Quantifying inter-dwelling air exchanges during fan pressurisation tests

Fan pressurisation tests (FPTs) are commonly used to measure air leakage in homes, to provide evidence for compliance with energy and ventilation standards in building regulations and inform energy models. The results are presented of 37 pressurisation and co-pressurisation tests on attached homes in the UK which measured inter-dwelling air exchanges during the FPTs. On average, 21% of the air leakage measured by the FPTs was found to be inter-dwelling rather than inside-to-outside air exchange, i.e. homes are more airtight than FPTs indicate, which is important when assessing energy efficiency and ventilation performance thresholds. Not accounting for inter-dwelling air exchanges poses a risk of under-ventilation and misclassification of homes deemed suitable for natural ventilation. Using the FPT result to replace default values for airtightness in energy models used to create Energy Performance Certificates (EPCs) for 11 of the case study homes improved their energy efficiency rating (EER), indicating default airtightness values used in EPCs used were overestimating the air leakage. Using the co-pressurisation value resulted in an additional EER point. These modest improvements represented a 5%, 8% and 3% reduction in predicted annual carbon emission, space heating demand and fuel bills, respectively. Practice relevance The airtightness of homes is fundamental to their energy efficiency and ventilation requirements. The FPT is commonly used to measure airtightness in homes; however, this research has shown that the FPT can overpredict air leakage in attached homes due to the elevated pressures during the test cause inter-dwelling air exchanges not experienced under non-test conditions. This may affect the accuracy of FPTs in attached homes and the appropriateness of using the FPT result to inform building regulation compliance, ventilation decisions and energy models. The research has implications for FPT standards, testing practitioners and professional bodies, energy modellers, ventilation designers, policymakers, and regulations. The development of further knowledge, industry guidance and protocols is required for inter-dwelling air exchange taking place during the FPT, particularly for different house type, form and construction.