by Karine Galla
Every professional builder knows about the damaging effects of unwanted moisture, as well as how challenging it is to control water intrusion in certain climates. However, What is not always understood is the role airflow plays in moisture control and how important air barriers are in managing it.
Water intrusion is bad for any structure, and air carries a lot of it. Keeping air from invading a building envelope is essential, and if there is leakage into the structure, one has to ensure this airflow does not get cold enough to condense and create moisture.
Managing the exterior flow of air is also important to dry out the wall system when moisture has found a way in. Additionally, restricting air from flowing unimpeded through a wall system can prevent the invasion of allergens, pollutants, and bacteria, which can compromise the health of building occupants.
This is why the air barrier should be vapor-permeable—the wall can ‘breathe’ and dry out, but the air cannot make its way past the sheathing and reach the inside wall cavity.
When insulation gets wet, it quickly loses its ability to resist thermal flow and has a negative effect on energy efficiency. Moisture-laden insulation reduces R-values, resulting in rising energy costs. There are also the damaging effects of capillarity (i.e. the tendency of water to flow through narrow spaces). However, by properly controlling air pressure inside the wall system, it is possible to reduce accumulation of moisture.
Air invariably moves. Moisture cannot be effectively controlled unless the air is first managed. Therefore, a highly airtight enclosure is necessary to provide conditioning, as well as temperature and humidity control. The best place to control this atmospheric element is on the outside of the sheathing, but under the insulation layer so the air temperature remains more or less static.
Air barriers vs. moisture barriers
At this point, it may be helpful to differentiate between a water-resistive barrier (WRB) and an air barrier. The function of a moisture barrier is to keep liquid H2O from entering the building enclosure. Combined with flashing, drain plane, sheathing, and other wall components, the moisture barrier provides a protective layer keeping water away from the stud cavity.
The role of an air barrier is to resist leakage. As explained by American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) Fellow Joseph Lstiburek, PhD, P.Eng. (founding principal of Building Science Corporation, a consulting and architectural firm), “Air barriers are systems of materials designed and constructed to control airflow between a conditioned space and an unconditioned one. The system is the primary air enclosure boundary separating indoor (conditioned) and outdoor (unconditioned) air.”
Air leakage loads are significantly greater than most designers and architects commonly realize. Many materials have been considered suitable air barriers, including gypsum wallboard, building felt, and concrete masonry units (CMUs). However, as building codes change and airtightness requirements increase, some of these materials may no longer be considered code-approved.
Vapor barriers should not be confused with air barriers. The former is designed to inhibit the flow of water vapor through a given material, and is intended to control the rate of diffusion into a building assembly.
Vapor dissemination has been traditionally considered a key component of moisture control. However, modern building science suggests air leakage—and not vapor diffusion—is the more serious problem. In fact, for a given wall, moisture transmitted via air leakage is 200 times higher than through vapor diffusion. (Read R. Quirouette’s The Difference Between a Vapor Barrier and an Air Barrier, published by National Research Council Canada in 1985.)
