Air sealing: find the leaks, then seal top and bottom first

How air and moisture escape

Air moves through a house wherever there is a hole and a pressure difference. In winter the stack effect does most of the work: warm air rises and pushes out through the ceiling and upper walls, while cold air is drawn in low, through basement windows and the rim joist. The effect grows with leakier houses, taller houses and colder weather, and wind and exhaust appliances add to it (NRCan ↗).

NRCan estimates about 25% of heat loss can be due to air leakage (NRCan ↗). Leaks carry moisture as well: one small air leak can let in 100 times more moisture than diffusion through the same area, and that moisture can condense or frost inside walls and attics (NRCan ↗). The same air movement is why many older houses are very dry in winter (NRCan ↗).

Where the leaks are

• Top: the attic hatch, plumbing stacks, wiring and light fixtures, chimneys, wall top plates, dropped ceilings, knee walls and balloon-frame walls open to the attic (NRCan ↗; NRCan ↗; CMHC ↗).
• Bottom: the sill plate and rim joist, service penetrations, basement windows, foundation cracks, floor drains and the base of the chimney (NRCan ↗).
• In between: outlets and switches (even on interior walls), trim and baseboards, window and door frames, fireplace dampers, exhaust fans, behind tubs and above pocket doors (NRCan ↗).

Signs of a leaky house

• Drafts at outlets, baseboards, trim and doors. On a cold, windy day, incense smoke shows where air is moving (NRCan ↗).
• Very dry indoor air in winter (NRCan ↗).
• Frost in the attic, ice dams, or discoloured patches in attic insulation where air has been filtering through (NRCan ↗; CMHC ↗).
• Condensation between storm and inner windows upstairs, where the stack effect pushes air out (NRCan ↗).
• After sealing: condensation on windows, stale air or lingering odours mean the house now needs more ventilation (NRCan ↗).

What an energy advisor checks

Every EnerGuide evaluation includes a blower door test (NRCan ↗). The fan depressurizes the house to 50 pascals, and the result in air changes per hour at that pressure (ACH50) shows how leaky it is; lower is tighter (NRCan researchers ↗; National Building Code ↗). Before the test, fuel-fired heating is turned down or off and the water heater turned down so neither fires while the fan runs, and ashes are cleared from wood-burning appliances (NRCan ↗). Smoke pencils and an infrared camera then pinpoint leaks and missing insulation (NRCan ↗); see thermal imaging.

Typical blower door numbers by house age

These are national averages from NRCan field data published in 2000; we found no newer official breakdown by house age. Regional results varied widely, from 23.6 for pre-1921 houses in BC to 9.1 in the Prairies, and many houses have been sealed since (NRCan researchers ↗). Today the R-2000 standard allows no more than 1.5 ACH50 (NRCan ↗). For energy modelling, the 2020 National Building Code assumes 3.2 ACH50 for a new house built to its basic rules, or 2.5 where the air barrier meets its detailed requirements, and its tiered energy path gives credit for tested levels from 2.5 down to 0.6 (National Building Code ↗).

Fixes, and the order to do them

1. Find the leaks: a blower door, or a DIY fan test with every exhaust fan running. Turn off fuel-fired space and water heaters first (NRCan ↗).
2. Seal the top and bottom of the house first: the attic floor and hatch, then the rim joist and basement penetrations. This order is our reading of NRCan's guidance rather than a rule it sets: the stack effect pushes air out at the top of the house and draws it in at the bottom (NRCan ↗; NRCan ↗).
3. Then the middle: outlets, trim, windows and doors. Caulk windows inside before outside (NRCan ↗).
4. Check ventilation and combustion air before calling the job done (NRCan ↗).

Air sealing and ventilation go together

Tightening a house raises humidity and leaves less air for fuel-burning appliances, so ventilation and combustion air are part of comprehensive air sealing (NRCan ↗). Most older houses still leak enough after sealing to get some fresh air (NRCan ↗), but NRCan says air leakage “does not make for good ventilation” and suggests a system able to change all the house air once every three hours (NRCan ↗). A heat recovery ventilator (HRV) typically recovers 70 to 80% of the heat from the outgoing air, according to Keeping the Heat In (NRCan ↗); NRCan's 2018 ventilation guide gives a wider range of 60 to 90% (NRCan ↗). Aim for indoor humidity of 30 to 50% (NRCan ↗).

Safety: backdrafting and carbon monoxide

When fans, dryers and fireplaces exhaust more air than leaks can replace, the house is depressurized, and a natural-draft furnace, water heater or fireplace can backdraft, spilling combustion gases, including carbon monoxide, into the house (NRCan ↗). Warning signs include soot around appliances, hot and muggy air near the furnace, combustion odours, melted plastic fittings on top of a water heater, and repeated headaches or flu-like symptoms (NRCan ↗).

• Install CO alarms. In homes with a fuel-burning appliance, code requires one inside each bedroom or within 5 m of each bedroom door (National Building Code ↗).
• Try NRCan's chimney flow test: with the appliance running, hold incense at the draft hood, then turn on every exhaust fan. If the smoke moves into the room, call a heating contractor (NRCan ↗).
• Ask for a combustion safety test with any air sealing job; many air sealing companies test for backdrafting (NRCan ↗).

Mistakes to avoid

• Stuffing fibreglass into gaps. Glass and mineral fibre are poor air sealing products (NRCan ↗).
• Ordinary caulk near heat. Chimneys, flues and fan motors need non-combustible sealants (NRCan ↗).
• Running a fan test with gas or oil heaters on, which risks backdrafting them (NRCan ↗).
• Exhaust-only ventilation in a tight house. Powerful exhaust fans can depressurize it enough to reverse chimney flow (NRCan ↗).
• Running several big exhausts at once, such as a high-capacity range hood and a clothes dryer (NRCan ↗).
• Caulking a bedroom window shut. Building codes require every bedroom to have at least one window that opens from inside (NRCan ↗).

Rebates and next steps

Programs open and close often, so check our rebates by province for what is available now. In Ontario, start with Home Renovation Savings assessments. Some programs include a post-upgrade EnerGuide evaluation that needs proof of the work, so keep paid invoices and receipts (NRCan ↗).

To compare this work with other upgrades at your local fuel prices, try the upgrade payback calculator, then put the order of work into your home plan. You can also find a registered energy advisor near you.