Condensation on windows: what it means and how to stop it
Water on the inside of the glass means your indoor humidity is too high for how cold that glass gets. Lower the humidity, warm the glass, or both.
Condensation between the panes is a different problem entirely — the seal has failed and the unit needs replacing. Condensation on the outside of the glass is harmless and actually a sign of good insulation.
Window condensation is worth paying attention to precisely because it is visible. Glass is almost always the coldest surface in a room, so it condenses first — days or weeks before the same thing starts happening on a wall behind your furniture, where you will not see it until mold appears.
Treat foggy windows as an early warning rather than a nuisance. The fix is the same fix that protects the parts of the room you cannot see.
First, work out which condensation you have
| What you see | What it means |
|---|---|
| Water on the room side of the glass, worst in the morning | Ordinary indoor condensation. Your air holds more moisture than the glass temperature allows. Everything below applies. |
| Misting between the panes, which you cannot wipe off | The sealed unit has failed and the gas fill is gone. Nothing you do to your air will change it — the glazing unit has to be replaced. |
| Water on the outdoor face on a clear morning | Harmless, and a good sign. Your glass is well insulated enough to stay cooler than the outdoor dew point. It clears as the sun comes up. |
| Water pooling on the frame and sill with black speckling | This has been going on long enough to grow mold. Clean it (see removing mold) and treat the cause as urgent — the frame is next. |
What to do, in order
Free actions first. Do not buy anything until these have had two weeks.
Today
Wipe the glass and sills dry, and keep doing it daily. A window vacuum removes about a pint per session and stops the water running into the frame.
Open the bedroom windows wide for 5–10 minutes when you get up. Overnight breathing is the source of most morning condensation.
Run the bathroom fan during and 20 minutes after every shower, with the door shut.
Put lids on pans while cooking and use the range hood — if it vents outside.
Move curtains and blinds clear of the glass during the day so room air can reach it. Closed curtains keep the glass colder than the room.
This week
Stop drying laundry on radiators and indoor racks. One load releases 2–4 pints.
Check that your dryer vents outdoors. An unvented one is among the biggest moisture sources in a house.
Fit a humidistat or timer switch on the bathroom fan so it runs long enough without anyone remembering.
Read your humidity with a hygrometer in the affected room, first thing in the morning, and compare it against the dew point calculator.
If it has not improved
If the reading stays above 60% after two weeks of the above, size a dehumidifier for the space.
If the glass is single-glazed or the frames are metal without a thermal break, the surface will always be cold. Secondary glazing or replacement units raise the surface temperature so the same air no longer condenses.
If condensation concentrates at the window reveal and lintel rather than the glass, that is a thermal bridge — see insulation and heating.
In freezing weather, accept a lower target. Below about 20 °F outside, even 40% indoors will wet the glass. Drop to 30–35% until it passes.
When it becomes a health issue
Condensation itself is not a health problem. What grows in it is. Once water sits on frames and sills long enough, mold colonizes the seal and the surrounding paint, and mold spores are a genuine trigger for asthma and allergic rhinitis.
The practical line: if you are wiping water off every morning, you have weeks rather than months before growth starts. Wet frames with black speckling mean it has already begun.
Common questions
Why do my windows have condensation in the morning?
Overnight the glass cools well below room temperature while your indoor air stays moist — a sleeping adult releases up to a pint of water a night. When the glass drops below the dew point of that air, water leaves the air onto the glass.
Is window condensation my fault or the window's fault?
On the room side, it is a combination: your humidity is high enough and the glass cold enough. On modern double glazing it is almost always a humidity issue. On single glazing or metal frames, the glass is cold enough that even reasonable humidity will condense.
Should I be worried about condensation between the panes?
It means the sealed unit has failed. It is not a damp problem and no amount of ventilation will fix it — the glazing unit needs replacing.
Does condensation on windows cause mold?
Not directly, but the water it leaves on frames, sills and seals does. Those surfaces stay wet long enough for growth to start, usually within a few weeks of daily condensation.