A draft near a window almost always comes from one of three places: the exterior joint between the frame and the wall, the weatherstripping on the opening sash, or the gap around the frame, behind the trim. Sometimes it isn't even air coming in: it's the cold glass chilling the air in the room. A simple test on a windy day lets you find the source in a few minutes.
The four possible sources
The exterior joint around the frame
This is the caulking between the window frame and the siding: brick, Canexel, vinyl or stone. Over time, the sealant hardens, cracks or pulls away. Outside air then gets into the cavity around the frame and comes out indoors, often through the trim or the corners.
Telltale sign: you feel air all the way around the window, even when it's closed and locked.
The weatherstripping on the opening sash
Weatherstripping is the flexible strip that seals the opening sash against the frame. It flattens and cracks over the years. Air then leaks along the opening sash, not around the frame.
That isn't a caulking issue. Weatherstripping can be replaced, often without changing the window.
The gap around the frame, behind the trim
Between the frame and the wall framing, there's a gap hidden by the interior trim. If it was poorly insulated at installation, air coming through a damaged exterior joint easily finds its way into the room. The exterior joint remains the first barrier: it's what keeps air and water out of that cavity.
Cold glass: the false draft
In extreme cold, room air cools against the glass and sinks down along it. You feel a stream of cold air at the bottom of the window, even though nothing is coming in from outside. It's more noticeable with single glazing or an insulated glass unit whose seal has failed.
If you see condensation between the two panes, the insulated glass unit is the problem, not the caulking. See our page on insulated glass replacement.
How to find the leak: the smoke test
Pick a windy day. Close and lock the window. Turn on the kitchen range hood and the bathroom fan: they create a slight negative pressure in the house, which makes leaks easier to spot.
- Light an incense stick, or grab a feather or a light tissue.
- Move it slowly along the frame, into the corners, then around the opening sash.
- Wherever the smoke moves or flattens, air is getting through.
- Note the exact spot: around the frame, or where the sash meets the frame.
Keep the incense away from curtains. No incense? A wet hand works too: damp skin picks up cold air more easily.
Three more simple checks
- **The paper test.** Close the window on a sheet of paper. If it slides out without resistance, the weatherstripping is no longer pressing at that spot.
- **The flashlight test.** In the evening, one person shines a light around the window from inside while another watches from outside. If light gets through, so does air.
- **A look outside.** Check the joint around the frame. Cracks, hardened or peeling sealant, holes, a visible gap between the frame and the brick: that's where it's getting in. Our article on the 5 signs it's time to recaulk shows what to look for.
What to do, depending on the source
- Air around the frame, damaged exterior joint: redo the exterior caulking.
- Air along the opening sash: replace the weatherstripping or adjust the window.
- Condensation between the panes: replace the insulated glass unit.
- Rotten or warped frame: the window may be at the end of its life. See caulk or replace your windows.
Sealing from the inside only can reduce the draft. But it doesn't fix a cracked exterior joint: rain keeps getting into the cavity around the frame.
Should you wait until spring?
No. The best time to caulk is now, whatever the season. We apply DOWSIL CWS silicone down to −29 °C, on a dry surface free of frost and ice. Our winter caulking page explains how we work in cold weather.
On every window, we fully remove the old sealant, clean and prep the joint, then apply the silicone and tool it neatly. All the details are on our window caulking page.
As for price, expect $30 to $60 per standard window, or $3 to $8 per linear foot. The exact price is confirmed at the inspection. Our caulking price guide explains what makes a quote vary.
In short
- The smoke test, on a windy day, shows where air is getting through.
- Air around the frame: exterior caulking.
- Air along the opening sash: weatherstripping.
- Condensation between the panes: insulated glass unit.
- No need to wait until spring: we caulk down to −29 °C on a dry surface.
Feeling air around your windows? The inspection and quote are free, and we get back to you within 24 h. Call us at 438-405-4195. 12-month interest-free financing, subject to credit approval (iFinance) is also available.