How to Stop Condensation on Windows in Winter

A wet window in January is easy to ignore. You wipe the sill, open the blinds, and hope the afternoon sun takes care of it. Sometimes it does. When it doesn’t, that moisture can stain wood, peel paint, and give mold a foothold on the coldest part of the wall.

The EPA treats condensation on windows, walls, or pipes as a signal: dry the wet surface and reduce the moisture source. It is what happens when moist indoor air meets a surface cold enough to drop that air below its dew point. In winter, your windows are usually that surface.

Why windows sweat when it’s cold

Air always contains some water vapor, and warm air can hold more of it than cold air. When indoor air touches a cold window, a thin layer of it cools. If it cools enough, it reaches the dew point, and the extra water condenses on the glass. The EPA puts the same idea in everyday terms: as the temperature goes down, the air is able to hold less moisture, which is why, in cold weather, moisture condenses on cold surfaces — for example, drops of water on the inside of a window.

Two things make winter condensation more likely. First, the glass is colder — especially on single-pane windows, metal frames, and poorly sealed older units, and especially at night. Second, indoor air is often wetter than you think. A tight house holds more of the moisture from showers, cooking, indoor laundry, houseplants, and a humidifier set too high. The EPA notes that a humidistat is set too high if excessive moisture collects on windows and other cold surfaces.

The practical work is the reverse of those two causes: keep indoor humidity in a sensible range, and make the interior surface of the glass a little warmer.

Not all window moisture means the same thing

Before you change anything, notice where the water is.

On the room side of the glass. This is the usual winter problem, and the one this article is about. Indoor air is too humid for how cold that pane is, or air is not circulating over the window. It often shows up overnight, especially at the bottom of the pane.

Between the panes of a sealed double- or triple-pane window. A faint haze you can’t wipe away from inside the house usually means the insulating-glass seal has failed. The unit typically needs to be repaired or replaced.

On the outdoor surface. Condensation on the outside of a window, often on a clear, cool morning, can be a sign that the window is insulating well. It usually evaporates as the day warms and is not an indoor humidity problem.

On the sill, the frame, or the wall under the window. Take this seriously. Standing water, darkened wood, peeling paint, bubbling drywall, or a musty smell at the sash are signs that moisture is lingering long enough to do damage.

Start with a humidity reading

Guessing is how people end up running a humidifier and a squeegee in the same week. Get a digital hygrometer and measure the room, not just the air next to the glass.

The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. The CDC says to keep it no higher than 50 percent all day long. In cold weather, extension services often recommend the low end of that range: NDSU Extension suggests 30 to 40 percent in winter, and notes that humidity above about 40 percent increases the potential for condensation on windows. The Minnesota Department of Health suggests 20 to 40 percent in winter in its climate. If your windows are wet at 50 percent, try 40. If they are still wet at 35 percent on a very cold night, you may also have a window or air-sealing problem. Our guide to ideal indoor humidity and how to measure it covers meters, placement, and calibration.

Cut the moisture you can control

This is the highest-leverage step, and it costs little.

Use exhaust fans that actually go outside. Run the bathroom fan during showers and for a while afterward, and a kitchen fan when cooking. The EPA and the CDC both stress that these fans should vent to the outdoors, not into an attic.

Watch the laundry. Clothes dryers should vent outdoors. Hanging laundry indoors adds a lot of water vapor; a Building America guide to interior storm windows, from the Pacific Northwest National Laboratory, flags indoor hang-drying as a considerable moisture source and recommends drying clothes outside or in a vented dryer.

Turn the humidifier down — or off. If you see moisture on windows, the EPA says to turn off humidifiers and unvented kerosene heaters. Long showers, simmering, houseplants, and unvented combustion appliances add vapor too; budget for them on the hygrometer.

Help the glass stay warmer, and let air reach it

Once you’ve cut indoor moisture, make the window less of a cold magnet.

Let air move across the glass. Heavy drapes, lowered blinds, and furniture pushed up to the sill can trap still, moist air on a cold surface. The EPA recommends increasing air movement, opening doors between rooms (especially colder closets), moving furniture out of wall corners, and using fans as needed. Raise the blinds during the day, and at night leave a little space at the sill so the glass is not sealed in a damp pocket.

Seal drafts around the window, but don’t confuse a draft with condensation. Weatherstripping and caulking cut the leak of cold air that chills the glass, and, as the EPA notes, storm windows and caulking keep the interior glass warmer and reduce condensation there. If you see daylight around a sash, fix that. Don’t seal a window in a way that traps water inside a wall if the flashing or frame is already leaking.

Add a layer: storm windows or better glazing. The EPA states that a storm window installed on the inside works better than one installed on the outside for raising the temperature of the cold surface. An interior storm panel creates an insulating air space and presents a warmer surface to the room.

That only works if the interior panel is fairly airtight. The Building America / PNNL guide explains the tradeoff: a good interior storm reduces condensation on the room side, but if humid indoor air leaks into the space between the storm and the original window, moisture can condense on the cold original pane. If you see water between the panel and the old glass, improve the storm’s seal first, and keep controlling indoor humidity.

Exterior storm windows can also help, especially on older single-pane units. University of Maryland Extension notes that a well-sealed storm window can reduce condensation and help prevent frost buildup in cold winters, and that exterior storms should include weep holes so any moisture that does collect can drain.

A modest temperature increase can help. The EPA lists “increase air temperature” among the actions that help prevent condensation. Bringing a chronically cold room up a few degrees is often enough when humidity is already under control. Rooms kept barely heated are common condensation spots. The EPA also recommends insulating cold water pipes, which can sweat for the same reason glass does.

What to do with the water that’s already there

Wipe it up and dry the sill. The EPA’s 24-to-48-hour drying window applies here as much as it does to a spill. Then fix the cause. If you only wipe the glass every morning, you have a daily moisture source, not a cleaning problem. Check the hygrometer, the bath fan, the humidifier, and the window coverings before you shop for a replacement window. New glass helps when a unit is underperforming or the seal has failed, but it will still condense if indoor humidity stays high.

What’s normal, and what isn’t

A thin film of moisture on the bottom of an older window on the coldest night of the year, which disappears after the room warms, is common. It is still worth lowering humidity a little so it doesn’t become a habit. Treat it as a problem when you see water pooling on the sill; ice on the inside of the glass; condensation that lasts well into the day; soft wood, peeling paint, or bubbling drywall; a musty smell or visible mold; or fog between sealed panes.

Homes in cold, dry-winter climates, including much of the mountain West, often see a sharp version of this pattern: very cold glass, indoor air humidified for comfort, and tight weatherization that holds that moisture inside. The physics is the same; the winter humidity target just needs to be more conservative.

A practical order of operations

1. Measure indoor humidity in the room with the wet window, morning and evening, for a few days.
2. Get that reading into the 30 to 40 percent range during cold weather, starting with fans, dryer vents, and the humidifier.
3. Let air and heat reach the glass. Open coverings during the day; don’t pin drapes to the sill.
4. Weatherstrip and caulk obvious leaks around the interior trim.
5. If the glass is still a cold, wet problem, add a well-sealed interior storm panel or plan for better glazing.
6. If the window, the wall, or the smell says moisture has been hanging around, stop treating it as a winter inconvenience and get the assembly looked at.

This article is general home-moisture guidance. It is not medical advice, a professional inspection, or a diagnosis of your house. If you have health concerns you think may be related to mold or dampness, talk with a clinician.