Across different territories there are various normative models for assessing energy demand of domestic dwellings, which use simplified approaches to account for the heat loss due to the air infiltration of a building. For instance, the United Kingdom uses a dwelling energy model, known as the Standard Assessment Procedure (SAP), and this utilises a process where the measured air permeability value (q50), is simply divided by 20 to provide an infiltration rate (subsequent modification factors are then used for factors such as sheltering etc.). This rule is commonly known as the divide-by-20 rule. As a starting point, building air leakage rate is measured through a steady pressurisation (blower door) test, and normalised by envelope area to provide a value of air permeability. In this study the air infiltration rate of five dwellings based in Nottingham, UK was investigated. Air infiltration was measured and calculated using the conventional approach with the divide-by-20 rule and also with tracer gas methods, which directly assessed air infiltration at ambient pressure levels. Results showed that for the test dwellings, a divide-by-39 (q50/39) rule would predict air infiltration more accurately (than the divide-by-20). It was also seen that the air change rate is overestimated by SAP when modifying factors are added. The errors in more than half of the properties are higher than 225%. The most significant differences were seen in the dwellings with more airtight building envelopes.