Draughty windows and doors in October: what caulk and weatherstripping fix

A draughty window in October usually means one of three things: a gap in the joint between the trim and the wall, a worn seal on the part of the window that opens, or air that is leaking in somewhere else and only showing up at the glass. They need different fixes, and buying a tube of caulk before you know which one you have is how a weekend gets spent sealing the wrong joint. Natural Resources Canada calls air leakage control the single most important retrofit in a house and says it should be done first. This guide covers how to find the leaks yourself, which material fits which gap, and why the calendar matters more for the outside of the house than the inside.

Is the cold air coming through the window, or being pulled in there?

Often it is being pulled in. In winter a house behaves like a chimney: warm air rises and escapes through the upper floors and the ceiling, and outside air is drawn in at the lower levels to replace it. NRCan describes this stack effect in its Keeping the Heat In guide. The practical result is that the cold stream you feel at a main-floor window or a basement outlet is partly driven by leaks you cannot feel at all, around the attic hatch, ceiling light fixtures and the plumbing stack.

So a window can be sealed well and still feel cold if the house is losing air at the top. That does not make window work pointless. It means the search should cover the whole house once, and the results decide where the caulk goes.

How do you find the leaks without paying for a test?

Use smoke. NRCan's own do-it-yourself leak detector is a few burning incense sticks held together. A strong leak scatters the smoke and makes the tips glow; a slow one makes the smoke trail toward or away from the gap. Pick a cold day, and a windy one if you can, because the pressure difference is larger and the leaks are easier to see.

1. Close every window and exterior door.
2. Turn off any fuel-burning space heater and the gas water heater, if you have one, before the next step (see the warning below).
3. Switch on every exhaust fan in the house: bathroom fans, the range hood, and the clothes dryer on its cool cycle. This pulls the house slightly below outdoor pressure, which makes every leak draw air inward.
4. Walk each room with two or three lit incense sticks. Hold them at the joint between the trim and the wall, along the edges of each sash, at the corners of the door frame, at outlets and switches on outside walls and on interior walls too.
5. Mark each leak with a small piece of painter's tape and write on it whether the smoke moved hard or slowly.
6. Turn the fans off and the heaters back on when you are done.

The tape marks decide what to buy. Count how many are on parts that move (a sash, a door) and how many are on parts that stay still (trim, frames, outlets). That split decides the materials.

Where to check besides the windows

NRCan's checklist of common leakage areas is long. These are the places on it worth the incense sticks on a typical GTA house:

• Around the door frame and the threshold.
• Electrical outlets and switches, on outside walls and on interior walls, because air can travel through partition walls or along floor joists.
• Baseboards and trim, and the corners where two walls meet.
• The attic hatch, ceiling light fixtures, and bathroom exhaust fans that do not close properly when they are off.
• The fireplace damper. Close it and check it with a flashlight; NRCan says a damper that does not close fully should be repaired by a certified chimney specialist.
• In the basement: where the wood sill plate sits on the foundation, and where the gas line, electrical service and other pipes pass through the wall.
• Behind the bathtub, under sinks, and around plumbing pipes and ductwork.

NRCan's guide carries a safety warning about asbestos and vermiculite insulation before any attic work. If the attic is full of loose, pebble-like, grey-brown insulation, leave it alone and read the guide's health and safety part first.

Caulk or weatherstripping: which one goes where?

The rule NRCan gives is simple: caulking, tapes and gaskets are for joints between materials that do not move, and weatherstripping is for joints that do. Caulk on the edge of a sash glues the window shut; foam tape along a trim joint falls off. The table below matches the leaks you marked to the product NRCan's materials tables point to.

On product choice, NRCan's materials section notes wide variation between brands of the same type of caulk and advises buying the premium grade for durability. The same table rates plain acrylic latex at 5 to 25 years depending on the brand, and acrylic latex with silicone at 30 years or more. Silicone holds up well but most types cannot be painted, and new silicone will not bond over old silicone, so the old bead has to come out completely.

Why the inside gets sealed before the outside

The instinct in autumn is to go outside with a caulking gun. NRCan puts the order the other way round, and the reason is moisture inside the wall.

Caulking on the outside of a window should be done only after interior sealing is complete. If the exterior is caulked first, it can trap warm, moist air in the wall, which over time, can damage the wall.

The same section calls exterior caulking the last and weakest defence against rain entering a wall. The main defence is properly applied flashing around the window and a properly detailed drainage plane behind the cladding; a bead of caulk on the outside only backs them up.

In practice: work through the inside marks first, with interior-rated caulk. NRCan warns that products labelled for exterior use only may contain volatiles that can be hazardous when breathed in a confined space over a long period, and that indoor caulking needs extra ventilation for two or three days while it cures. Then do the outside cracks you can reach safely from the ground or a short step ladder.

Weatherstripping by window type

Double-hung and single-hung windows

Weatherstrip the sides, top and bottom of the sash that moves, and caulk the leaks around the fixed part. NRCan's method for V-strip: open the window and slide the strip up the gap between the sash and the frame with the open side of the V facing outside, running it about 25 mm above the top of the closed sash. The gap where the two sashes meet in the middle is done by taking out the lower sash and applying V-strip to the upper one from the inside.

Sliding windows

Sliders usually use brush weatherstripping that sits in a slot. The sash comes out, the old brush is pulled from its slot, and the new one is cut to length and snapped or slid in, with each end tacked or glued so it does not creep. On newer windows, NRCan suggests prying out a sample and taking it to the manufacturer or supplier so the replacement is the same profile.

Casement and other hinged windows

These seal like doors: the weatherstrip goes on the frame or the stop, and the closed window presses against it. If a casement still leaks with good seals, look at the hardware. A crank or lock that no longer pulls the sash tight will leave a gap no strip can close, and NRCan lists servicing latches, cranks and locks as an upgrade in its own right.

Doors: the top, the sides and the gap at the bottom

For the top and sides of a door frame, NRCan names good-quality V-shaped vinyl as the easiest and most effective option, because it keeps contact with the edge of the door as the door warps from season to season. The bottom is harder and is often the largest draught in the room. The choice is between a seal on the door and a seal on the sill:

• A door sweep screws to the door. It is easier to fit than a threshold and lasts longer, but NRCan rates its seal as less effective.
• A door bottom wraps the bottom edge of the door and needs 8 to 13 mm of clearance under it.
• A threshold seal on the sill gives the best seal, but foot traffic and weather wear it down, so pick a model with replaceable inserts.
• Magnetic strips are best on modern steel doors and may not seal well in cold weather, because condensation and frost form on them. Spring metal strips can ice up for the same reason.

If the door is out of square with its frame, no strip will fix it. Adjust or repair the hinges and the door first. A sliding patio door that nobody uses in winter can be closed with removable sealant or covered with heat-shrink film until spring, which NRCan lists as an interim measure for older sliders.

What the October weather allows, and what has to wait

Indoor sealing can be done any time of year. Outdoor caulking is limited by the product. DAP's technical data sheet for its Alex Plus acrylic latex caulk with silicone, one of the most common tubes on a hardware shelf, says to apply it above 40 °F (about 4 °C) and not when rain or freezing temperatures are forecast within 24 hours. Other brands and types have their own limits printed on the tube, and they are the ones to follow. Compare that window with the long-term averages at Toronto Pearson:

That is the reason this is an October job. Check the forecast for the next 24 hours, not just today's high, and make sure the joint is clean and dry: the same data sheet asks for a surface free of old caulk, dirt, dust and grease. The data sheet limits a joint to 1/2 inch (about 13 mm) wide and deep, and asks for backer rod where it is deeper.

If December arrives before the outside is done, NRCan's temporary option for a leaky window is an interior heat-shrink film kit. Double-sided tape holds the film to the trim, a hair dryer tightens it, and it seals well. The guide also lists the drawbacks: the tape can lift paint when it comes off, in most cases the film can be used only once, and the window cannot be opened until the film is removed.

What changes in the house after you seal it

A tighter house keeps more moisture in. NRCan warns that as the envelope is tightened, indoor humidity rises and can cause condensation problems, and that less fresh air leaks in for fuel-burning appliances. Two things to watch over the first cold weeks: water or frost on the inside of the glass in the mornings, and whether the bathroom fan and range hood actually run when you cook and shower. Persistent condensation on a sealed window points to indoor humidity rather than to the window itself, and that is covered in why windows streak and fog.

If the house has a gas furnace, water heater or fireplace and you have sealed a lot of leaks, have the appliances checked for combustion air. NRCan notes that air sealing companies often test for backdrafting as part of the job.

When to do it yourself and when to call someone

Most of this is a homeowner job. NRCan says homeowners can usually do a relatively effective air sealing job if they have the time and patience. V-strip, door sweeps, outlet gaskets and interior caulking need a utility knife, a screwdriver and a caulking gun with a pressure-release lever. The incense test costs almost nothing.

Call a professional for three situations. A whole-house problem you cannot locate is the job of an air sealing contractor or energy advisor, who uses a blower door and often an infrared camera to find leaks the incense misses. A window that is rotted, out of square or has a failed sealed unit needs a window and door supplier. And exterior caulking above the first storey is ladder work; the two-storey window cleaning guide explains where a ladder job should stop.

While the screens are out for the season, the sill, the sash edges and the outside of the frame are all in reach at once, which makes it the easiest time to inspect the weatherstripping. What to look at with the screens off is in taking window screens off before winter. Caulk only sticks to a clean, dry surface, so if the glass and frames are grimy from the summer, wash them first.