Water on the inside of your windows is not a window fault. It is a message: the air in that room holds more moisture than the coldest surface in it can tolerate. Glass is almost always the coldest surface in a heated room, so it condenses first — which makes your windows an accidental humidity gauge.
That gives you exactly two levers. Lower the humidity in the room, or raise the temperature of the glass. Everything that actually works is one of those two things.
The only thing that causes it: dew point
Air can hold a certain amount of water vapour, and that amount depends on temperature. Warm air holds more. When air touches a surface cold enough, it cools, its capacity drops, and the surplus has to go somewhere — it condenses onto the surface as liquid water.
The temperature at which this starts is the dew point. It is not a fixed number: it moves with both the room temperature and the relative humidity. In a room at 70°F, the dew point looks like this:
| Room at 70°F, relative humidity | Dew point | Condensation starts on any surface below |
|---|---|---|
| 30% | 37°F | 37°F |
| 40% | 45°F | 45°F |
| 50% | 50°F | 50°F |
| 60% | 55°F | 55°F |
| 70% | 60°F | 60°F |
Read that last column again, because it is the whole article in one line. At 60% humidity, anything colder than 55°F in that room will get wet. At 40%, the surface has to fall to 45°F before anything happens. You have not changed a single window — you have changed what the windows can survive.
Why the windows and not the walls
An insulated wall keeps its inner surface within a few degrees of room temperature. Glass does not. Even good double glazing conducts heat out of the room far faster than the wall around it, so the inner pane sits well below room temperature whenever it is cold outside. Single glazing is worse still: the inner surface tracks the outdoor temperature closely.
This is why condensation shows up on the glass first, then the frame, then the corner of an exterior wall behind a wardrobe — in order of how cold each surface is. If you are seeing it on the walls too, the humidity problem is well past the point where window films and trickle vents will help.
What humidity you can actually run in winter
The colder it gets outside, the colder your glass gets, and the lower your indoor humidity has to be to stay ahead of it. These are approximate ceilings for a room held at 70°F:
| Outdoor temperature | Single glazing | Older double glazing | Modern low-e double |
|---|---|---|---|
| 40°F | ~45% | ~70% | Not limiting |
| 30°F | ~35% | ~65% | ~80% |
| 20°F | ~25% | ~60% | ~75% |
| 10°F | Below 20% | ~50% | ~70% |
| 0°F | Not achievable | ~45% | ~70% |
Two things fall out of that table. First, if you have single glazing and a real winter, some condensation is close to unavoidable — the fix is glazing, not humidity. Second, if you have decent double glazing and your windows still stream every morning, your indoor humidity is genuinely high and worth measuring. Our room-by-room humidity guide covers the targets, and the humidity and mold chart covers what happens if you ignore them.
Where all that water comes from
People routinely underestimate this. A household of four puts several gallons of water into the air every day without noticing:
| Source | Roughly how much |
|---|---|
| Breathing and perspiration | 2–3 pints per person per day |
| A shower | 0.5 pint each |
| Cooking, three meals | 2–3 pints per day |
| Drying one load of laundry indoors | 4–9 pints |
| A clothes dryer venting indoors | 10+ pints per load |
| Unvented gas or kerosene heater | ~1 pint per hour of use |
| Houseplants | 0.2–0.5 pint each per day |
Bedrooms are the classic case. Two adults breathing for eight hours in a closed room with the heating off is roughly a pint of water released into a small, cooling space. That is why bedroom windows are wet at 7am and dry by lunchtime.
How to stop it, cheapest first
- Stop drying laundry indoors. Single biggest avoidable source in most homes. If you have no choice, dry it in one room with the door shut and a dehumidifier running.
- Check the dryer is actually vented outside. A disconnected or crushed duct dumps every load into the house.
- Run the bathroom fan during the shower and for 20 minutes after. Most people switch it off with the light, which is exactly when it is needed.
- Use the kitchen extractor and lids on pans. A covered pot puts a fraction of the moisture into the room.
- Open bedroom curtains and blinds during the day. Closed curtains trap a cold pocket of air against the glass, dropping its temperature by several degrees and guaranteeing condensation overnight.
- Leave interior doors open, or crack a window briefly each morning. Five minutes of full cross-ventilation exchanges the air without meaningfully cooling the structure.
- Keep the heating steady rather than swinging. Rooms that go cold overnight and warm in the morning condense during the cold half.
- Run a dehumidifier. This is the direct lever on humidity, and the only one that works reliably in a house where the moisture load cannot be reduced. Our size calculator will tell you what capacity you need, and the running cost guide tells you what it will cost.
- Improve the glass. Secondary glazing, thermal blinds fitted with a gap, or replacement units raise the surface temperature and move the whole problem out of reach. The most expensive fix, and the only permanent one for single glazing.
If you want the fuller list of humidity reduction methods beyond windows specifically, see 12 ways to lower humidity in your house.
When condensation means something worse
Water between the panes. If the misting is inside the sealed unit and you cannot wipe it off, the perimeter seal has failed and the inert gas has escaped. No amount of humidity control will touch this — the unit needs replacing.
Black spotting on the frame or sill. That is mold, and it means the surface has been staying wet long enough for growth to establish. Clean it, then fix the humidity, in that order — see what a dehumidifier can and cannot do about mold.
Soft or discoloured wood at the bottom of the frame. Repeated wetting has got into the timber. This is rot, and it will keep going after the condensation stops.
Condensation in only one room. Usually the room with the moisture source, or the coldest one. If it is a room you barely heat, the answer is heat and airflow rather than a dehumidifier.
Condensation everywhere, all winter, in a well-glazed house. Something is adding water that you have not accounted for — a leak, a wet crawl space venting upward, or a failed vapour barrier. Our guide to why your house is so humid works through the possibilities in order.
Common questions
Will a dehumidifier stop condensation on windows?
Yes, if the humidity is the binding constraint. Dropping a room from 60% to 45% moves the dew point from about 55°F down to about 48°F, which is enough to keep most double glazing dry through a normal winter. It will not help with single glazing at 20°F outside, because the glass gets colder than any liveable dew point.
Should I just wipe the windows every morning?
It protects the frames and sills, so it is worth doing while you fix the cause. But wiping puts the water back into the room on the cloth, and it does nothing about the same moisture condensing behind furniture and inside wardrobes where you cannot see it.
Why are my windows wet when the house feels dry?
“Feels dry” tracks temperature more than humidity. A cool room at 60% feels drier than a warm room at 45%. Buy a cheap hygrometer and put it in the room that condenses — it is the only way to know what you are dealing with.
Is condensation on the outside of the glass a problem?
No, the opposite. Dew on the outer pane on a clear morning means the glass is losing almost no heat from inside — it is a sign the window is performing well.
The short version
Measure the humidity in the room that condenses. If it is above 50% in winter, cut the moisture at source and, if that is not enough, run a dehumidifier. If it is already below 45% and the glass still streams, the glazing is the limiting factor and no humidity control will fix it.
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