Crawl space encapsulation seals the space beneath your floor from ground moisture and outside air, bringing it inside the building envelope instead of leaving it open to the weather.
Crawl space encapsulation covers the earth floor and walls with a heavy polyethylene vapour barrier, sealed at every seam, then closes the vents, air-seals and insulates the rim joist, and controls the remaining humidity with a dehumidifier. It stops soil moisture evaporating into the space and rising into the living area above.
Why an open crawl space affects the whole house
Air moves upward through a building. Warm air rises and escapes at the top, which draws replacement air in at the bottom - the stack effect.
In a house over an open crawl space, the bottom is the crawl space. A meaningful share of the air circulating through your living rooms entered through it, picking up whatever is down there on the way.
Bare earth in a crawl space evaporates water continuously. That moisture, and the mould spores and soil odour it carries, goes up into the house.
Which is why the musty smell people spend years trying to trace in the main floor frequently originates somewhere they have never actually looked.
Why venting made it worse
Building codes, including the Ontario Building Code, required crawl space vents for decades on the reasoning that outside air would dry the space.
In a Toronto summer it does the opposite. Warm humid outdoor air enters through the vents, meets surfaces cooled by the ground below, and condenses on them. The vents deliver water rather than removing it.
This is why so many vented crawl spaces are at their wettest in July and August, when the theory says they should be driest.
What encapsulation involves
Clear the space and address any standing water first. Encapsulating over an active water problem seals the water in.
A heavy polyethylene vapour barrier is laid across the entire floor, run up the foundation walls, and mechanically fastened and sealed near the top. Seams are overlapped and taped. Piers and posts are wrapped and sealed individually.
Vents are closed and sealed. The rim joist - the perimeter where the floor framing sits on the foundation, and the single largest air leakage path in most houses - is air-sealed and insulated.
Then humidity control: a dedicated dehumidifier, and in spaces with a history of standing water, a small sump.
What it costs
Crawl space encapsulation is quoted per site rather than from a published range, and it is worth explaining why, since we publish verified ranges for nearly everything else on this site.
Cost is driven by three things that vary enormously between houses: the area to be covered, the working height - a 24-inch crawl is several times slower to work in than a 48-inch one - and whether there is an existing water problem to resolve first.
Any per-square-foot figure we published would mislead half the houses that read it, so this one is measured before it is priced.
The materials, by name
The barrier is reinforced polyethylene sheet, string-reinforced so it resists puncture from kneeling and dragging tools across it during installation. Thin builder's poly tears and is not the same product.
Seams are overlapped and sealed with seam tape rated for the material, and the barrier is fixed at the wall with a termination bar and sealant rather than tape alone, because tape on damp masonry releases within a season or two.
At the rim joist, the perimeter where the floor framing bears on the foundation, the air seal is the point of the exercise. Closed-cell spray foam or rigid board cut to fit and sealed at the edges both work; batt insulation stuffed into the cavity does not, because it stops neither air nor vapour.
Finally the space is conditioned rather than vented: a dedicated dehumidifier draining to a condensate pump or the sump, so the humidity is controlled rather than merely sealed in.
Deal with the water first
If your crawl space takes standing water after rain, encapsulation is the second step, not the first.
A vapour barrier laid over ground that periodically floods traps water underneath it. The drainage has to work before the space is sealed - which usually means the same footing drainage a basement needs, covered on interior waterproofing.
And as with any damp-space problem, the cause is worth establishing before the remedy is chosen. The full diagnostic sequence is on our foundation repair overview.