Dry, sealed crawlspaces built for Manitoba winters

The space under your floor is not sealed off from the rooms above it. Air moves between the two continuously, through gaps in the subfloor, around plumbing and electrical penetrations, through duct chases and along the rim joist. During our long heating season, the stack effect gives that movement a consistent direction: air enters low in the house and leaves high, which means crawlspace air is being drawn upward into your living space for months at a time. Whatever is in that air — moisture, mould spores, rodent waste, radon — comes with it.

That is the part homeowners here tend to find surprising, because a Winnipeg crawlspace does not feel like a humid place. For most of the winter it is not. The problem is that an unconditioned crawlspace spends the year swinging between two destructive states. In January it runs close to outdoor temperature, so warm, moist indoor air moving down into it condenses and freezes on the cold underside of the subfloor and on the rim joists. In July the direction reverses: humid outdoor air comes in through open vents, meets soil and framing that are still cool, and condenses there instead. Either way, wood that holds moisture above roughly 19 percent is wood that decay organisms and mould can use.

The structural side runs in parallel. Our design frost depth is roughly 1.8 to 2.4 metres, and the Lake Agassiz clay under most of the city is highly plastic — it swells when it takes on water and shrinks when it dries. Anything founded above the frost line gets lifted every winter and dropped every spring. Crawlspace additions built on shallow sonotube piers, which is most of them, move on that cycle for decades. The sloping sunroom floor and the patio door that binds in January are not cosmetic complaints; they are the visible end of a support problem under the floor.

None of this is unfixable, and none of it requires guesswork. A crawlspace that has been sealed properly, insulated at the perimeter rather than between the joists, drained where water actually arrives, and supported below frost depth will hold a stable humidity through both the melt and the August peak. The framing dries out and stays dry. The floor above stops being cold. The smell goes away because its source is gone. And the house stops losing heat to a space that was, in effect, outdoors.

What we will not do is sell you a system you do not need. Plenty of the crawlspaces we inspect need nothing more than longer downspout extensions, better grading away from the wall, and another reading next spring. If that is what we find, that is what we will tell you — in writing, with the numbers we measured, so you can judge it yourself.