Estimation of the infiltration rate of UK homes with the divide-by-20 rule and its comparison with site measurements

Abstract

Buildings are responsible for 40% of the global energy usage to which infiltration-caused heat losses are responsible for 30%. Air infiltration is the unintended flow of air through leakage paths and fundamentally determined by the airtightness of a building. In the United Kingdom, building airtightness is conventionally measured through a blower door test and used to predict air infiltration in conjunction with the divided-by-20 rule, which is a rule of thumb that has been adopted by SAP (Standard Assessment Procedure: a UK government's recommended method system for measuring the energy rating of residential dwellings) for the estimation of the infiltration-caused heat losses for dwellings. This paper assesses the representativeness of this rule of thumb by carrying out blower door and tracer gas tests in twenty one dwellings located in the East Midlands Region of the United Kingdom. Results showed that a divide-by-37 rule would be more representative. It was also seen that the air infiltration rate is overestimated by SAP when modifying factors are added. The errors are as high as 500% in some cases. The most affected dwellings were the tighter ones. A revision of the usage of the divide-by-20 rule and the modifying factors is advised.