Condensation on the Inside of Windows: Causes and Fixes
Condensation on the room side of a window means the air in the room is humid and the glass is cold; it is a humidity problem, not a broken window, and it is fixed by lowering the moisture and moving more air. Fog trapped between the two panes is different: that means the window’s seal has failed. Here is how to tell which one you have, and what to do about each.
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The 30 second test: where is the moisture?
Wipe the glass with a dry cloth on the room side, then on the outside if you can reach it.
It wipes off inside
Room-side condensation. The room is too humid for how cold the glass is. Read on for the fixes.
The moisture is between the panes. The seal has failed and the glass unit needs replacing. See foggy window repair.
It is on the outside
Dew on the outside on cool mornings. Usually a sign the window insulates well. It clears as the sun warms the glass.
Why windows sweat on the inside
Moisture from daily life
Showers, cooking, drying clothes, plants and breathing all add water to the air.
Cold glass
In winter the glass is the coldest surface in the room, so the moisture lands there first.
Too little air movement
Tight homes, closed blinds and bathrooms without a working fan trap the humid air.
Single pane glass gets much colder than insulated glass, so it fogs at lower humidity.
The EPA’s brief guide to mold, moisture and your home treats condensation on windows as a sign of high humidity, and recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, and drying wet surfaces quickly.
How to stop it
- Measure the humidity. A cheap hygrometer tells you whether you are above the range above.
- Run the fans. Use bathroom and kitchen exhaust fans while showering and cooking, and for a while after.
- Turn down humidifiers in winter, and dry laundry outside or in a vented dryer.
- Let air reach the glass. Open blinds and curtains during the day so warm room air can reach the window.
- Use a dehumidifier in damp rooms and basements if the fog keeps coming back.
- Dry the frame. Wipe water off the sill each morning so it doesn’t soak into wood or paint.
When the glass itself is the problem
If humidity is in range and one window still fogs on the inside, the glass may be too cold. Single pane glass and old double pane units without a low-E coating stay cold enough to sweat in normal winter air.
Window condensation questions
More questions? Browse our glass repair FAQ or contact us.
Is condensation on the inside of windows bad?
A little on very cold mornings is normal. Heavy condensation that runs onto the sill every day can lead to mold, peeling paint and rotting wood, so it is worth fixing.
How do I know if my window seal has failed?
Wipe both sides of the glass. If the fog won’t wipe off from either side, it is between the panes and the seal has failed.
What humidity stops window condensation?
The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. In very cold weather, the lower end of that range helps.
Do new windows stop condensation?
Insulated glass with a low-E coating stays warmer, so it fogs less. But if the room is very humid, any glass can still sweat.
Why does only one window fog up?
Usually it is in a humid room like a bathroom or kitchen, or the glass in that window is colder, such as single pane glass or an older unit.
Fog that won’t wipe off?
Send a photo of the window, or call for a free estimate.