Why Do My Windows Have Condensation on the Inside?
Wet windows on a cold morning are a humidity gauge you did not ask for. Here is the physics, the humidity your windows can handle at each outdoor temperature, and what actually works.
Quick answer
Condensation forms when the inside of the glass is colder than the dew point of your indoor air. In winter that usually means humidity is too high for the outdoor temperature. Bring indoor humidity down (run bathroom and kitchen fans, lower or switch off humidifiers, use a dehumidifier) and let warm room air reach the glass by opening curtains and blinds.
- Difficulty
- Easy DIY
- Cause
- Glass colder than the air’s dew point
- Double pane, 20°F outside
- Keep humidity below about 37%
- Fastest fixes
- Exhaust fans, lower humidifier, open curtains
- Between the panes
- Failed seal; the glass unit needs replacing
On this page
Warm air holds more water vapor than cold air. When room air touches a surface colder than its dew point, it cannot hold all that vapor and some condenses into droplets. In winter, the inside surface of window glass is usually the coldest surface in the room, so that is where the water appears first, starting at the bottom edges where the glass is coldest.
Two things decide whether your windows sweat: how much moisture is in the air, and how cold the glass gets. You can change both.
How much humidity your windows can handle
The colder it is outside, the colder the glass and the lower the indoor humidity has to be. These limits were calculated for a room at 70°F (21°C), using the coldest part of the glass near the edges.
| Outdoor temperature | Single pane | Double pane, clear | Double pane, low-E with argon | Triple pane |
|---|---|---|---|---|
| 40°F (4°C) | 45% | 56% | 71% | 83% |
| 30°F (-1°C) | 33% | 46% | 63% | 78% |
| 20°F (-7°C) | 25% | 37% | 56% | 74% |
| 10°F (-12°C) | 18% | 30% | 49% | 69% |
| 0°F (-18°C) | 13% | 24% | 44% | 65% |
| -10°F (-23°C) | 9% | 19% | 38% | 61% |
| -20°F (-29°C) | 7% | 15% | 34% | 57% |
Our window condensation calculator works out the limit for your own temperatures and glass, in °F or °C. Heavy curtains and aluminum frames make glass colder than this model assumes, so treat the numbers as an upper bound.
Where the moisture comes from
A family adds a surprising amount of water to indoor air every day. The biggest sources:
- Showers and baths
- Cooking, especially boiling water without a lid or range hood
- Drying laundry indoors
- Humidifiers set too high, including furnace-mounted whole-house humidifiers
- Breathing and sweating, which is why bedroom windows fog overnight
- Unvented gas heaters, aquariums and lots of houseplants
- Damp basements and crawl spaces, and new construction that is still drying out
How to stop it
- Measure first.
A digital hygrometer costs about $10 to $20 and tells you whether humidity is actually high. Compare it with the table above.
- Use exhaust fans.
Run the bathroom fan during showers and for 20 minutes or so afterward, and the range hood while cooking. Make sure they vent outside, not into the attic.
- Turn humidifiers down or off.
In cold snaps, a whole-house humidifier set to 40 percent can be far above what your windows can handle. Lower it as the temperature drops.
- Let warm air reach the glass.
Open curtains and blinds during the day and leave a gap at night. A ceiling fan on low helps move air across windows.
- Run a dehumidifier
in rooms or basements that stay damp. Set it to around 45 to 50 percent.
- Warm the glass.
Interior window insulation film or storm windows add a layer that keeps the inside pane warmer. For a longer-term fix, low-E double or triple glazing raises the humidity your windows can tolerate.
Condensation between the panes
Fog or droplets trapped between the two panes of a double-glazed window are a different problem. The seal around the edge has failed and humid air has leaked into the gap. Lowering indoor humidity will not help and the fog will not wipe off. The insulated glass unit, not necessarily the whole window, needs replacing; check whether the window is still under warranty.
When condensation becomes a problem
Light fogging that clears by midmorning is harmless. The EPA notes that ongoing moisture problems lead to mold growth, so take action if you see water running down the glass and pooling on sills every day, black spots on sills or around the frame, peeling paint, or soft, darkened wood. Dry the sills each morning while you bring humidity down.
Frequently asked questions
Is condensation on windows bad?
Occasional fog that clears by midmorning is harmless. Water that runs down the glass and pools on sills every day can rot wood frames, peel paint and feed mold on sills and surrounding walls.
Why do only some windows get condensation?
Rooms with more moisture, such as bedrooms with people sleeping, bathrooms and kitchens, fog first. So do windows behind heavy curtains, which keep room air from warming the glass, and older or single-pane windows.
Will new windows stop condensation?
Better windows have warmer inside glass, so they tolerate more humidity before fogging. They help a lot, but if indoor humidity is very high, even good windows will sweat. Fix the moisture source first.
Why is there condensation on the outside of my windows?
Exterior condensation on cool, clear mornings means the outer pane is colder than the outdoor dew point, a sign that efficient glass is keeping heat inside. It is not a problem and clears as the day warms.
Sources
- A Brief Guide to Mold, Moisture and Your Home U.S. Environmental Protection Agency
- Mold Course Chapter 2: Moisture and condensation U.S. Environmental Protection Agency
- Understanding Window Energy Performance Ratings National Fenestration Rating Council
- Moisture Control U.S. Department of Energy, Energy Saver (archived copy, 2025)
First published October 6, 2026. Last reviewed October 6, 2026. Spot an error? Tell us and we will check it against the source.