Estimating the Background Ventilation Rates in New-Build UK Dwellings – is n50/20 appropriate?

In the UK, a rule of thumb applied to air permeability is commonly employed when estimating background ventilation rates from pressurisation test data. However, this may lead to significant errors in estimating the infiltration rates in UK new-build dwellings, resulting in poor estimation of the dwellings in-use energy and CO2 emissions, and the adoption of ventilation strategies leading to either unacceptable indoor air quality or unnecessary energy consumption. In this paper, a preliminary investigation into the applicability of the rule of thumb is undertaken. Background ventilation rates in four new-build dwellings in the UK are determined using the tracer-gas decay method and also the pressurisation (blower-door) method coupled with both the conventional n50/20 and (in the UK) q50/20 rule of thumb, and Sherman’s modified rule of thumb, which takes into account other building-related factors. The conventional method over-estimated the air-change rate in two of the dwellings and under-estimated it in the other two dwellings. The modified rule of thumb produced comparable results for two of the dwellings, but significantly underestimated the air-change rate in the other two dwellings. These results suggest that more work needs to be done to devise appropriate climate and building-related correction factors for the UK