Stand next to a window on a windy day and put your hand near the frame. If you feel cold air moving, you’ve found one of the most common ways heat leaves a house. And if you’ve got a window that looks permanently dirty on the inside, with a hazy film trapped between the two panes of glass that no amount of wiping touches, you’ve found a different problem entirely. Those are two separate issues with two separate fixes, and I’ll walk through both. The good news: the drafty one is cheap and quick, and you’ve got time to do it before it gets really cold.
Part 1: The drafty window
Step 1: Find the leak (the incense test)
Close the window and lock it. On a breezy day, light a stick of incense (or hold a lit candle, carefully, well away from curtains) and slowly move it around the edges of the frame and the sash. Where the smoke wavers or gets pulled sideways, that’s your leak.
Check the usual suspects:
- The gap where the window frame meets the wall (inside trim)
- Between the sash (the part that moves) and the frame
- Around the lock, where the two sashes meet
- Cracked or shrunken seals where the glass sits in the frame
No incense? Hold a piece of tissue by the edges and watch it flutter. You can also just feel with the back of your hand.
Step 2: Match the fix to the leak
Gap between the trim and the wall → caulk. Use a paintable, flexible acrylic latex caulk with silicone (the label usually says “window and door”). Cut the tip small, apply a steady bead, smooth it with a damp finger or a caulk tool, and let it cure. Remove old, cracked caulk first with a utility knife or caulk remover tool. Check the label for the minimum application temperature, since many caulks want it above roughly 40°F, and cold weather is exactly why you should do this now and not in January.
Moving parts leaking (the sash closing against the frame) → weatherstripping.
- Self-adhesive foam tape is the cheap beginner option (about $5 to $8). Clean the surface first with rubbing alcohol so it sticks. It works, but it wears out in a season or two.
- V-strip (tension seal) is a thin folded strip of metal or vinyl that springs against the gap. It lasts longer and works well on double-hung window sides.
- Replace worn existing weatherstripping if your window has a rubber or felt seal that’s compressed, cracked, or falling out.
A gap at the sash lock, or loose-fitting window → check the lock. Sometimes a window just isn’t being pulled tight when it’s latched. Tighten the lock screws, or adjust the strike plate. A window that locks firmly pulls the sash against its seals.
A temporary seal for a window you don’t open in winter → rope caulk. This is a soft putty-like rope (about $5 a roll) that you press into the gap between the sash and the frame. It’s the fastest fix for a drafty window you won’t use until spring. Peel it off in the spring; it leaves little residue.
Step 3: Add a layer for the worst windows
Shrink-wrap window insulation film kits (about $10 to $20 for several windows) use double-sided tape and a hair dryer. You tape clear film over the interior of the window frame, then shrink it tight with the dryer, creating a still-air layer that cuts drafts and heat loss.
Tips:
- Clean the frame first and let it dry, since the tape won’t stick to dust or moisture.
- Apply the tape to trim or the frame, not to fresh paint or wallpaper, which can peel off when you remove it.
- You can’t open a window that’s been film-covered, so choose windows you won’t need for ventilation.
Thermal curtains are another no-tools layer. Close them at night and open them on sunny winter days to let free solar heat in.
Part 2: The foggy window (condensation between the panes)
This one needs a different explanation, because it’s not a draft. Modern windows are double-pane (or triple-pane) units: two sheets of glass with a sealed gap between them, filled with air or an inert gas, which is what gives them their insulating value. If that seal fails, moist air gets into the gap. Then you get fog, droplets, or a hazy mineral film that shows up between the panes and you can’t wipe it away because it’s inside the sealed unit.
How to tell what kind of fog you have
- Fog on the inside surface of the window (the side facing the room): that’s indoor humidity. Run a bathroom or kitchen fan, use a dehumidifier, and it will clear. The window isn’t broken.
- Fog on the outside surface: that’s normal outdoor condensation on a cold morning, and it’s actually a sign the glass is insulating well.
- Fog or haze trapped between the panes: this is the failed seal.
What your options are
- Do nothing. A failed seal doesn’t make the window unsafe. It just means the window insulates less than it did, and it looks cloudy. If it’s a seldom-used window, many people live with it.
- Replace just the glass unit (the “IGU”). The frame is usually still good. A glazier can replace only the sealed glass panes and reuse your existing frame. It costs far less than a new window, and it’s often the best value fix. If your window is still under warranty, check this first, since many manufacturers cover seal failure for 10 or 20 years.
- Replace the whole window. This makes sense if the frame is rotting, warped, or draft-prone, or if you’ve got several failing windows.
- Defogging services or kits. Some companies drill tiny holes to vent and clean the gap. These can clear the visible fog, but the insulating gas is gone, so it doesn’t restore the original performance. Know what you’re buying before you pay.
For a fogged window, take a photo and note the manufacturer name (it’s often printed on the spacer between the panes or etched in a corner of the glass), then call a local glass company for a quote.
A quick safety note
If you’re working on windows on an upper floor, don’t lean out or stand on a chair at the window opening. Keep both feet on the floor and use a stable stepladder for upper trim. And if the house was built before 1978, caulk or trim paint you’re scraping may contain lead. Wet the area, don’t dry-sand it, and use a lead test kit before disturbing old paint.
Before you go
An hour with a caulk gun and a roll of weatherstripping can noticeably change how a room feels, and it costs less than a tank of gas. It’s one of the best weekend projects to do before the first deep freeze.
Next up: what to do when your water heater runs out of hot water too fast, and the 10-minute fix that might get you your showers back.
Talk soon, Homeowner Help
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