Rim joists, foundation walls, crawl spaces, the room over the garage: where spray foam is the right choice, where it is not, thickness, vapour barrier, preparing the house and how long before you can go back in.
The Toiture & Isolation Provinciale team · Published
Spray foam is not the right insulation for every spot in the house. In the attic, we recommend blown-in insulation. But in the basement, the crawl space, at the rim joist or under a bedroom built over the garage, it does in a single pass what other insulations take three steps to do: insulate, stop air and hold back vapour. Here is where it is the right choice, where it is not, and what to know before a crew comes to spray at your home.
It is a closed-cell polyurethane foam made of two components mixed at the gun, which expands and hardens on the surface within seconds. The one we install most often, Huntsman's Heatlok Soya HP (sold in Quebec as Airmétic Soya HP), gives about R-6 per inch and acts as insulation, air barrier and vapour barrier at once. It bonds to concrete, wood and masonry, and leaves no seam for air to slip through.
Wherever cold air gets in at the bottom of the house and space is tight. Four places in particular:
We also use it in cathedral ceilings and flat roofs, where a ventilated attic has no place. Those applications are covered on our spray foam page.
Because that is where the house's lumber meets the foundation concrete, and that joint lets outside air in all the way around the basement. A batt pushed between the joists insulates a little, but air flows around it. Spray foam fills every cavity, bonds to the wood and the concrete, and closes the gap while it insulates. You feel the difference first on the ground-floor floor.
If the basement is already finished, the rim joist can often be done on its own: we remove the old insulation between the joists, spray, and the finished walls stay in place.
The best time is when you are redoing the basement. Spraying directly on the concrete, before the new framing and drywall, gives continuous insulation with no thermal bridge, plus an air barrier and a vapour barrier in a single application. On an older foundation, stone or uneven, the foam follows the surface where a rigid board would leave gaps behind it.
The part of the wall above ground loses the most heat, because it is exposed to outdoor air. On a sloped lot, one wall can have much more of it than the others: we account for that in the planned thickness.
There are two ways, and spray foam suits both. You can insulate the walls of the crawl space and the rim joist: the crawl space then becomes part of the heated house, and the pipes running through it stay warm. Or you can insulate the floor above it: the crawl space stays cold, which means protecting any pipes down there. We recommend one or the other after the visit, based on height, access and what runs through the space.
On a dirt floor, a membrane laid on the ground and sealed with spray foam around the perimeter cuts the contact between the soil and the air in the house, along with the moisture rising from it.
A room built over an unheated garage has a floor exposed to the cold from below, and it is often colder than the rest of the house in winter. We spray foam on the garage ceiling, between the joists, which insulates the floor and cuts off garage air at the same time. The wall the garage shares with the house is handled the same way. For Rénoclimat, that floor is an exposed floor, which can qualify for a grant (see below).
We say it as plainly as we recommend it. Spray foam is not the right answer:
Thickness is chosen zone by zone, based on the target insulating value, the space available and what will cover it. In a 2x4 stud wall, for example, the foam does not go past the stud's 3½ inches. The Heatlok Soya HP data sheet gives these long-term thermal resistance values (CAN/ULC-S770 test):
| Sprayed thickness | Long-term insulating value |
|---|---|
| 2 in (50 mm) | R-11.7 |
| 3 in (75 mm) | R-18.5 |
| 4 in (100 mm) | R-25.6 |
That is what backs the roughly R-6 per inch rule we use in our quotes. Past a certain thickness, we spray in several passes, as the manufacturer requires: a layer that is too thick at once gets too hot as it cures. The planned thickness is written in your quote, and the installer measures what was applied, surface by surface, in the job report.
Usually not. The National Building Code (Article 9.25.4.2) defines a vapour barrier as a material that lets less than 60 ng/(Pa·s·m²) of water vapour through. According to Huntsman, Heatlok Soya HP drops below that threshold at 1¼ inches (32 mm), and the manufacturer recommends applying at least 1½ inches. At those thicknesses the foam works as the vapour barrier, with no polyethylene to add over it.
Yes. Inside a building, the manufacturer's data sheet requires the foam to be covered with a thermal barrier that meets the Code, drywall for example. In a basement being finished, the new drywall does that job. For a crawl space or a garage ceiling, the quote specifies the covering planned and who installs it, before the work and not after.
Preparation is simple, but it matters. Before we arrive:
On the day, our crew protects the surrounding surfaces, shuts off the forced-air ventilation and seals its registers in the zone, then ventilates under negative pressure, as Huntsman's best practices call for. Only the installers, wearing their respiratory protection, stay in the house during the application.
For Heatlok Soya HP, the manufacturer's data sheet sets the minimum time before re-occupancy at 25 hours (CAN/ULC-S774 test on volatile organic compound emissions). If your project uses another product, we give you the delay from its own data sheet before setting the date. How long the application takes depends on the area and the access: we confirm it with the appointment.
Yes. The manufacturer offers seasonal versions of the product: a winter version, applied between -10 and 10 °C, and a "super winter" version down to -20 °C, those being the temperatures of the air and the surface during the application and the 24 hours that follow. In a heated basement the question rarely comes up; it matters mostly for a cold crawl space or a garage.
No, not on its own. Spray foam reduces the entry routes for radon: it seals the rim joist, cracks, the joint between the slab and the walls, and plumbing penetrations. Health Canada notes that sealing entry routes can help, but that its effectiveness is limited. If a long-term test exceeds the Canadian guideline of 200 Bq/m³, the recognized fix is sub-slab depressurization, installed by a professional certified by the Canadian National Radon Proficiency Program (C-NRPP). The honest sequence stays the same: test, seal, retest. The details are in our article on radon in the basement.
Yes, but not the one most people think of first. Hydro-Québec's LogisVert funds only one insulation measure, the roof combined with air sealing: nothing for the basement, the rim joist or the crawl space. It is Rénoclimat, the Quebec government program, that covers this work. Its new version, in force since October 1, 2026, sets a minimum insulating value to add and a minimum area, then pays by the square foot:
| Work | Value to add | Minimum area | Grant |
|---|---|---|---|
| Basement walls, rim joist included | R-15 | 300 sq. ft. | $2.50/sq. ft. |
| Crawl space walls, rim joist included | R-23 | 300 sq. ft. | $3.50/sq. ft. |
| Floor above the crawl space | R-23 | 300 sq. ft. | $3.50/sq. ft. |
| Exposed floor, above an unheated space such as a garage | R-23 | 150 sq. ft. | $3.50/sq. ft. |
For a house, the total grant for foundations and floors cannot exceed $2,500, and never the amount paid before taxes. The rim joist is no longer a separate measure: it counts within the basement or crawl space walls. At about R-6 per inch, adding R-15 works out to roughly 2½ inches of spray foam, and R-23 to a little under 4 inches. That is a calculation, not a promise: we check with you, before the work, whether the planned thickness reaches the threshold.
Other rules changed at the same time. The energy evaluation before the work is now optional, the work must be done by a contractor holding the appropriate RBQ licence who fills out the certificate of compliance form, and the application is filed online no later than 12 months after the work ends. Only work done from October 1, 2026 onward qualifies. The rest is in our grants guide.
We do not publish a price per square foot, because a number without your house says very little. The price depends on the thickness (the product is calculated per square foot per inch applied), the actual area to spray, the access (a low crawl space or a cluttered basement takes longer), the preparation (old insulation to remove, surfaces to protect) and the covering required. An expert measures at your home and hands you a detailed fixed-price contract, scope by square foot, firm price.
Spray foam work is guaranteed for 2 years, in writing, and the product carries its manufacturer's warranty. Our end-of-job report documents what was applied, where and at what thickness, with photos. Coverage details are on our warranty page.
An expert inspects the rim joist, the foundation and the crawl space, measures, photographs, and hands you a detailed fixed-price quote.
Get my detailed quoteSources checked October 31, 2026. Grant programs can change: the eligibility decision always belongs to the program administrator.
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