Introduction
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.
Background
Air permeability and pressure testing were introduced into Part L of the Building Regulations in 2006 [1] in order to improve the quality of construction and control heat loss through adventitious ventilation. A limit for design permeability was set at 10 m3/(h.m2) at 50 Pa, although homes are commonly designed to achieve better airtightness as standard. Compliance with Part L is demonstrated by testing a sample of dwellings under construction. NHBC Foundation guidance on achieving airtight dwellings is available in the NHBC Foundation publication A Practical Guide to Building Airtight Dwellings.
Up until the end of 2009, NHBC’s Air Leakage Testing Services team had undertaken over 5500 tests. Results from these tests indicated that air permeability results far better than 10 m3/(h.m2) were being achieved on a regular basis and that there is an improving trend.
Due to the natural variability in housing construction, it is to be expected that a small proportion of homes will achieve better as-built air permeability than they were designed to. However, as can be seen from Figure 1, very high levels are being obtained in a substantial proportion of homes tested. This is of concern because, if homes are more airtight than they were designed to be and do not have additional ventilation provision, the indoor environment may not be satisfactory – in particular, the air quality may suffer and humidity levels may be too high, increasing the risk of mould growth. The topic of indoor air quality is discussed in the NHBC Foundation publication Indoor Air Quality in Highly Energy Efficient Homes – A Review.
The Study
Based on the results of these air permeability tests, NHBC has become concerned that some homes – probably a small proportion – are likely to be under-ventilated, although there is no evidence to suggest that this is causing a problem in practice. It has been suggested that this may be because within the first months and years after completion small gaps and cracks open up in the building fabric, causing the air permeability to increase and thus providing more adventitious ventilation.
This study aimed to test this hypothesis by re-testing 25 homes that were reasonably airtight at completion (i.e. they achieved a low level of air permeability). The re-tests took place one to three years after the original test and the opportunity was taken to inspect the homes for signs of condensation and mould growth and to make enquiries to establish the occupants’ behaviour in terms of use of trickle vents, extractor fans, etc.
The site work was accompanied by a desk study to establish and evaluate previous related work in this area.