Q: The interior of a building with full masonry walls was gutted after a fire and is being rebuilt. What is the best way to air-seal the brick wall?
A: Foster Lyons, an engineer and building-science consultant, responds: There are many variables that could affect the answer to your question. But if the plan is to leave the interior exposed with no framed wall over the masonry, then repoint the exterior and plaster the inside of the wall with a cement-based, vapor-open, three-coat plaster.
Of course, if you intend to add framing and insulation on the interior, that interior plaster recommendation would be financially wasteful. But, the basic idea is the same: Make sure the exterior side of the wall is well-pointed, then apply something on the interior side to stop air ingress while still allowing water-vapor transfer. This air-control material should be vapor semi-permeable (permeance between 1 and 10).
There are many commercially available products that can achieve this combination of interior requirements, and they generally fall into two categories: liquid-applied membranes and trowel- or brush-applied slurries. Some of these may require a primer. If the interior of the brick wall is lumpy and uneven, you first need to parge it with a Portland cement–based stucco to create a surface that is smooth and continuous enough to allow the air-control material to achieve continuous coverage.
If the building is in a northern climate (any place where the heating system runs regularly, such as climate zone 5 or above), then the insulation must not be air permeable, or you must add a smart vapor barrier on the interior side of your air-open insulation. (A smart vapor barrier changes permeance with changes in humidity to keep the wall cavity dry.) These precautions will prevent moisture-laden, warm air from getting to the cold interior surface of the brick wall and allowing condensation to form. If the building is air conditioned, then do not use vinyl wallpaper or epoxy paint as the interior finish, as these products will inhibit vapor movement.
If your building is in a southern climate (any place where the air-conditioning system runs twice as much as the heating system, such as climate zone 4 or below), then any type of insulation would be acceptable, but it’s still not a good idea to apply vinyl wallpaper or epoxy paint on the interior. The right thing to do is to allow the water vapor to pass through the interior finish and eventually make its way to the cooling coils in the AC system, where it will condense and drain out of the building. For a more in-depth discussion of this topic, check Building Science Insight #105, “Avoiding Mass Failures” by Dr. Joseph Lstiburek, of Building Science Corporation, June 2018.
