Air sealing means closing the gaps, cracks and holes that let warm indoor air escape and cold outdoor air sneak in, and it's usually the least expensive way to cut heating bills and make a Canadian home more comfortable. A few hundred dollars of caulk, foam and weatherstripping, applied in the right places, can noticeably reduce drafts. The trick is knowing where the biggest leaks hide, which often isn't around windows, and making sure your home still gets enough fresh air afterward.
Key takeaways
- In older homes, air leakage can account for a large share of heat loss, often estimated at a quarter to a third or more.
- The biggest leaks are usually in the attic floor and basement rim joists, not windows.
- DIY materials typically cost roughly $100 to $500; professional air sealing often runs about $1,000 to $3,500 (approximate).
- Seal before you insulate: insulation doesn't stop air movement.
- Tighter homes need planned ventilation, and fuel-burning appliances need a safety check.
Why air leaks matter so much
Warm air rises. In winter, it pushes out through the top of your house, which pulls cold air in through the bottom. This "stack effect" is strongest in cold weather and tall homes, which is exactly when and where Canadians feel it most. Wind adds to the problem by pressurizing one side of the house.
Air leakage doesn't just waste heat. It causes:
- Drafts and cold floors, which tempt people to turn up the thermostat.
- Moisture problems, when warm, humid indoor air leaks into cold attics and walls and condenses, leading to frost, mould and ice dams.
- Dust, pollen and noise entering from outside.
Where homes really leak
Many people start with windows and doors, but the biggest leaks are usually hidden.
| Location | Why it leaks | Typical fix |
|---|---|---|
| Attic floor (top plates, pot lights, plumbing stacks, wiring holes, attic hatch) | Stack effect pushes air out at the top | Spray foam, caulk, weatherstripped insulated hatch |
| Basement rim joists | Gaps where the house frame sits on the foundation | Rigid foam cut-and-cobble with foam sealant, or spray foam |
| Plumbing and wiring penetrations | Holes cut through walls and floors | Caulk or foam |
| Chimney and flue chases | Large gaps around chimneys | Sheet metal and high-temperature sealant (never regular foam) |
| Electrical outlets and switches on exterior walls | Gaps in drywall | Foam gaskets behind cover plates |
| Windows and doors | Worn weatherstripping, gaps in trim | Weatherstripping, door sweeps, caulking trim |
| Dryer vents, exhaust fans | Missing or broken dampers | Replace with proper dampers |
The attic and basement are the two big priorities. A blower door test during an EnerGuide evaluation can pinpoint where your house leaks most.
DIY air sealing: a practical plan
Materials you'll need
- Acrylic or silicone caulk for gaps under about 6 mm
- Low-expansion spray foam for gaps around windows and doors
- Standard spray foam for larger holes in attics and basements
- Weatherstripping and door sweeps
- Foam outlet gaskets
- Rigid foam board for larger openings such as rim joist bays
- High-temperature sealant and sheet metal for chimney and flue gaps
Where to start
- Basement rim joists: Often the easiest big win. Cut rigid foam to fit each bay and seal edges with spray foam. Check your local fire code; exposed foam in some situations must be covered.
- Attic floor: Seal around plumbing stacks, wiring holes, the tops of interior walls and the attic hatch. Do this before adding insulation. Be careful with older pot lights, which may need covers rated for insulation contact.
- Doors and windows: Replace worn weatherstripping and add door sweeps.
- Outlets and penetrations: Add gaskets and caulk where pipes and wires pass through exterior walls.
Safety first
- Vermiculite insulation: Some older Canadian attics contain vermiculite that may contain asbestos. Don't disturb it; have it tested and, if needed, professionally removed.
- Knob-and-tube wiring: Don't bury or seal around old wiring without an electrician's assessment.
- Attic work: Step only on joists, wear a mask and eye protection, and avoid working in extreme heat.
When to hire a professional
Professional air sealers use blower doors and infrared cameras to find leaks, and have tools for hard-to-reach areas. Consider a pro if:
- Your attic is hard to access or full of loose insulation.
- You want measured results before and after.
- You're combining air sealing with insulation upgrades.
Approximate professional costs in Canada range from about $1,000 to $3,500 for a typical detached home, depending on scope. Some provincial and utility programs have supported air sealing alongside insulation; check our rebate finder for current options by address.
Build tight, ventilate right
A well-sealed house still needs fresh air. Air sealing reduces random, uncontrolled leakage; good ventilation provides controlled fresh air where and when you need it.
- Use your bathroom and kitchen exhaust fans, vented to the outdoors, not into the attic.
- Consider a heat recovery ventilator (HRV) or energy recovery ventilator (ERV) if your home becomes quite tight. These bring in fresh air while recovering much of the heat from outgoing air.
- Watch for signs of too little ventilation: persistent window condensation, stale air or lingering odours.
Combustion safety
If you have a naturally vented gas or oil furnace, water heater, wood stove or fireplace, tightening the house can cause exhaust to backdraft into the home when exhaust fans run. Before and after major air sealing:
- Install and test carbon monoxide alarms on every level.
- Have a qualified technician check combustion appliances for proper venting.
- Consider upgrading to sealed-combustion or electric equipment, such as a heat pump, over time.
How air sealing fits with other upgrades
Air sealing is the foundation for nearly every other upgrade. Sealing before insulating makes the insulation work properly, and a tighter home lets a smaller heating system, including a heat pump, keep you comfortable. Our whole-home upgrade sequence explains why envelope improvements usually come first.
Frequently asked questions
How much can air sealing save on heating bills?
Savings vary widely with how leaky the home starts, but older, drafty homes can often see meaningful reductions in heating energy, sometimes in the range of 10 to 20 percent or more when combined with insulation. A before-and-after blower door test gives the most accurate picture.
Can a house be too airtight?
A house can't really be too airtight if it's properly ventilated. Problems arise when tight homes lack mechanical ventilation, leading to moisture and indoor air quality issues, which is why HRVs and ERVs are standard in high-performance homes.
Should I use spray foam or caulk?
Use caulk for small, narrow gaps and spray foam for larger gaps and holes. Never use standard spray foam near chimneys or flues; use sheet metal and high-temperature sealant instead.
The bottom line
Air sealing is the cheapest, highest-impact place to start making a Canadian home more efficient and comfortable. Focus on the attic floor and basement rim joists first, seal before you insulate, and pair a tighter house with good ventilation and working carbon monoxide alarms.
Sundamentals is an independent, non-profit education platform. Program details and prices change often — check current programs for your address in our rebate finder.