Air sealing a house: where the leaks are, and the safety step people skip

Insulation gets the attention, but NRCan's Keeping the Heat In puts air leakage control first in any upgrade. The failures come from two directions: sealing the obvious spots and missing the big holes, or sealing well and then finding that the water heater spills exhaust or the windows stream with condensation.

Why air leaks matter

NRCan explains that air leaks occur where there is a hole in the building envelope and a pressure difference. In winter the house acts like a chimney: air enters low and leaves high, through the upper floors and the ceiling. That is why the attic and the basement usually hold the largest leaks, while drafts felt in living rooms are often the air being pulled in to replace what escapes upstairs.

Where to look

NRCan's checklist covers living areas (window sashes and casings, door frames and thresholds, outlets and switches even on interior walls, exhaust fans that do not close, ceiling light fixtures, baseboards, joints where wood framing meets masonry or chimneys, attic hatches, fireplace dampers, behind bathtubs and around pipes and ducts), the attic (plumbing stacks, wires and light fixtures through the ceiling, ducts, partition tops, chimneys, areas above stairwells) and the basement (the sill plate on the foundation, pipe and wire penetrations, leaky ducts, window framing, cracks, floor drains and the base of the chimney). It says not to limit the search to these places.

How to find them

On a cold, windy day, hold two or three lit incense sticks near suspected leaks and watch the smoke. NRCan describes a rough fan test: close windows and doors and run all exhaust fans and the dryer on cool, after first turning off fuel-fired space and water heaters to prevent backdrafting. Professional air sealers use a blower door with smoke pencils and an infrared scanner, which NRCan calls a powerful tool for locating leaks and missing insulation.

Seal with the right material

NRCan advises sealing cracks and penetrations on the inside of exterior walls, ceilings and floors. Outside, caulk only cracks that would let in water or insects; do not plug siding joints. Use silicone or polysulphide near heat sources, special high-temperature products at flues and where a chimney meets the wall, and a fully closing damper or tight glass doors with non-combustible caulk at fireplaces. At outlets, turn off and test the circuit before sealing. The attic hatch should be sealed and weatherstripped like an exterior door, then insulated.

The safety step

Fuel-burning appliances, including furnaces, water heaters, fireplaces and wood stoves, need air for combustion and exhaust. NRCan warns that without enough, the chimney or flue can spill dangerous gases into the house. It says controllable whole-house ventilation and a combustion-air supply are essential after tightening, that humidity rises and can cause condensation and mould, and that many air sealing companies offer backdraft testing. Older houses may need mould cleanup first. Before disturbing attic insulation, NRCan points to its asbestos and vermiculite guidance.

Prevention Checklist

• Start with the attic and basement, where the largest leaks usually are.
• Turn off fuel-fired heaters before any fan or depressurization test.
• Use high-temperature, non-combustible products near chimneys and flues.
• Turn off and test circuits before sealing outlets.
• Ask the contractor about backdraft testing and ventilation after sealing.
• Check for vermiculite or suspect insulation before disturbing the attic.

References

1. Natural Resources Canada. Keeping the Heat In - Section 4: Comprehensive air leakage control in your home, Web page, Introduction, 4.1 finding leaks, 4.1.2 checklist, 4.2 materials, safety considerations. Accessed 2026-11-03.

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