Crawl Space Insulation in Ontario: Sealed vs Vented and How to Do It Right

Crawl space insulation in Ontario works best, for most year-round homes, by sealing and insulating the crawl space walls and covering the ground with a vapour barrier, rather than insulating the floor above and venting the space to the outdoors. A sealed crawl space keeps pipes and ducts inside the warm part of the house, reduces moisture and usually makes the floors above noticeably warmer.

Crawl spaces are common under additions, older bungalows, sunrooms and cottages across the province. This guide explains the two approaches, the steps for each, typical costs and the moisture, radon and fire-safety points to watch.

Why do crawl spaces matter for energy?

A crawl space that is open to outdoor air in winter, or simply uninsulated, can make the floor above cold, freeze water lines and let damp air and soil gases into the house. Because warm air rises, the house tends to draw air up from lower levels, a process called the stack effect. In winter, a leaky crawl space feeds that flow.

Common complaints that point to the crawl space:

Cold floors in one room or an addition
Frozen pipes in January
Musty smells or condensation on windows
High humidity in summer
Rodents or insects

Crawl space insulation: walls or the floor above?

	Sealed (conditioned) crawl space	Vented crawl space with floor insulation
Where insulation goes	Perimeter walls and rim joists	Between floor joists above the crawl space
Vents to outdoors	Closed and sealed	Left open
Ground cover	Sealed vapour barrier, required	Vapour barrier still recommended
Pipes and ducts	Inside the warm zone	Outside the warm zone; need insulation or heat trace
Moisture control	Ground sealed; small amount of conditioned air or dehumidification	Relies on outdoor air, which can bring in humid summer air
Typical fit	Year-round homes, additions with ducts or plumbing below	Cottages, seasonal buildings, homes on piers
Rough cost (estimate)	$3,000 to $10,000 for a typical crawl space	$2,000 to $6,000

For most year-round homes the editorial team recommends the sealed approach. The wall area of a crawl space is usually smaller than the floor area, the work is easier to do well, and it protects mechanical systems. Floor insulation makes sense where the crawl space is very shallow, very open or not connected to the heated home, such as a seasonal cottage.

How to insulate a sealed crawl space

Step 1: Fix water first

Insulating a wet crawl space traps moisture against the structure. Before you start:

Confirm gutters and downspouts direct water at least 1.5 to 2 m (5 to 6 ft) from the foundation
Make sure the soil outside slopes away from the house
Repair leaks and any standing water; a sump may be needed in some cases

Step 2: Lay a ground vapour barrier

Cover the floor with polyethylene, at least 0.15 mm (6 mil), overlapping and taping seams by about 300 mm (12 in) and running it up the walls and piers. A thicker reinforced sheet is more durable if the space is used for storage or regular access.

Step 3: Air seal and insulate the rim joists

Rim joists, where the floor framing sits on the foundation, are usually the leakiest spot. Spray foam or cut-and-sealed rigid foam boards work well here. Our air sealing guide explains the options.

Step 4: Insulate the walls

Material	How it is used	Notes
Rigid foam board (EPS, XPS or polyiso)	Glued or fastened to walls, seams taped	Common DIY choice; must usually be covered with a thermal barrier
Spray foam (closed-cell)	Sprayed on walls and rim joists	Good air and moisture control; installer must follow code for coverings
Mineral wool board	Fastened to walls	Non-combustible; needs an air and vapour control layer

A target of about R-20 is reasonable for crawl space walls, similar to the basement wall target used in many Ontario retrofits. Check what your municipality requires if you are applying for a permit.

Caution: Most foam insulation must be covered by an approved thermal barrier, such as drywall or a listed coating, when installed in accessible spaces. Building code requirements vary for crawl spaces that are rarely entered. Ask your installer or building department.

Step 5: Close the vents and control humidity

Seal crawl space vents with rigid foam and caulk. The space then needs a small amount of conditioned air, either a supply register from the duct system or a small dehumidifier, to keep relative humidity in check. Many contractors aim for below about 60%.

Insulating the floor above a vented crawl space

If you choose this approach:

Lay the ground vapour barrier anyway.
Fill the joist bays with batts or blown insulation held tight against the subfloor. Gaps below the subfloor let cold air in and reduce performance.
Support the insulation with netting, strapping or a rigid board so it does not sag over time.
Seal penetrations in the subfloor for plumbing, wiring and ducts.
Insulate water pipes and consider self-regulating heat cable on lines that freeze. Ducts in the crawl space should be sealed and wrapped.

This works best when nothing that can freeze is in the crawl space.

Radon and soil gases

Parts of Ontario have elevated radon, an odourless radioactive gas from the soil that is a leading cause of lung cancer among non-smokers. Health Canada's guideline is 200 Bq/m³ for indoor air.

Test your home with a long-term (at least three-month) test during the heating season.
A sealed vapour barrier helps, but if levels are high, a sub-membrane depressurization system vents the gas outdoors.
Sealing a crawl space is a good time to rough in a radon pipe, even if you test later.

What savings can you expect?

Savings vary widely. A sealed crawl space under a 40 m² (430 ft²) addition might cut that area's heat loss substantially, but the effect on a whole-house gas bill may be modest, often in the range of $50 to $200 a year with gas heat at 45¢ per m³ (estimate). The main payoffs are often comfort, avoiding frozen pipes and moisture control. Savings are higher with electric, oil or propane heat.

Insulation may qualify through the assessment stream of the Home Renovation Savings Program, which requires pre- and post-retrofit assessments and at least two upgrades. Confirm details on saveonenergy.ca (opens in a new tab), and use our insulation savings calculator for a rough estimate.

Frequently asked questions

Can I just close the crawl space vents?

Not without also sealing the ground and providing some humidity control. Closing vents over bare soil can trap moisture and lead to mould and rot.

Is spray foam on the crawl space floor a good idea?

Foam is not usually sprayed on the soil. A sealed polyethylene vapour barrier is the standard ground cover. Spray foam is mainly used on walls and rim joists.

Do I need a permit to insulate a crawl space?

It depends on the scope and your municipality. Simple insulation and vapour barriers often do not, but structural, drainage or ventilation changes may. Check with your local building department.

How does this differ from basement insulation?

The principles are similar: control water, seal air leaks and insulate the walls. Our basement insulation guide covers full-height basements and finishing.