss around openings such as doors and windows was identified as a particular problem showing a need for developers to improve detailing, specification, and workmanship.
Despite the differences in energy use, indoor temperature profiles in the heating and non-heating season were similar in Passivhaus and Non-Passivhaus dwellings, with mean indoor temperature ranging from 20.1oC-24.6ºC. However consistently higher mean monthly CO2 concentrations were observed in Non-Passivhaus dwellings than in Passivhaus dwellings, with the difference being more significant in bedrooms. Comfort and satisfaction with indoor temperature are found to be greater in winter than in summer across Passivhaus and Passivhaus dwellings. Indoor temperature perceived to be too hot during the summer.
The study emphasises that the future of UK housing must include measuring and monitoring to quantify and minimise performance gap. Developers, builders, and design teams require upskilling and re-training. Involving residents in building performance studies is vital to ensure energy systems are understood, operated, and controlled as intended.
With the UK committed to achieving net zero emissions by 2050, the report provides building professionals with key insights to inform future design of homes. Most importantly, it highlights the gains to be made from measuring and testing of new homes. What is clear is that building performance evaluation studies of housing are socio-technical in character, requiring an interdisciplinary approach.
Housing performance map showing the distribution of housing performance studies across the UK.
The full State of the Nation study is available here.
Professor Rajat Gupta, Low Carbon Building Research Group, Oxford Brookes University, rgupta@brookes.ac.uk