Airtightness in masonry homes

Air leakage from the uncontrolled movement of air through joints and gaps in the building fabric can be a significant source of heat loss, particularly in a well insulated home where the impact will be proportionately higher.

The introduction of an air leakage limit in the 2006 edition of the Building Regulations has gone a long way towards ensuring a minimum standard of airtightness in new dwellings. This was set at a maximum of 10m3/(h.m2), and the limit is likely to remain the same in the 2010 revisions to Part L1. However, the introduction of tougher CO2 emission requirements in Part L mean that actual air leakage rates will typically need to improve on the backstop value by a significant margin for dwellings to achieve their overall emissions target.

Appropriately designed and constructed masonry dwellings will achieve air leakage levels below 2m3/(h.m2). Of equal importance, is the ability to use a wet plaster finish or parging coat on masonry walls, offering a very simple and robust primary air barrier, helping ensure dwelling remain airtight throughout their life.

The use of a primary air barrier is central to achieving a good standard of airtightness. There are three basic options for masonry homes:

1. Dry-lining

This can be used to form the air barrier through careful edge sealing around perimeters and junctions. To achieve this, plasterboard is mounted on a continuous perimeter ribbon of adhesive. With a high standard of workmanship, dry-lining can be reasonably effective at meeting the requirements of a pressure test, with anecdotal evidence suggesting air leakage rates as low as 2.5m3/(h.m2). However, compared to other options, it may not offer such a robust, long-term solution. Another limitation is that air can still permeate the blockwork and potential leakage paths may remain behind the plasterboard, allowing air to pass into ceiling cavities, floor voids and partitions.

2. Parging coat

This is basically a sand and cement render which, when applied, seals the blockwork, creating a primary air barrier. Once applied, the wall can then be dry-lined in the usual way. Edge sealing of the plasterboard may further reduce air leakage, although the benefit is likely to be limited if the parging coat has been applied effectively.

3. Wet plastering

Traditional wet plastering seals blockwork to provide an effective, durable air barrier. An air pressure survey of masonry housing showed that, of the houses tested, those built in the 1980s, when dry-lining was not ubiquitous, tended to perform better than newer homes built in the 1990s which were all dry-lined. It was also found that the best performing masonry dwellings in a 1984–1994 sub-set performed as well as the best of the framed houses with an air leakage index of 3 to 4m3(h/m2). Although wet plastering takes longer than dry-lining, projection systems which spray-apply the plaster, can significantly increase productivity and are growing in popularity.