Air Sealing

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Air sealing closes the cracks, gaps and holes that let heated air escape from a house and cold outdoor air leak in. In a Canadian winter, warm air rises and pushes out near the top of the house, at the attic hatch, around ceiling fixtures and along the tops of walls, while cold air is drawn in low, at the rim joist, around basement windows and through service openings. The result is drafts, cold floors, frost in the attic and a heating system that works harder than it should.

It is usually the least visible part of an energy retrofit, and one of the most important. The work uses caulking, spray foam, gaskets, weatherstripping and rigid materials, and it is most effective when a blower door test shows where the air actually escapes. Because a tighter house holds more moisture and leaves less air for fuel-burning appliances, a proper job also checks ventilation and combustion safety.

This page explains why air leakage comes first, where the biggest leaks hide, which materials go where, how blower-door-guided sealing works, what to check afterward and which programs can help.

Air sealing vs adding insulation: why air leakage comes first

Natural Resources Canada's retrofit guide ranks air leakage control as the most important single upgrade, the one to plan before any other. Fibrous insulation slows heat moving through it but lets air pass, so warm air keeps escaping through an unsealed ceiling however much insulation lies on top. Sealing first also means reaching leaks before new insulation or finishes cover them.

Moisture is the other reason. Indoor air carries water vapour, and when it leaks into a cold attic or wall cavity it can condense or freeze on cold surfaces. That is behind much of the attic frost, mould on roof sheathing and ice damming seen in older homes, and insulation added without sealing makes those surfaces colder and the problem worse.

Where the biggest air leaks are: attic hatch, top plates, penetrations, rim joist, pot lights and fireplaces

The largest leaks are usually at the top and bottom of the house, where winter pressure pushes warm air out and pulls cold air in hardest. These spots, all on Natural Resources Canada's checklist, are where a contractor looks first.

The attic hatch or pull-down stairs, often a loose panel with no weatherstripping or insulation at the highest point of the house.
Top plates, where interior and exterior walls meet the attic floor; in older houses with open wall cavities, air can travel from the basement all the way to the attic.
Plumbing stacks, wiring holes and ducts that pass through the ceiling, along with the chases and gaps around chimneys and flues.
The rim joist and sill plate, where the framing sits on the foundation; NRCan lists the joint between the sill plate and the foundation among the main basement leaks.
Recessed lights and ceiling fixtures under an attic, and bathroom fans whose housings are rarely sealed to the ceiling.
Fireplaces, where a damper that doesn't close tightly, or is left open, lets heated air flow straight up the chimney.

Air sealing materials: caulking, spray foam, gaskets, rigid panels and weatherstripping

No single product seals everything. The right one depends on how wide the gap is, whether the parts on each side move, and how hot the surface gets.

Caulking fills narrow, stable cracks around trim, baseboards and window frames; Natural Resources Canada compares several types for indoor and outdoor use, and their durability varies widely.
Near heat sources such as chimneys, light fixtures and fan motors, NRCan calls for non-combustible caulking, noting that silicone or polysulphide sealants usually work well and that high-temperature silicones exist for flue pipes.
Spray foam sealant fills wider gaps and penetrations and comes in low- to high-expansion types; low-expansion foam is the usual choice around window and door frames, which too much pressure can distort.
Gaskets and backer rod handle specific spots: foam strips under the sill plate, foam gaskets behind outlet and switch plates on exterior walls, and compressible foam rope pushed into deep joints before caulking.
Rigid insulation, drywall, plywood or sheet metal close large openings such as open wall tops and chases, with every edge sealed, and sheet metal with a heat-rated sealant goes next to chimneys.
Weatherstripping seals the parts that move, on doors and operable windows; at the bottom of a door, NRCan notes that a threshold gives the better seal and a door sweep is easier to install.

Blower-door-guided air sealing: how a contractor finds and fixes leaks

Sealing by guesswork misses the leaks you can't see. A blower door, the fan an energy advisor fits into an exterior doorway during an EnerGuide evaluation, depressurizes the house so outdoor air rushes in through every gap, which makes leaks easy to find and gives a measurement to compare against once the work is done.

Before testing, fuel-fired space heaters and water heaters are turned off, since Natural Resources Canada warns that depressurizing the house can backdraft combustion gases.
With the fan running, the contractor finds leaks with smoke pencils and often an infrared camera, which NRCan notes also reveals missing insulation.
Hazards come first: suspected vermiculite, old or damaged wiring and mould go to the right professionals before anyone crawls through the attic.
The big leaks at the top and bottom of the house are sealed first, then the walls, windows and doors.
A second blower door test shows the improvement, and programs such as Rénoclimat require a post-retrofit evaluation once the work is finished.

Ventilation, combustion appliances and backdrafting after air sealing

Tightening a house changes how it breathes. Natural Resources Canada explains that as the envelope gets tighter, indoor humidity rises and less air is left for fuel-burning appliances, and that a chimney or flue without enough air can spill dangerous gases into the house. Exhaust fans, range hoods and clothes dryers add to the risk by pulling air out of a house that no longer lets much in.

After significant sealing, have a qualified heating technician check any naturally vented furnace, boiler, water heater or fireplace for spillage. Controlled ventilation, such as a heat recovery ventilator with its filters and core kept clean as CMHC describes, replaces the fresh air that used to leak in by accident. Health Canada recommends carbon monoxide alarms, with the hallway outside the bedrooms as the most important spot; if one sounds, leave the home right away and call for help from outside.

Managing indoor humidity in a tighter house through a Canadian winter

A well-sealed house keeps the moisture from cooking, showers, laundry and breathing inside, where it has to be removed on purpose. Health Canada recommends keeping relative humidity between 30% and 50%, using a dehumidifier when needed. In the coldest weeks, heavy condensation on windows or frost in closets on exterior walls means the air is too humid for the house, a cue to ventilate more rather than to stop sealing.

Run bathroom fans during showers and, as Health Canada suggests, for at least 30 minutes afterward.
Use the range hood whenever you cook, preferably on its highest setting, as Health Canada advises.
Make sure every exhaust fan and the dryer vent discharge outdoors, never into the attic or a wall cavity.
Keep the heat recovery ventilator running and maintained, and have it rebalanced after major air sealing.

Air sealing rebates: Rénoclimat, LogisVert, federal and Ontario programs

Air sealing is often funded as part of a broader retrofit, and many programs rely on blower door results to confirm it. Rules change, and several programs require an energy evaluation before any work starts, so check the official page before booking a contractor. When this page was reviewed, these programs were active:

Rénoclimat, from the Government of Québec, which covers airtightness work alongside insulation, with energy evaluations before and after the retrofit.
Hydro-Québec's LogisVert program, whose list of eligible measures includes caulking.
The Canada Greener Homes Affordability Program, offering no-cost retrofits to eligible low-to-median-income households through participating provinces and territories, including Quebec.
In Ontario, the Home Renovation Savings Program from Enbridge Gas and Save on Energy, where air sealing rebates require a home energy assessment.

Air Sealing: frequently asked questions

Is air sealing worth doing if I'm not adding insulation?

Usually, yes. Sealing the attic floor, the rim joist and other large leaks cuts drafts and keeps moist indoor air out of the attic and walls, whether or not insulation follows. It is also the natural first step if you plan to insulate later, since the leaks are easier to reach now.

Can a house be too airtight?

Tightness itself isn't the problem; a tight house without enough controlled ventilation is. As a house gets tighter, humidity and indoor pollutants build up and fuel-burning appliances have less air, so air sealing should come with a ventilation plan and a combustion safety check.

Can I air seal my house myself?

Some jobs suit a careful homeowner, such as weatherstripping doors, caulking interior trim and adding outlet gaskets. Attic and chimney work is different: it involves heat-rated materials, recessed lights, wiring and possibly vermiculite, and after major sealing, fuel-burning appliances need a technician's check. Leave those to qualified professionals.

Do pot lights need to be replaced for air sealing?

Often. Natural Resources Canada says standard recessed lights are difficult to air seal effectively and can be a fire hazard under insulation, and recommends approved airtight fixtures or approved covers. Replacing fixtures is electrical work for a licensed electrician.

Will air sealing stop ice dams and attic frost?

It tackles the main cause, warm, moist air leaking into the attic, so it often makes the biggest difference. Enough insulation and clear soffit vents complete the fix, and a roof that is already damaged should be checked by a roofer.

Sources

Written by the LokalMatch editorial team. Last reviewed September 10, 2026.

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What affects the cost of air sealing

Prices depend on the details of your project. We only publish price ranges when they’re backed by real LokalMatch quote data or reliable sources. Until then, here’s what usually changes the price:

Area to insulate: attic, walls, basement or crawl space
Type of insulation
Removal of old insulation
Air sealing work
Access to the space
Ventilation changes

How to compare insulation contractors before you hire

Choose a contractor who talks about air sealing and ventilation, not only adding more insulation.
Ask them to check the attic or walls for moisture, mould, old wiring or insulation that may contain asbestos before starting.
Get a written quote that states the type of insulation, the areas covered and the target insulation level.
For spray foam, ask about their training, curing time and when it's safe to be back in the home.
Consider an energy audit before and after the work to see where heat is being lost.

Questions to ask insulation contractors before you hire

What type of insulation do you recommend for this area, and why?
Will you seal air leaks before insulating?
How will you keep the attic properly ventilated?
What will you do if you find old insulation that may contain asbestos, mould or old wiring?
For spray foam, how long do we need to stay out of the house?
Does this work need a permit or an inspection?

When to call a pro for air sealing

Rooms that stay cold or drafty, or ice dams forming on the roof
Frost or condensation in the attic in winter
Loose, pebble-like insulation in the attic: leave it undisturbed and have it assessed by a qualified professional.

Air sealing permits and local rules

Some air sealing work needs a permit or has to meet local bylaws. Rules vary by municipality, so ask your pro whether a permit is needed and who will apply for it — and check with your city or town if you’re unsure.

Permits and licensing

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