Building better basements

Comfort, durability and healthier indoor air

The musty basement smell that we have come to accept as unavoidable is actually quite avoidable. The solution is really very simple – don’t build below grade with walls that were designed for above grade, because they can’t possibly do what you want them to do.

By Mike Reynolds

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The key to building better basements is to understand that there is a completely different set of challenges facing walls built underground, and different building practices need to be employed for them to work.

The greatest source of moisture that above-grade walls must contend with is the warm, humid air in our homes that wants to get out. Below grade, it is that big porous sponge called concrete that is sucking up moisture from the ground, and that moisture has nowhere it can go but in. When you change the problem, you need to change the solution.

Polyethylene vapour barriers can play a positive role in walls that are above ground level, never below. [But, not always the case. See “Build a poly-free house,” in the Summer, 2013 issue of ecoHouse Canada.] Above grade, we most commonly build walls intended to dry to the exterior. That is impossible below grade, yet we build as if it were. The ground is wet, wood studs are wet and concrete is wet. How then, is covering something with a sheet of plastic going to keep it dry if it’s already wet?
Above grade, we focus on preventing moisture from getting out; in the basement we first of all need to prevent it from coming in, but also allow the moisture that is already in there to get out. Covering walls with a sheet of plastic certainly won’t help you there.

Mike Reynolds is a former home builder, a LEED for Homes Green Rater and the editor of Ecohome.net.