You notice it sometime in January.
One room never quite warms up, or you walk past the front window and feel the air change around your ankles. The instinct is to blame the glass.
Most of the time the glass is fine.
Air does not usually come through a window.
It comes around it. Until you know which gap you are dealing with, anything you buy is a guess.
Find the leak before you seal anything
Pick a cold, windy day. That is when leaks are easiest to find.
Close every window and exterior door, then turn on the bathroom fans, the kitchen exhaust and the range hood. That pulls the house into slight negative pressure, so outside air pushes in through whatever gaps exist.
Now go around with a lit incense stick.
Hold it a few inches from the edge of each window and door and watch the smoke. Where it bends or scatters, you have found air movement.
Work the whole perimeter rather than the obvious spots.
Check where the sash meets the frame, where the frame meets the trim, and where the trim meets the wall.
Those are three separate joints and they fail for different reasons.
A note on scale. A gap an eighth of an inch wide, running the length of a window, moves roughly as much air as leaving that window open six inches. It is easy to underestimate how small a gap has to be before it costs you money.
The three places air gets in
Through the moving parts. Where the sash slides or presses closed. This is a weatherstripping problem.
Around the frame. Between the window frame and the wall opening it sits in. This is an air sealing problem, and it is the one most people get backwards.
Through the wall itself. Air enters behind the siding, travels through the wall cavity, and comes out around the inside edge of the window. You can seal the outside perfectly and still feel this one.
What to use where
For moving parts, weatherstripping. The material matters more than people expect.
Plain foam strip is cheap and easy to fit, and it lasts a season or two before it compresses flat and stops doing anything. Plain brush strip lets a surprising amount of air straight through the bristles.
A plain plastic fin folds over under pressure and stays folded.
The one that holds up is brush with a flexible fin running through the middle of it, because the brush takes the wear while the fin does the sealing.
For the joint between frame and wall, caulk. And this is where it goes wrong.
The mistake almost everyone makes
Nearly everyone seals the outside and stops there.
Exterior caulk keeps water out and it matters.
It does not stop the air that is already inside your wall.
Air gets in past the siding, moves through the cavity, and exits around the inside edge of the window frame. Sealing the exterior does nothing to that path.
In an existing house, the seal that stops the draft is the one on the inside.
Run a continuous bead of interior caulk where the trim meets the wall, at the mitred corners of the trim, and in the joint between the trim and the window frame.
Use an interior caulk rather than an exterior one. It needs to stay flexible and take paint.
Continuous is the operative word.
A bead with a two inch gap hiding behind the trim is not a seal.
It is also why drilling and injecting foam after the fact so often fails to fix a draft. The foam goes in, but it rarely goes in everywhere, and the leak keeps running through whatever it missed.
At Delco, the callbacks we get in January are almost never about glass.
They are about a joint somewhere that nobody sealed on the inside.
The temporary fixes, rated honestly
Rope caulk. Genuinely effective. You press it into the gap and peel it out in spring.
Two warnings. On glossy paint, or on trim that gets direct sun and warms up during the day, it can leave an oily residue. Press it in gently rather than hard.
Clear silicone weatherstrip tape. Sticks to the frame, nearly invisible, peels off clean.
Slower to fit because it has to line up straight, but it is the tidiest of the temporary options.
Shrink film. It works. It also makes the window look like it has been packaged for shipping, and you cannot open it until spring. Fine for a spare room, harder to live with in a living room.
Draft stoppers and heavy curtains. These make a room feel warmer without sealing anything.
Useful, as long as you know what they are doing. The air is still coming in, you are just no longer standing in it.
What condensation is telling you
Three situations, three different meanings.
On the inside of the glass. Indoor humidity is high and the glass is cold.
Manage the humidity with ventilation, and keep curtains and blinds from trapping still air against the glass.
On the outside of the glass. The window is doing its job. The outer pane stays cold because the window is not letting your heat through to warm it. This one is a good sign.
Between the panes. The sealed unit has failed. Moisture got in, the insulating gas got out, and the window has stopped performing.
Look for a white haze on the glass as well, which is mineral deposit left behind by repeated condensation. That film means a failed seal even on a day when you cannot see any moisture.
A failed sealed unit cannot be caulked or resealed from the outside. The unit itself has to be replaced.
The one that catches people out
Here is something Natural Resources Canada points out that surprises most homeowners.
Old, leaky windows vent your house.
They let humid indoor air escape without anyone noticing.
Replace them with tight modern ones and the humidity that used to leak away now stays inside.
Some people fit new windows, get more condensation than they had before, and assume the windows are defective.
They are not. The house just got tighter and the ventilation has to catch up.
Run the bathroom fan longer, use the range hood every time you cook, and stop drying laundry indoors where you can.
When sealing stops being the answer
Sealing is the right first move. It stops being the right move when:
- You have sealed the same window two or three winters running and the draft keeps coming back
- The sealed unit is fogged, or has that white film on the glass
- The frame is warped, rotted or no longer square in its opening, so nothing seals against anything
- The window is single glazed, or 20 to 25 years old and never performed well to begin with
At that point, weatherstripping and caulk are managing a symptom. A window that old may still open and close, but it stopped being efficient a long time ago.
A short list to run before the cold sets in
- Do the smoke test on a windy day with the exhaust fans running
- Replace any weatherstripping that is compressed flat or torn
- Run a continuous interior bead where the trim meets the wall, and where the trim meets the frame
- Check that the weep holes in door sills and storm windows are open, so meltwater drains out instead of freezing in the track
- Clear grit and leaves out of sliding door tracks
- Note which windows are fogged between the panes, so you are planning for next year rather than reacting in February
Most of this is an afternoon of work and under fifty dollars of materials. It will not make an old window new. It will stop you from heating the outside.
Author bio
Edan Sher is the owner of Delco Windows and Doors, a Canadian manufacturer and installer of windows and doors, building and fitting its own products since 1989.
