How to insulate a crawl space against cold and damp

Why does a crawl space need insulation?

Cold floors, rising heating bills and a damp, musty smell often have the same source: a crawl space that is open to winter air and bare soil. Warm air from the house escapes through the floor, and moisture from the ground rises into the space and then into the house. Insulating alone will not cure damp, so the order of work matters. The wider logic of where to spend first is in our home insulation upgrade priorities guide.

Vented or unvented: which crawl space do you have?

A vented crawl space has openings to the outside and is meant to dry through air exchange. An unvented, or conditioned, crawl space is closed off, treated as part of the house envelope and sometimes given a small supply of conditioned air or a dehumidifier. Many older houses have vents that were never closed in winter. Their purpose was to remove moisture in summer, but in a cold climate they also chill the floor and any pipes in the space.

Building science guidance has moved toward closing and insulating the walls, because the vented approach performs poorly in humid summers and cold winters alike. Rules differ, though, so ask your municipal building department which approach your code accepts before you start.

Should you insulate the walls or the floor?

Insulating the floor above a vented crawl space is the older method. Batts between the joists are cheap to buy, yet they can sag, absorb damp and leave gaps, and ducts and pipes in the cold space can still freeze. The wall method brings the space inside the thermal envelope: rigid foam or spray foam on the foundation walls, with the rim joist insulated and air-sealed, and the vents closed.

• Floor insulation. Lower upfront cost and simpler, but depends on keeping the space cold, dry and vented.
• Wall insulation. Better for pipes and ducts and for comfort, but needs careful moisture control and attention to fire and code requirements for exposed foam.

Materials are compared in spray foam insulation and cellulose versus fibreglass.

What comes before the insulation?

Fix water first. Check that the ground slopes away from the house, gutters discharge well away from the foundation and there is no standing water or active leak inside. Then cover bare soil with a durable ground sheet, sealed at the seams and up the walls, to stop moisture rising. Seal gaps around plumbing, wiring and the rim joist, as heated air leaking through them carries moisture into the cold space. The air sealing guide covers these details.

If you see mould, rot or signs of pests, deal with them before covering anything. A sealed crawl space also affects combustion appliances and ground gases such as radon, so ask a qualified contractor and your local authority about testing and ventilation requirements.

How much does it cost and who should do it?

Cost depends on the size of the space, access, the material, the amount of moisture work and the region, and quotes decide. A small, dry, easy-access space is a manageable DIY job with rigid foam, while a damp or tight one is better left to a contractor who understands moisture. Ask for the plan in writing: ground cover, wall or floor method, rim joist treatment, vent handling and the R-value target. You can compare recommended levels in R-values by region, and the basement insulation guide covers the same principles for a full basement. A blower door test, explained in our air tightness guide, can confirm the result.

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