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Crawlspace Insulation in Ontario: Vented or Conditioned, Walls or the Floor Above, the Ground Sheet, Radon and Moisture, and What It Costs on a 36×28 House
Insulation & Air Sealing
Crawlspaces: Bring It Inside the House or Keep It Outside, but Stop Leaving It Halfway
A crawlspace in Ontario is a short basement that nobody wants to go into, and that is how it gets built: dirt floor, a couple of vents, batts stapled under the floor joists, and the water heater and the ductwork sitting in there as if it were a heated room. It is not. The vents let in July air that condenses on everything, the batts fall down by the second winter, the ducts lose a fifth of their heat, and the smell comes up through the floor. There are two honest ways to do it. Insulate the walls and seal the ground, which makes the crawlspace part of the house, or insulate the floor above properly and treat the crawlspace as outdoors. The first costs about $5,400 on a 36×28 house before HST and is what I do; the second is cheaper and only works if nothing in there needs to stay warm.
Vented and insulated above, or conditioned and insulated at the walls
| Vented crawlspace, floor above insulated | Conditioned crawlspace, walls insulated | |
|---|---|---|
| Where the insulation goes | Between the floor joists, R-31 to meet the exposed-floor number, held up with wire or mesh | On the foundation walls, 2 in. closed cell, plus the rim, with a sealed ground sheet over the dirt |
| What the crawlspace is | Outdoors. Vents open, cold in winter, humid in summer. | Indoors. Vents closed and sealed, a small supply of house air or a dehumidifier, same temperature as the basement would be. |
| Ducts, pipes, water heater down there | In the cold. Pipes freeze, ducts lose heat, and all of it has to be wrapped separately. | Inside the insulated envelope, where they belong. |
| Moisture | Summer air at 25 C and 70% humidity condenses on the 12 C floor framing. The batts get wet and sag. | Ground sheet stops the soil vapour; no outdoor air; the framing stays at room humidity. |
| Cost on the 36×28 house | about $3,000 | about $5,400 |
| When it is the right answer | A seasonal cottage closed in winter with the water drained, or a crawlspace with no mechanicals and a floor you can reach | Every year-round house, and anything with a furnace, ducts or plumbing in the crawlspace |
The OBC lets you build either. The exposed-floor number under the common SB-12 package is R-31; the crawlspace wall, when the crawlspace is heated, carries the basement wall number, R-20 effective full height. A crawlspace that is neither, vented but with ducts in it, meets no package and is the one you have now.
The conditioned crawlspace, priced
$6 to $92 in. closed cell, per sq ft of wall
$0.906 mil ground sheet, taped and sealed, per sq ft of floor
$1.25ignition barrier over the foam, per sq ft
$5.50rim joist, per lf, usually in the wall rate here
36×28 house, 4 ft crawlspace walls, dirt floor, furnace and ducts down there. Wall area is 128 lf times 4 ft, 512 sq ft. Two inches of closed cell on the wall, over the top of the wall and up into the rim, at $7.50 a square foot, is $3,840. The crawlspace rate is higher than the $3.00 basement rate for the same foam because the crew is on their knees, the masking is everything, and the rim is included in the number. Ground sheet, 36 by 28 is 1,008 sq ft, at $0.90, $907, taped at every lap, sealed to the foam at the wall and around every pier. Ignition barrier over the foam, 512 sq ft at $1.25, $640. Total $5,387 before HST, $6,087 with.
That gets you R-12 of continuous foam with a full air seal on the walls, the rim done, the dirt sealed, and the furnace and ducts inside the house. The vents get blocked with foam and sealed. A supply register from the furnace, or a continuous bath-fan-sized exhaust, keeps the air moving, and a dehumidifier the first summer tells you whether the ground sheet is doing its job. On a block wall, foam the open cores at the top first; the spray foam basics page covers what the crew does and what to check.
What the number does not include: a ground that is actually a puddle, which needs drainage and a sump before anything else, and a crawlspace too low to work in, which some crews will not enter below about 24 in. and all of them charge for. The why basements leak page is as true at 4 ft as at 8.
The floor-above option, and when it is honest
Batts between the floor joists, R-28 or R-31 to hit the exposed-floor number, with wire supports or a mesh so they do not sag out of the bay in year two, run $2.50 to $3.50 a square foot of floor. On the 1,008 sq ft house at $3.00 that is $3,024, and the crawlspace stays vented. It is a real assembly in the code and it is the right one for a cottage that is drained and closed for the winter, where there is nothing down there to freeze.
It is the wrong one for a house with the furnace in the crawlspace, which is most of them, because you have just insulated the furnace out of the house. Every duct joint leaks into the cold, every pipe needs heat tape, and the batts hang against floor framing that sweats all summer. A 1 in. layer of foam board on the underside of the joists, under the batts, as an air barrier and a vapour retarder on the cold side, is the one detail that makes a floor-above job last, and almost nobody prices it. The insulation materials hub has the batt choice; mineral wool holds its shape better upside down.
Moisture, radon and the things a crawlspace hides
- The ground sheet is not optional. Bare soil puts litres of water a day into the air above it. 6 mil poly, lapped 12 in., taped, run up the wall and sealed to the foam, weighted or pinned so it stays put. $0.90 a square foot is the cheapest moisture control in the house.
- Radon comes up through dirt faster than through a slab. A sealed ground sheet is also the first half of a radon barrier; a test afterward tells you whether it needs a fan. The OBC radon page has the rules for new work and the test for old.
- Check the floor framing before the foam goes on. Black joists, soft sills, a beam pocket with water in it: a crawlspace that has been vented and batted for thirty years has usually got some of this, and foam over rot is foam over rot.
- Combustion appliances. A furnace or water heater that drew its air from the vents now needs a combustion air supply, or a sealed-combustion unit. Closing the vents without this is how backdrafting starts.
- Foam exposed in a crawlspace needs an ignition barrier, the $1.25 coating, or it needs to be a crawlspace nobody and nothing enters. Write it in.
- Pests. A conditioned crawlspace with a sealed sheet is less attractive to mice than a vented one full of batts, but the vents still get screened and foamed, not just closed.
If you are building new
An ICF crawlspace wall, which is how I build them when a client wants a crawlspace rather than a basement, is already R-22 and needs nothing on the walls. The ground still gets the sheet, or better a 4 in. slab over foam and poly, and the rim still gets foamed, so the crawlspace insulation line on a new ICF house is the ground, the rim and the ignition barrier, roughly $1,600 on this house. A conventional poured crawlspace gets the full $5,387 above. The basement and foundation hub has the rest of the foundation choices, and the slab page has the under-slab foam if you are pouring a floor down there.
Questions people ask
Should a crawlspace be vented or sealed in Ontario?
Sealed and conditioned, with foam on the walls and a taped ground sheet, for any year-round house and any crawlspace with a furnace, ducts or plumbing in it. Vented with the floor above insulated only for a drained seasonal building. Vented with ducts in it is the common one and the wrong one.
How much does it cost to insulate a crawlspace?
About $5,400 before HST on a 36×28 house to foam the walls at 2 in., seal the ground and coat the foam; about $3,000 to insulate the floor above with batts and leave the crawlspace vented. Drainage, a sump or rot repair are on top.
Can I spray foam a crawlspace ceiling?
You can, and it makes a floor-above job airtight, but it leaves the ducts and pipes in the cold and seals the floor framing against inspection. Foam on the walls is the better use of the same money in most houses.
Does a conditioned crawlspace need ventilation?
A little. A supply register from the furnace, or a small continuous exhaust fan, moves enough air that it does not go stale. It does not need outdoor vents, and the ground sheet does most of the moisture work.
What about radon in a crawlspace?
Bare soil is the easiest path radon has into a house. The sealed ground sheet is half the fix, and a long-term test afterward says whether a fan under the sheet is needed. The OBC radon rules apply to new work; the test applies to every house.
Price the walls and the rim together
The spray foam cost calculator prices 2 in. of closed cell on crawlspace walls with the rim, the masking and the minimum charge, and shows whether the job clears the minimum on its own. If you have a quote for the floor above and one for the walls and cannot tell which is the better house, the $99 review will.
Sources and further reading
- Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
- Radon: Health Canada: why a tighter basement should be tested, how, and what level calls for mitigation.
- Ontario’s Building Code, Supplementary Standard SB-12: the energy-efficiency packages that set the R-values for attics, walls, basements and slabs in a new or renovated Ontario house.
