During a typical Canadian winter, interior air often contains more moisture than the exterior air can accommodate, so the vapour drive is to the exterior. Without a vapour control layer, water vapour would be driven outwards through the wall assembly, concentrating in building materials as the dewpoint is reached. This of course can lead to mould, rot, and many other undesirable results. To prevent this, the construction specifier must provide a vapour control layer in the wall assembly—a building material whose composition prevents vapour diffusion. In house construction, this is most often polyethylene sheeting, but in commercial construction various available material options can meet these needs.
The vapour control layer must be placed at the correct location within the wall assembly. As noted earlier, warmer air can hold more moisture, vapour drive is typically from the warm side of the wall to the colder side of the wall. In Ontario, builders typically place the vapour control layer at the interior side since most of the year the vapour drive is outwards. However, this also means for some days in the summer, when it is hot and humid outside and comfortably air conditioned inside, the vapour drive is reversed. In most parts of the country, this condition is much less common than an outward vapour drive, but designers should still accommodate for these conditions with moisture-tolerant materials capable of drying out should vapour drive add to their moisture content.
Accidentally introducing multiple vapour control layers in the wall assembly is a frequent problem. Take, for example, a building with a polyethylene vapour barrier at the interior, insulation in a stud wall cavity, then a layer of exterior insulation with its joints taped using a vapour-impermeable tape. If this insulation layer does not allow vapour diffusion, such as a foil-faced polyisocyanurate (ISO) insulation, then the building is left with a second vapour barrier at the exterior side of the building. Since construction is never perfect, some moisture movement is likely, but if water vapour makes its way to the space between these layers it cannot escape—the two vapour control layers prevent drying to the interior or the exterior. Therefore, it is not only important to make sure a wall assembly has no more than one vapour control layer, but also to ensure it has one to begin with.
Air control layer
While a lot of attention is always given to the vapour control layer, in this author’s opinion even more important is the air control layer. It prevents air from passing through the wall assembly between the interior and exterior spaces. Air movement through gaps and cracks from the exterior to the interior can result in uncomfortable drafts, but even more serious is the movement of air from the interior to the exterior.
Remember, for much of the year warm, moist interior air would reach its dewpoint somewhere between the interior face of the building wall and the exterior face of the wall assembly. If air can exfiltrate out through the wall, when it reaches this temperature moisture will condense out, depositing water on the colder component of the building enclosure. If this colder component is, for example, wood framing, the condensed water can result in mould, rot, or other undesirable conditions.
If the colder components are, for example, the steel anchoring system of a precast concrete cladding system or the steel lintel supporting brick masonry, the moisture can promote rust deterioration of the anchor or lintel, potentially compromising the structural integrity of the heavy cladding system.
Vapour diffusion is equal across the entire wall assembly; therefore, a missing vapour control layer would result in diffusion across a wide area. However, in the case of a failed air control layer, the escaping air is typically concentrated in a few localized spots, so the water deposition by condensation is most often localized as well. This can result in excessive amounts of water deposition in small areas, leading to serious localized failures. In opposition to the vapour control layer, a building cannot have too many air control layers. The location of the air control layer(s) does not matter either—they can be located anywhere within the wall assembly if they are continuous.
