Crawl spaces: insulating the floor alone freezes pipes, and the ground cover comes first

A crawl space is the part of the house nobody wants to visit, so it is often insulated in the quickest way: batts pushed up between the floor joists. NRCan's Keeping the Heat In explains why that can make things worse, and what has to happen before any insulation goes in.

Two approaches

NRCan describes two ways to insulate a crawl space: insulate the walls, inside or outside, so the crawl space becomes a heated area, or insulate the floor above to keep heat out of it. A heated crawl space should be treated as a short basement. Decide which one you are building before buying material, because the vents, the barrier location and the pipe protection all follow from that choice.

Heated crawl spaces

For a heated crawl space, NRCan says not to use vents that open to the outdoors, because condensation risk can increase in summer; integrate the space with whole-house ventilation or use a dehumidifier, and permanently seal existing vents in a full renovation that heats the space. Walls can be insulated outside, following the basement method, with at least 600 mm (24 in.) of overlap where an obstruction forces part of the wall to be insulated inside. Inside, rigid board or spray foam can be used; ask the building inspector whether foam needs a fire-protective covering. If footings are above the frost line, insulate outside to keep them warm and avoid frost heave.

Vented crawl spaces and the floor-only trap

NRCan warns that insulating only the floor joists can lead to frozen pipes, frozen ground and rot at joist ends, so floor insulation should be combined with foundation-wall insulation. The air and vapour barrier goes on the warm, top side of the floor insulation, and the rim joist should be sealed carefully. Batts must fit tightly against the underside of the floor and be held up with breathable wrap, wire or board. NRCan gives a vent ratio of 1 to 500 of vent area to floor area, opened in spring and closed and insulated in winter, and says to check humidity and condition at least monthly.

Ground cover

NRCan's minimum ground cover is 0.10 mm (4 mil) polyethylene, clear or opaque, with joints overlapped, caulked and taped, mechanically fastened to the walls and posts and sealed to any wall barrier. White sheet makes leaks, mould and pests easier to see. Hold it down with smooth boards or scrap, not sand or gravel, and cover walkways with a plastic floor mat.

Moisture, radon and pipes

Fix water leaks and sources first. NRCan's plastic-sheet test, about 1 m square taped down for a day or two, shows whether moisture comes from the air (condensation on top) or the ground (underneath). Never vent a clothes dryer into a crawl space. NRCan notes that radon may be present in any home; see ArtBuild's radon article. Insulating the walls is preferred to relying on pipe insulation; insulated pipes can still freeze, so NRCan suggests self-regulating heat trace or a small thermostat-controlled heater, and taping and insulating ducts.

Prevention Checklist

• Decide whether the crawl space will be heated or vented before buying insulation.
• Fix leaks and run the plastic-sheet moisture test first.
• Lay at least 4 mil polyethylene, lapped, taped and fastened.
• Do not insulate only the floor in a vented crawl space; add wall insulation.
• Seal outdoor vents if the crawl space becomes heated.
• Protect water pipes against freezing and never vent a dryer into the crawl space.

References

1. Natural Resources Canada. Keeping the Heat In - Section 6: Basement insulation, floors, walls and crawl spaces, Web page, Crawl space parts: heated, partially heated, ground cover, moisture, pipes. Accessed 2026-11-03.

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