Air Sealing Your Ontario Home: Where the Leaks Are and How to Fix Them

Drafts are not just uncomfortable. In a typical older Ontario house, uncontrolled air leakage can account for a large share of heat loss, often somewhere around a quarter to a third of it (an estimate that varies widely by house). Every cubic metre of warm air that escapes is replaced by cold outdoor air that your furnace or heat pump has to heat.

Air sealing is usually inexpensive compared with other upgrades, and much of it can be done by a careful homeowner. This guide explains how leakage is measured, where to look, what to seal, and what to watch for as your house gets tighter.

How air leakage is measured

The standard test is a blower door test, done by a Registered Energy Advisor during a home energy assessment or by an air sealing contractor. A powerful fan is fitted into an exterior door frame and depressurizes the house to 50 pascals. The result is reported as ACH50: air changes per hour at 50 Pa.

Home type	Typical ACH50
Older Ontario home (pre-1960), never upgraded	10–15 or more
1960s–1980s home	7–10
Older home after a thorough air sealing job	Often 4–6
New home built to current code	About 2.5–3.5
Passive House standard	0.6 or less

A blower door test also lets the advisor walk around the house with a smoke pencil or infrared camera while it is depressurized, which makes leaks easy to find. It is worth doing before and after major work, so you can see what was achieved.

Where the biggest leaks are

Windows and doors get the blame, but in most Ontario homes the largest leaks are hidden. Warm air escapes high up (through the attic floor) and cold air is drawn in low down (around the basement and foundation). This is called the stack effect, and it is strongest on the coldest days.

Priority areas, roughly in order of typical impact:

1. Attic floor and hatch: gaps around plumbing stacks, wiring, chimneys, recessed lights and the tops of interior walls.
2. Basement rim joist: the band of framing sitting on top of the foundation wall. Often uninsulated and leaky.
3. Penetrations through exterior walls: dryer vents, hose bibs, gas lines, cable and air conditioner line sets.
4. Fireplaces: an open or poorly sealed damper can leak like an open window.
5. Windows and doors: worn weatherstripping, gaps between the frame and the rough opening.
6. Ductwork in unheated spaces: leaky joints in attics or garages.
7. Electrical outlets and switches on exterior walls: individually small but they add up.

DIY vs professional air sealing

Task	DIY-friendly?	Typical materials	Notes
Weatherstripping doors and window sashes	Yes	V-strip, foam tape, door sweeps	Cheap; replace when compressed or torn
Caulking interior trim and baseboards	Yes	Acrylic latex caulk	Good rainy-day job
Foam gaskets behind outlet covers	Yes	Pre-cut gaskets	Turn off the breaker first
Sealing rim joist	Moderate	Spray foam kits, rigid foam with sealed edges	Ventilate well; follow product safety instructions
Attic floor air sealing	Harder	Low-expansion foam, caulk, rigid board	Cramped, hot or cold work; care with wiring and vermiculite
Whole-house sealing with blower door guidance	Pro	Various	Best results, measured outcome

DIY materials for a typical house might cost $100–$400. A professional air sealing job varies widely, but roughly $500–$2,500 is a common range depending on scope and access. Prices are estimates; get quotes.

Tip: On a cold, windy day, hold the back of your hand or a stick of incense near suspect areas. Moving smoke or a cold draft points straight to the leak.

What air sealing saves

Results vary with how leaky the house was and how it is heated. As a rough illustration, taking an older gas-heated home from about 10 ACH50 to about 6 ACH50 could reduce heating energy by something like 10–20%. With gas at an example 45¢/m³ and use of 2,400 m³ per year, that is roughly $100–$200 a year. In homes heated with electric baseboards, oil or propane, the dollar savings are higher.

Comfort improvements are often what homeowners notice first: fewer cold floors, less draft near the stairs, more even temperatures upstairs and down, and less dust entering from outdoors.

Ventilation and safety: do not skip this

A tighter house holds onto moisture, cooking odours and indoor pollutants longer. It also changes how air pressure behaves indoors.

• Fresh air: Very tight homes need mechanical ventilation. Many newer Ontario homes have a heat recovery ventilator (HRV). If your home gets much tighter, ask whether an HRV makes sense.
• Combustion safety: Older natural-draft gas water heaters and furnaces rely on the chimney to draw exhaust out. In a tight house, a strong kitchen range hood or dryer can pull exhaust back indoors (backdrafting). A combustion safety test is a sensible step after major air sealing. Appliances and venting should be serviced by a TSSA-registered contractor with a licensed gas technician (G2 or G1).
• Carbon monoxide: Ontario law requires CO alarms near sleeping areas in homes with fuel-burning appliances, fireplaces or attached garages. Test them monthly.
• Humidity: Aim for roughly 30–40% relative humidity in winter. Persistent condensation on windows suggests you need more ventilation, not less sealing.

Rebates

According to Save on Energy, the Home Renovation Savings Program assessment stream has included up to $250 for air sealing, as one of at least two upgrades verified by pre- and post-retrofit energy assessments. Income-eligible households may receive air sealing at no cost through the Enbridge Home Winterproofing Program or the Energy Affordability Program. Confirm current terms on saveonenergy.ca and enbridgegas.com, and see our rebates directory.

Air sealing checklist

• Book a blower door test, or do a hand-and-smoke survey on a windy day.
• Seal the attic floor before adding attic insulation.
• Seal and insulate the basement rim joist (see our basement insulation guide).
• Replace worn weatherstripping and door sweeps.
• Close and seal unused fireplaces properly; consider a chimney balloon or a tight-fitting damper.
• Check that bathroom fans vent outdoors and run them during showers.
• Arrange a combustion safety check if you have older gas appliances.

Frequently asked questions

Can a house be too airtight?

A house cannot be too airtight if it is properly ventilated. The rule used by building scientists is "build tight, ventilate right". Problems arise when a home is sealed up without a plan for fresh air and safe appliance venting.

Should I use expanding spray foam around windows?

Use low-expansion foam made for windows and doors. High-expansion foam can bow frames and make windows hard to open.

Is air sealing worth it if I am replacing my windows soon?

Yes. Window leaks are usually a small share of total leakage. Sealing the attic and rim joist gives benefits no matter what you do with windows.

Does air sealing help in summer too?

Yes. It keeps hot, humid outdoor air out, which reduces the load on your air conditioner and helps upstairs rooms stay cooler.