Ageing and airtightness; How dwelling air permeability changes over time

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Routine air leakage testing has demonstrated that some new homes designed to achieve high standards of air permeability are actually achieving very high ‘as-built’ standards – standards at which additional ventilation provision would normally be advisable. NHBC and others share some concerns about the potential consequences of living in highly airtight homes that do not have provision of additional ventilation, although there is little evidence to support these concerns.

In response to suggestions that new homes become leakier as they age because of shrinkage and settlement and that this provides additional adventitious ventilation, this research subjected a small number of dwellings to re-testing one to three years after completion to establish how their air permeability had changed. The research found that, whilst two-thirds of homes did become leakier, the remaining third actually became more airtight. It appears that the type of dwelling, construction, heating and ventilation all have a bearing on the extent to which air permeability changes, although the small sample size means that firm conclusions cannot be drawn.

The re-tests demonstrated that most of the dwellings (83%) remained airtight, achieving a re-test result tighter than 5 m3/(h.m2), yet there was little evidence to suggest that the low air permeability without additional ventilation provision was causing a problem in practice. In spite of the fact that good ventilation guidance was widely being disregarded by occupants, only three dwellings showed signs of mould growth and in all cases, it was minor.

This research was intended only to provide an indication of the performance as the sample size was limited. This should be borne in mind when interpreting the results.