Is there a crawl space under my garage that needs insulation, and how would that work?

Crawl spaces under garages are not as common as crawl spaces under houses, but they do exist in Ottawa, particularly in homes built on sloped lots where the garage is at a different elevation than the rest of the foundation, or in older properties where the garage was added after the original house was built and sits on a shallow foundation rather than a full slab on grade. If your garage floor feels bitterly cold in winter even when the space is heated, or if you can access an area beneath the garage slab through a hatch or from an adjacent basement area, you may have a crawl space that is contributing to heat loss.

The approach to insulating a garage crawl space follows the same principles as insulating any crawl space in Ottawa, with a couple of garage-specific considerations. You have two options: insulate the crawl space walls and treat it as a semi-conditioned space, or insulate the underside of the garage floor and leave the crawl space cold. In most cases, insulating the crawl space walls is the preferred method because it is more effective and easier to do properly.

To insulate the crawl space walls, you would apply rigid foam insulation board, typically two inches of extruded polystyrene giving you R-10, directly to the interior face of the foundation walls. The foam is adhered with construction adhesive and mechanically fastened, and all seams are taped with sheathing tape. The foam should extend from the top of the foundation wall all the way down to the footing, and then at least two feet horizontally across the floor to capture the heat that the surrounding soil provides. A six-mil polyethylene vapour barrier covers the entire crawl space floor, lapped up the walls behind the foam and sealed, to prevent ground moisture from entering the space.

The garage-specific complication is moisture from vehicles. If any water from the garage floor above drains or seeps into the crawl space, you need to address that before insulating. This might mean sealing cracks in the garage slab, adding or improving the floor drain, or ensuring that the crawl space has its own sump pit if water intrusion is a recurring issue. Insulating over a wet crawl space traps moisture and creates conditions for mould and wood rot that can compromise the structural framing.

If the crawl space is too shallow to work in comfortably, which is anything under about eighteen inches of clearance, insulating becomes significantly more difficult and expensive because it essentially requires someone to work lying down in a confined space. In those cases, some contractors will blow loose-fill insulation into the space through access holes, or spray foam the walls and floor from the access point using extended applicator hoses. These low-clearance jobs cost more per square foot due to the difficult working conditions, so expect to pay a premium of thirty to fifty percent over standard crawl space insulation pricing.

For a typical Ottawa garage crawl space with reasonable access, professional insulation including the vapour barrier and rigid foam on the walls runs $2,000 to $4,500 depending on the size and condition of the space. This is a job where professional installation is strongly recommended because crawl spaces involve working in confined areas with potential moisture issues, and getting the vapour barrier details wrong can cause more problems than leaving the space uninsulated.

After insulating, the crawl space should have some connection to the heated space above it, either through intentional vents in the garage floor or through natural air movement around the perimeter, so that a small amount of heat keeps the space above freezing and prevents the conditions that lead to frost heave under the garage foundation. A fully sealed and insulated crawl space that receives no heat at all can actually cause foundation problems in Ottawa's freeze-thaw climate.

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