Drafty house? Fix air leaks and insulation while the walls are open

The first cold nights of fall are when most people notice the cold floor by the back door, the whistle around a window or the bedroom that never quite warms up. The air leaks were there all summer. What changes is the pressure that drives air through them.

Natural Resources Canada describes it as the stack effect. Warm indoor air rises and pushes out through gaps near the top of the house, around upper windows and through the ceiling into the attic. That creates lower pressure near the bottom of the house, which pulls cold outside air in through basement windows, the rim joist and other low openings. The colder it gets outside, the harder that cycle runs, and wind adds its own push on top.

That’s why sealing the top and the bottom of a house usually does more than chasing individual windows. It’s also why a renovation is such a good opportunity: the places where air leaks are often the same places we open up anyway.

Natural Resources Canada’s air leakage guide lists the usual suspects, and they match what we find when we open up older GTA houses. A simple test on a cold, windy day is to hold a couple of lit incense sticks near each spot and watch where the smoke drifts.

• The attic hatch, which often has no weatherstripping and no insulation on its back
• Pot lights, bathroom fans, plumbing stacks and wiring holes that pass into the attic
• The tops of interior walls where they meet the attic floor
• The sill and header, often called the rim joist, where the house framing sits on the foundation
• Gas, water and cable entry points, dryer vents and exhaust fan housings
• Electrical outlets and switches, including on some interior walls
• Windows and doors, around the sash and the frame as well as the glass
• Chimneys, foundation cracks, floor drains and old mail slots

This is the step people miss, and it can cost them the rebates. The Ontario-supported Home Renovation Savings program, run by Enbridge Gas and Save on Energy, offers rebates on insulation for attics, walls, foundations and exposed floors, air sealing, ENERGY STAR windows and doors, and heat pump water heaters. For all of those, the program requires an initial home energy assessment before any work begins, and you need to complete at least two of the recommended upgrades. Its help page is blunt: if you’ve already started installing those upgrades, you’re not eligible for the bundled rebates.

The timing matters for a second reason. The assessment includes a blower-door test that measures how leaky the house is, and Natural Resources Canada says the building envelope has to be intact for it, with any renovation under way limited to interior partitions. Once we’ve stripped an exterior wall or opened the attic floor, the advisor can’t test the house.

So the order is: assessment, then renovation, then a follow-up assessment with the same service organization once the upgrades are in. You pay for the assessments up front, and the program offsets that cost after you’ve completed it. A few upgrades, such as attic insulation on its own, heat pumps and smart thermostats, don’t need an assessment at all. To qualify you need to own the home, and detached, semi-detached and row houses are all eligible.

In Toronto, the City’s Home Energy Loan Program can finance insulation, air sealing and similar improvements. It also starts with an energy assessment and asks for contractor quotes with the funding request, so it’s another reason to get the assessment done early.

When we plan a renovation in a drafty house, we look at every opening the work creates and ask what can be sealed and insulated while it’s exposed. Your energy advisor’s report tells you which upgrades count toward the program; this table is about using the access you’re already paying for.

If you stand in an unfinished basement on a January morning and run your hand along the top of the foundation wall, you’ll often feel the draft before you see it. That band of wood where the floor joists sit on the foundation, the rim joist, is thin, full of holes for pipes and wires, and usually poorly insulated, if at all. In a finished basement it’s hidden behind the ceiling and easy to forget.

A basement renovation is the ideal time to fix it. With the ceiling open at the perimeter, each bay between the joists can be sealed and insulated properly, and the holes where services pass through the wall can be closed. Before we seal anything, though, we look at the wood itself. Soft spots, staining or signs of insects mean a repair comes first, because sealing in a moisture problem only hides it.

If you’re planning to finish the basement, our basement renovations page explains how we deal with moisture before any wall goes up.

The older the house, the more care air sealing and insulation need. Pre-war houses in Toronto neighbourhoods are often solid brick, and adding insulation on the inside of a solid masonry wall changes how that brick dries out after rain and freezes in winter. That’s a design decision to make with someone who understands masonry, not a default to apply to every wall.

Old attics deserve a careful look before anyone climbs in with a hose of new insulation. Health Canada warns that vermiculite insulation, the loose, pebbly grey-brown kind, may contain asbestos. If you have it, don’t disturb it or store things in the attic, and use professionals trained to handle asbestos before a renovation goes near it.

Wiring is the other question. Many older GTA houses still have sections of knob-and-tube or other original wiring in the walls and attic. Before insulation surrounds old wiring, have a licensed electrician look at it. If new wiring goes in while the walls are open, the Electrical Safety Authority rough-in inspection has to happen before insulation, vapour barrier and drywall cover it, so the electrician and the insulation work need to be booked in that order.

Sealing leaks is only half the job. Natural Resources Canada points out that as a house gets tighter, humidity rises and condensation can follow, and less air is available to fuel-burning appliances. The drafts were doing a crude job of ventilating the house; once they’re gone, ventilation has to be deliberate.

In practice that means bathroom fans and a kitchen range hood that actually vent outside, run on timers or humidity controls where they’re needed. In a house that becomes much tighter, a heat or energy recovery ventilator brings in fresh air while recovering much of the heat from the stale air it removes. If your furnace or water heater vents up a chimney, have it checked after the air sealing, because a tighter house with strong exhaust fans can pull combustion gases back down the flue. Many air-sealing contractors and energy advisors can test for that.

Keep an eye on the windows in the first cold spell after the work. A little fog at the bottom corners on a very cold morning is common; water running down the glass every day means the house needs more ventilation. Health Canada recommends indoor humidity between 30% and 50%, and a hygrometer makes it easy to check.

Whoever does the work, these questions make sure the energy side of the renovation is planned rather than improvised.

• Has the initial home energy assessment been done, and do we have the advisor’s report?
• Which upgrades in the scope are meant to count toward a rebate or loan?
• Does the price include sealing at outlets, top plates and the rim joist, or only insulation?
• What insulation is planned for each wall, ceiling and floor, and does it meet the levels in the report?
• Is the electrical rough-in inspection booked before insulation goes in?
• How will bathrooms and the kitchen be ventilated once the house is tighter?
• When is the follow-up assessment, and who needs to be there?

Read how we scope a multi-room renovation, compare quotes on the same terms, and see what else to plan for when the work happens in winter.