Walls Below Grade

Dampproofing (usually a bituminous coating) acts as the primary barrier (the first plane of protection) against water penetration from the exterior side of the foundation wall. The concrete foundation wall can be considered a second plane of protection. The use of a drainage composite product can also provide improved drainage at the exterior surface of the wall. Neither of these planes of protection will address water penetration control at joints or cracks in the concrete. So if the site is wet with some probability of hydrostatic pressure occurring, additional measures must be taken to provide adequate water penetration control (such as using a waterproof membrane, and crack and control joint waterproofing details).

Concrete walls or polyethylene sheet can serve as an air barrier. The below-grade wall air barrier must be sealed to those of both the basement floor and the walls above grade; and should be caulked or sealed to the air barrier at the floor framing (for example, by wrapping the joist header with spun-bonded polyolefin as described in the next section) to make the air barrier continuous. If the basement walls are insulated on the interior, it is important to restrict the movement of air to the face of the concrete foundation wall because it will be cold, significantly increasing the potential for condensation. This can best be accomplished by placing rigid insulation directly onto the concrete and sealing insulation board joints.

If insulation is located on the exterior (see Figure 15), a vapour barrier is not required. If there is insulation on the interior wall (see Figure 16), an interior dampproofing membrane may be required, if moisture-sensitive materials (for example, framing and batt insulation) are situated close to the concrete foundation walls. In this case, a vapour barrier is required to prevent vapour from entering the space between the wall finish and the concrete wall. The vapour barrier is typically sheet polyethylene.

Figure 15

Concrete wall with ridgid insulation on outer face

Figure 16

Source : Canada Mortgage and Housing Corporation (CMHC)