How to air seal a house, step by step

Why does air sealing matter as much as insulation?

Insulation slows heat conducted through a surface, but it does little to stop moving air. In winter, warm indoor air rises, escapes through gaps at the top of the house and pulls cold air in at the bottom. That flow, called the stack effect, makes up a large share of the heat loss in many older houses, and it also carries moisture into attics and walls. Loose-fill insulation laid over an unsealed attic floor will be bypassed, so seal first. The insulation upgrade priorities guide follows that same order.

Where are the leaks in a typical house?

Most of the loss comes from a handful of places.

• The attic floor. Gaps around the hatch, chimney chase, plumbing stacks, wiring holes, bath fan housings and the tops of interior walls.
• The rim joist. The band where the floor frame meets the foundation.
• Recessed lights and ceiling fixtures. Many are open to the attic unless they are rated airtight.
• Windows and doors. Gaps between the frame and wall, worn weatherstripping and thresholds, see door weatherstripping.
• Penetrations. Dryer vents, gas lines, cable holes and electrical outlets on exterior walls.

How do you find the leaks?

On a cold, windy day, hold a stick of incense or a smoke pencil near suspected gaps and watch where the smoke moves. A thermal camera, when available, shows cold streaks on walls and ceilings. The most complete method is a blower door test, which depressurizes the house so that leaks stand out and can be measured. It is described in our blower door guide. An energy adviser usually does one as part of a home energy audit.

What materials and methods work?

Use the right product for the gap. Flexible caulk suits small cracks around trim and between materials that move. Low-expansion can foam fills larger gaps around pipes and wiring. Gaskets and weatherstripping handle moving parts such as doors and attic hatches. Sheet materials such as rigid board or flashing, sealed at the edges, close big openings and chimney chase gaps. Near flues and chimneys, use only non-combustible materials and fire-rated sealants, and keep the required clearances. Spray foam can do much of the work in rim joists and cathedral ceilings, as discussed in spray foam insulation.

Start at the attic and work downward. Once sealing is done, the attic is ready for the extra layer described in attic insulation in Canada.

What safety points come with a tighter house?

A tighter house has less natural ventilation, so humidity can rise and combustion appliances can draw air from the wrong place. Test your carbon monoxide and smoke alarms, use bathroom and kitchen fans, and have gas or oil appliances checked if you seal heavily. A heat recovery ventilator supplies fresh air with little heat loss, as explained in the HRV guide. Ask a qualified contractor whether your house needs ventilation upgrades after sealing. Some programs have offered support for air sealing and testing; check the Government of Canada energy-efficiency pages, your province and your utility for current terms.

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