Condensation on the Inside of Windows: Causes and How to Stop It

Why windows sweat on the inside

Air always holds some water vapor. The warmer the air, the more vapor it can hold. When that air touches a surface that is cold enough, it can no longer hold all of its moisture, and the excess turns into droplets. The temperature at which this happens is called the dew point. The EPA describes it with a familiar example: take a cold bottle out of the refrigerator on a hot day and water forms on the outside of the bottle.

In winter, the window is the cold bottle. Glass is usually the coldest surface in a heated room, especially at the bottom edge and corners, where cold air settles and the frame conducts heat outside. That is why condensation almost always starts low on the pane and along the edges.

Two things decide whether you get water on the glass: how humid the indoor air is, and how cold the inner surface of the glass gets. Your home adds moisture through showers, cooking, breathing, drying laundry and houseplants. The outdoor temperature, the type of window and whether curtains trap cold air against the pane decide how cold the glass gets.

Inside, outside or between the panes: what it means

Where the water sits tells you what kind of problem you have. Check with a fingertip: if you can wipe it away from the room side, it is inside condensation.

Where you see it	What it usually means	What to do
Inside surface (room side)	Indoor air is humid for how cold the glass gets	Lower humidity, improve air flow, keep glass warmer (this article)
Between the panes of a double- or triple-pane unit	The seal of the insulated glass unit has failed	Humidity changes will not fix it; the glass unit or sash needs repair or replacement
Outside surface (outdoor side)	Outdoor air is humid and the outer pane is cool, often on clear mornings	Nothing; it is not an indoor moisture problem and it clears as the day warms

The Building America Solution Center, a U.S. Department of Energy resource run by Pacific Northwest National Laboratory, notes that insulated glass unit seal failure shows up as condensation or “fogging” in the space between the two layers of glass, and that it is commonly caused by the seals sitting in contact with water for long periods. Wiping or ventilating will not clear it.

The usual moisture sources in a home

Before buying anything, find out where the extra moisture is coming from. The EPA lists these everyday sources and how to handle them:

• Showers and baths: run the bathroom fan or open the window while showering.
• Cooking and dishwashing: use the range hood or another exhaust fan, or open a window.
• Clothes dryers: vent them to the outside, not into the garage, attic or living space.
• Unvented combustion heaters: kerosene heaters and similar appliances release water vapor and raise humidity unless they are vented outside.
• Leaks and wet areas: a plumbing leak, a damp basement or water entering around the foundation keep feeding moisture into the air.

Other common contributors are drying laundry on racks indoors, many houseplants in one room, aquariums, and a humidifier set too high. Bedrooms often show the worst window condensation in the morning: a closed door, a few people breathing all night and a cold window add up.

How to stop condensation on the inside of windows

Work through these steps in order. The first ones cost little or nothing and solve most cases.

1. Measure humidity. A digital hygrometer tells you whether the air is actually humid. The EPA notes that these meters are small and inexpensive, and recommends keeping indoor humidity below 60%, ideally between 30% and 50%. The CDC’s advice for mold prevention is to keep it no higher than 50%.
2. Remove moisture at the source. Run bath and kitchen exhaust fans while showering and cooking, and for a while afterward. Make sure the dryer vent goes outdoors and is not blocked.
3. Let air reach the glass. Open blinds and curtains during the day, and leave a gap at night. Heavy curtains trap a pocket of cold air against the pane. Don’t block heating registers under windows with furniture.
4. Move air. The EPA suggests increasing ventilation or air movement by opening doors and windows when practical and using fans as needed. Keeping interior doors open helps drier air mix into bedrooms.
5. Keep rooms evenly warm. The EPA also lists raising the air temperature as a way to prevent condensation. A very cold spare room with its door closed will collect moisture from the rest of the house.
6. Dehumidify if the numbers stay high. If humidity stays above your target after these steps, the EPA recommends air conditioners and dehumidifiers when needed. In winter, a dehumidifier in the dampest room is usually the practical option.
7. Turn down the humidifier. If you use one, lower its setting in cold weather. Condensation on windows is one of the easiest signs that it is set too high.

When the window itself is part of the problem

Older single-pane windows and double-pane units with aluminum spacers have cold inner surfaces, so they sweat at lower humidity than modern units. The Building America Solution Center points out that traditional aluminum edge spacers are the weak thermal link in most double-glazing units; “warm-edge” spacers with a thermal break reduce heat loss at the edge of the glass, which is exactly where condensation starts.

If you are replacing windows anyway, two ratings help:

• Condensation Resistance (CR): an optional rating on the NFRC label, scored from 0 to 100. The higher the number, the better the window resists water build-up on its inner surface.
• U-factor: how much heat the window lets through. A lower U-factor means a warmer inner surface. The Building America Solution Center suggests considering triple-pane windows in cold climates, both for energy efficiency and to minimize condensation on window interiors.

If replacement is not on the cards, interior storm windows or panels are a lower-cost option. The same DOE resource notes that they reduce the potential for interior condensation on the window system, but warns that condensation can form between the storm panel and the original window if the storm panel is not airtight.

Mold, rot and when to worry

A light fog that clears by mid-morning is mostly a nuisance. Water that pools on the sill every day is different: wood frames and sills can rot and stain, and mold can grow on damp surfaces. The EPA advises drying wet or damp materials within 24–48 hours to prevent mold growth. Wipe the glass and the sill with a cloth or squeegee in the morning until you have brought the humidity down.

If you already see mold, the EPA says that in most cases you can clean an area smaller than about 10 square feet (roughly 3 ft by 3 ft) yourself; larger areas, or mold after significant water damage, call for more care and possibly a professional. For questions about mold and health, see the CDC’s mold pages and talk to your doctor; this guide does not give medical advice.