The relationship between airtightness and ventilation in new UK dwellings

Jenny Crawley, Jez Wingfield, Cliff Elwell

Summary

The UK’s Air Tightness Testing and Measurement Association competent persons scheme collects pressure test data and metadata from the majority of new build dwellings in the UK. This article uses the dataset to investigate the importance of the ventilation strategy in airtightness design and construction. Design and measured airtightness were tested for association with declared ventilation strategy. It was found that ventilation strategy makes a statistically significant difference to airtightness; however, this difference is too small to be practically relevant. Properties with mechanical ventilation and heat recovery (MVHR) were shown to have a mean designed air permeability only 0.46 m³/m²h lower than naturally ventilated dwellings. Seventy-three per cent of homes with MVHR have design airtightness greater than or equal to 5 m³/m²h and 17% of naturally ventilated dwellings have design airtightness less than 5 m³/m²h. We discuss how current design is not maximising the CO₂, cost and air quality benefit of each ventilation strategy. A new approach to regulatory compliance is proposed, which explicitly links the designed airtightness and chosen ventilation system. It is suggested that compliance could then be achieved using a set of airtightness ranges linked to appropriate ventilation strategies. This could be expected to result in reduced energy consumption and carbon emissions for new build homes compared to the current approach and would also potentially lead to better outcomes for occupants in terms of indoor air quality.

Practical application: Analysis of a large database of the airtightness of new UK dwellings found that ventilation strategy makes very little difference to airtightness design. For dwellings with MVHR, the results suggest that infiltration levels are too high to maximise the energy savings; for naturally ventilated homes, there may be air quality issues. Coupling airtightness design and ventilation strategy can reduce a dwelling’s energy demand and can support achieving the required energy performance rating.

Banner photo credit: Alireza Attari on Unsplash