An Airtight Case

By Luc Saint-Martin

Building an exterior wall assembly so that it will stay dry is not easy, but it can be done with attention to detail. However, details cannot be specified without an understanding of how
wind, rain and air interact on the outside of the wall and how warm air laden with moisture behaves on the inside. It is also important to understand the difference between the role of the vapour barrier and the role of the air barrier. This article focuses mainly on indoor conditions and the role of the air barrier.
The vapour barrier - in most cases the polyethylene that is installed toward the warm side of the assembly - is there to limit the diffusion of water vapour from the interior of the house into the assembly. Diffusion, driven by moisture and temperature differences, is a relatively slow process and the amount of water capable of being transferred this way is not large. In reality, gypsum board with a few coats of low-permeance paint will do an effective job in most instances.
The more worrisome issue is that of air leakage, brought about by the action of wind and temperature differences between interior and exterior conditions in winter. The temperature difference in winter creates pressure for outside air to leak into the house at the lower levels and air from the inside to "escape" from the house at the upper levels through small deficiencies - cracks and discontinuities - in the building envelope. In essence, the house is like a chimney. As inside air travels through those small openings, it carries with it moisture from cooking, washing and other activities, into the wall assembly where the moisture will condense on cold surfaces: typically the inside face of the sheathing.
Research has shown that vapour diffusion through a two-centimetre hole in a wall can result in about one litre of water making its way into the wall assembly over an entire heating season. In contrast, air leakage through a hole that size over the same period would result in 30 litres of water accumulating in the wall cavity. This is why air-tightness is so important. The continuity of the air barrier over the entire building envelope, including the foundation and the attic, is particularly important with regard to restricting the movement of inside air to the outside.
Walls sometimes fail because rainwater has found its way in through deficiencies in the cladding, improper flashing, missing or damaged drip caps and poorly installed or deteriorated caulking at openings such as electrical outlets. A common location for such damage is the area immediately beneath a window. Leakages due to gravity and capillary forces at the window/cladding junction can be made worse by discontinuities in the air barrier system at the opening, since air leakage can lead to water being transferred with the air.
Regardless of where the airtight element of the air barrier system is located, it must be sealed to the window frame. Whether or not a sheathing membrane is intended to serve as the airtight element of the air barrier system, it is important to wrap the sheathing membrane across the jambs and sills and head of rough window openings. A self-adhered membrane can serve the purpose of protection at the windowsill. It is best to have this done by the workers who are installing the membrane or insulating panels because the window installers may not feel it is their responsibility. NRC-IRC is currently doing research to evaluate various wall/window junction installation methods with a view to providing information for best-practice installation guidelines.
The Canadian Construction Materials Centre (CCMC) has evaluated many of the commercial air barrier membranes and insulating panels currently in use. The CCMC reports include a section on the Usage and Limitations of the product and, in many cases, an Appendix that contains the manufacturer's recommendations for installing the product as part of the air barrier system. See http://irc.nrc-cnrc.gc.ca/ccmc/index_e.html.
For builders, "attention to detail" translates into time and money. Time to read and understand specifications and manufacturers' recommendations for installation, plus extra hours for site supervision. In the end, however, this effort delivers a superior product and saves both time and money by reducing recalls and warranty claims.

Luc Saint-Martin is a technology transfer advisor at the Institute for Research in Construction, National Research Council, Canada's leader in construction research. He can be reached at luc.saint-martin@nrc-cnrc.gc.ca.