Condensation on Windows Inside: Why It Happens and How to Stop It
Foggy, dripping windows on winter mornings are a humidity warning. Here's how to bring moisture down, keep sills mold-free, and tell when a window seal has failed.
Condensation on the inside of windows means the air indoors is holding more moisture than the cold glass can handle. When warm, humid air touches a surface below its dew point, the water vapor turns to droplets. The fix is to lower indoor humidity (ENERGY STAR suggests 30% to 40% in winter in colder climates), ventilate during showers and cooking, and wipe the glass and sills dry before mold starts. Fog between the panes of a double-glazed window is different. It means the window’s seal has failed.
Why it gets worse in winter
Glass is often the coldest surface in a room. When it’s freezing outside, the inner pane of a window can be cold enough that even normal indoor humidity condenses on it. Everyday activities add moisture to the air:
- Showers and baths
- Cooking, especially boiling pots without a lid or range hood
- Drying laundry indoors
- Breathing while you sleep. Bedrooms with closed doors often fog up overnight.
- Humidifiers set too high
- Lots of houseplants or an aquarium
- Unvented gas heaters
Newer, tighter homes trap more of that moisture. That’s why a house can suddenly start fogging after new insulation or windows go in.
Where is the condensation?
| Location | What it means | What to do |
|---|---|---|
| Inside surface of the glass, facing the room | Indoor humidity is high for the outdoor temperature | Lower humidity, ventilate, increase air movement |
| Between panes of double or triple glazing | The insulating seal has failed | Can’t be wiped. Contact the window maker about warranty, or have the glass unit replaced. |
| Outside surface | Cool morning glass meeting humid outdoor air | Normal, and often a sign of efficient glass. Nothing to fix. |
How to reduce window condensation
- Measure your humidity. An inexpensive digital hygrometer gives you a real number. The EPA recommends keeping indoor humidity below 60%, ideally 30% to 50%. ENERGY STAR suggests aiming for 30% to 40% during the heating season in colder climates to prevent condensation on windows.
- Turn down or turn off the humidifier if you’re above those numbers. The EPA says to lower its setting or use it less if water condenses on windows. See how to clean a humidifier.
- Run exhaust fans in the bathroom during and after showers, and use the range hood when cooking.
- Dry laundry outside or in a vented dryer when you can.
- Let warm air reach the glass. Open heavy curtains and blinds during the day. Keep furniture off radiators and vents under windows.
- Air out the house briefly. Opening a couple of windows for a few minutes swaps moist indoor air for drier winter air.
- Use a dehumidifier in rooms that stay damp, such as basements. Our guide explains how dehumidifiers help with mold.
Cleaning mold on sills and frames
Water that runs down the glass every morning collects on sills, in tracks and around rubber seals. That’s where black spots appear.
What you need
- Dish detergent and warm water
- Microfiber cloths, a soft brush and cotton swabs for corners
- Gloves, and a mask if there’s more than a few spots
- A dry towel
Steps
- Wipe up standing water first.
- Scrub the sill, frame and tracks with detergent and water. Use the brush in the corners.
- Rinse with a damp cloth and dry everything thoroughly.
- For stains on vinyl or aluminum frames, check the window maker’s care instructions before using bleach or abrasive cleaners. Some finishes and seals can be damaged.
- Painted wood sills: wipe gently and dry. If the wood is soft or black deep in the grain, see how to clean mold off wood.
If you use a bleach-based mildew remover, follow the label, open the window, wear gloves and keep it off fabric curtains. Never mix bleach with vinegar, ammonia-based glass cleaners or any other product. Rinse with water before switching products.
If someone swallows, inhales or gets a cleaning product in the eyes, call Poison Control at 1-800-222-1222 (US) or emergency services.
When condensation signals a bigger problem
- Fog between the panes: A failed seal means the insulating gas has leaked out and the window insulates less well. Check the warranty, since many manufacturers cover seal failure for a set number of years.
- Water damage below the window: Peeling paint, soft drywall or stained floors under the sill mean water has been running down for a while.
- Mold on walls, not just windows: Humidity is high throughout the home. See how to prevent mold in winter.
- Condensation everywhere, even at modest humidity: Very old single-pane windows or poorly insulated frames may need storm windows or replacement.
Find the EPA’s moisture tips in its Brief Guide to Mold, Moisture and Your Home.
Bottom line
Inside condensation is a humidity signal, not a window defect. Measure your humidity, aim lower in winter, ventilate showers and cooking, and dry sills daily during cold snaps. Clean mold spots early with detergent and water. Fog between the panes is the exception. That’s a failed seal, and only repair or replacement fixes it.
FAQ
Is a little morning condensation normal?
A thin band at the bottom of the glass on very cold mornings is common. Heavy droplets that run onto the sill every day mean humidity is too high.
Will a dehumidifier stop window condensation?
It can help in a damp room or basement. Ventilation and controlling moisture sources matter just as much.
Can I fix fog between double-pane windows myself?
Not really. The sealed unit has failed. Window makers or glass shops can usually replace just the glass unit instead of the whole window.
Does plastic window film help?
Interior insulating film adds an air layer that keeps the inside surface warmer, which can reduce condensation on old windows. Follow the kit’s instructions, and keep humidity in check.
