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Condensation and mold on windows in winter
Updated 2026-11-03. Advice comes from U.S. EPA, university extension and energy-efficiency sources (read on 2026-11-03); items marked Wrenly suggestion are our own advice as a cleaning service. Peoria temperatures are NOAA 1991-2020 normals. Prices come from our live price list. Disclosure: Wrenly Home sells house cleaning in the Peoria, IL area. We don't do mold remediation.
Short answer: water on the inside of the glass in winter means the indoor air is too humid for how cold the window is[4][1]. Lower the humidity (EPA: ideally 30 to 50 percent[1]; in cold weather keep to the low end, under 35 percent when it's 10 to 20°F outside[5][6]) with bathroom and kitchen fans vented outside and the humidifier turned down[4][7], and warm the glass with plastic film or storm windows[4][7]. Wipe the sills dry. If mold has started, scrub it off with detergent and water and dry completely; bleach isn't required[2][1].
Why windows sweat in winter
Warm air holds more water vapor than cold air. When humid indoor air touches a surface colder than its dew point, the vapor condenses into water, frost or ice on that surface[7], and in cold weather that happens on window glass and frames, wherever humid indoor air meets a cool surface[6]; a single-pane window is the classic spot[7]. NDSU Extension calls heavy condensation or frost on windows a sign of too much indoor moisture or a cold window[4], and EPA lists condensation on the inside of a windowpane as a sign of high humidity[1].
It often starts right about now. Houses hold a lot of moisture from damp fall weather, so windows commonly steam up when the weather suddenly turns cold in late fall or early winter; University of Minnesota Extension says that is usually temporary and fixed by running ventilation fans several hours a day or opening windows slightly on warm days[6]. In Peoria the normal low falls from 33.0°F in November to 23.2°F in December and 17.6°F in January (NOAA 1991-2020 normals at the Peoria airport)[8], so the glass keeps getting colder through midwinter.
Left alone, the water does damage. Dripping condensation can peel paint, swell or split wood, rot window frames and sills, stain, and grow mold[6][4].
The right humidity for the outdoor temperature
EPA's general guidance is to keep indoor humidity below 60 percent, ideally between 30 and 50 percent, and a $10 to $50 hygrometer from a hardware store measures it[1]; EPA also says not to humidify above 50 percent[3]. In cold weather the windows set a lower limit: University of Minnesota Extension says to move to the low end of that range during very cold weather, and that window condensation tells you when[6]. Minnesota's Center for Energy and Environment gives this winter guide[5]:
| Outdoor temperature | Keep indoor humidity below |
|---|---|
| 20 to 40°F | 40% |
| 10 to 20°F | 35% |
| 0 to 10°F | 30% |
| -10 to 0°F | 25% |
| -20 to -10°F | 20% |
| -20°F or below | 15% |
On a normal Peoria January night (low 17.6°F[8]) that means under about 35 percent. Don't overcorrect: NDSU Extension says below 30 percent can dry skin and irritate nasal passages[4]. If your windows are single-pane, Missouri Extension says condensation is probably a common problem[7].
Where the moisture comes from
University of Missouri Extension estimates that a family of four can add more than 6 gallons of water to the air a day[7]:
| Source | Pounds per day |
|---|---|
| Breathing and perspiring | 16.8 |
| Clothes dryer vented into the house | 26.4 |
| Washing clothes | 4.3 |
| Mopping floors | 2.4 |
| Cooking on an electric range without an exhaust vent | 2.0 |
| Showers or baths | 2.0 |
| Washing dishes by hand | 1.0 |
| House plants | 1.0 |
A humidifier set too high, an unvented kerosene heater or gas stove, firewood stored indoors and a wet basement or crawl space add more[6][1].
How to stop it
- Run the fans. Use the bathroom fan during and after showers and the kitchen fan while cooking, and make sure they vent outside, not into the attic or crawl space[7][4][1].
- Turn the humidifier down or off if the windows are wet[4][3].
- Vent the dryer outside, avoid hanging wet laundry indoors in cold weather, and cover pots while boiling[7].
- Keep air moving across the glass. Don't block heat registers with furniture, use a ceiling fan, and leave curtains open in the day[7].
- Warm the glass. Plastic window film or storm windows on single-pane windows raise the glass temperature[4][7]; weatherstrip and caulk around the window[6].
- Don't count on a dehumidifier in winter. NDSU says most only bring humidity down to about 50 percent, not low enough to stop winter condensation[4].
- Wipe it every morning while you work on the cause: a microfiber cloth or a squeegee and a towel along the sill, so water doesn't sit in the corners and tracks. (Wrenly suggestion)
If only a few windows sweat while the rest of the house is dry, those windows are the weak spot rather than the humidity: look at their weatherstripping, storms or glazing[6].
Cleaning mold off window sills
Black, gray or green spots in the corners of the glass, on the sill or along the caulk are mold or mildew feeding on dust and water. EPA's steps for small areas (under about 10 square feet) that you can clean yourself[1][2]:
| Step | What to do | Source |
|---|---|---|
| Protect yourself | Wear gloves (ordinary household rubber gloves are fine with water and a mild detergent) and goggles; EPA says an N-95 respirator limits how much mold you breathe in. | EPA[1] |
| Scrub, then dry | Scrub mold off hard surfaces such as glass, vinyl and painted or sealed trim with detergent and water, then dry the surface completely. | EPA[2][1] |
| Bleach isn't required | EPA says a biocide such as chlorine bleach is not recommended as a routine part of mold cleanup; a background level of spores always remains and won't grow once the moisture problem is fixed. If you do use bleach, ventilate to the outdoors and never mix it with ammonia or cleaners that contain ammonia (toxic fumes). | EPA[1] |
| Don't paint or caulk over it | Clean and dry the mold first. Paint applied over moldy surfaces is likely to peel. | EPA[2] |
| Know when it's bigger than a sill | Porous materials (drywall returns, soft or rotted wood, curtains) may have to be replaced if they become moldy. If mold covers more than about 10 square feet (roughly 3 by 3 feet), or there was a lot of water damage, EPA points to its guide for larger jobs and professional help. | EPA[2][1] |
| Fix the moisture | Mold comes back if the sill keeps getting wet. Wipe condensation as soon as you see it and bring the indoor humidity down (below). | EPA[1] |
Vacuum the window track before you wipe it, so you're not turning dust into mud, and finish with a dry cloth into the corners. On a painted wood sill, use a soft cloth rather than an abrasive pad so you don't scrub through the paint. (Wrenly suggestion) EPA notes mold can leave a stain that cleaning won't fully remove[1].
Is window mold dangerous?
Molds come in many colors, and EPA shows both white and black molds[1]. EPA says mold can cause allergic reactions in sensitive people, such as sneezing, runny nose, red eyes and skin rash, can trigger asthma attacks in people with asthma, and can irritate the eyes, skin, nose, throat and lungs; dead mold can still cause allergic reactions, so it should be removed, not just killed[1]. Clean small spots promptly with protection, and talk to a doctor about any health concerns, especially for someone with asthma or a weakened immune system.
What a Wrenly clean covers
Every deep clean and move-out clean wipes window sills and vacuums window tracks, and dusts blinds; a move-out clean also cleans the inside of the glass (see the deep cleaning checklist and move-out checklist). We don't do mold remediation, replace caulk, repair sills or wash exterior windows. If something on the checklist is missed, tell us within 48 hours under our re-clean guarantee.
What it costs
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| Home | Deep clean | Then every 2 weeks (optional) |
|---|---|---|
| Studio / 1 bed (1 bath incl.) | $228 | $155 |
| 2 bed (2 baths incl.) | $272 | $189 |
| 3 bed (2 baths incl.) | $350 | $233 |
| 4 bed (3 baths incl.) | $441 | $306 |
| 5 bed (3 baths incl.) | $558 | $337 |
| Each extra bathroom | +$33 | +$20 |
Book online with your bedrooms, bathrooms and preferred day; we confirm the date by email within one business day.
Frequently asked questions
Why is there condensation on the inside of my windows in winter?
In winter the window glass is one of the coldest surfaces in a room. When warm, humid indoor air touches glass colder than its dew point, the water vapor condenses into droplets or frost. Extension sources call heavy or frequent window condensation a sign of too much indoor moisture, a cold window, or both.
Is condensation on the inside of windows normal?
A little, briefly, is common: University of Minnesota Extension says windows often steam up in late fall and early winter, when the house still holds moisture from damp fall weather and the weather suddenly turns cold, and that running ventilation fans a few hours a day usually fixes it. Condensation that runs down the glass or stays for hours, especially on many windows, means the humidity is too high for the outdoor temperature.
What indoor humidity should I keep in winter?
EPA's general advice is below 60 percent, ideally 30 to 50 percent. In cold weather, lower is better for the windows: Minnesota's Center for Energy and Environment suggests keeping it under 40 percent when it's 20 to 40°F outside, under 35 percent at 10 to 20°F and under 30 percent at 0 to 10°F. NDSU Extension says under 40 percent in winter, but not below 30 percent, which can dry skin and nasal passages.
How do I stop condensation on my windows in winter?
Lower the humidity and warm the glass. Run the bathroom fan during and after showers and the kitchen fan while cooking (vented outside, not into the attic), turn down or switch off humidifiers, never vent a dryer indoors, cover pots while boiling, keep air moving across the glass (don't block heat registers; a ceiling fan helps), and add plastic window film or storm windows on single-pane windows. A dehumidifier helps less in winter: NDSU says most only bring humidity down to about 50 percent.
Why do I get condensation on my bedroom windows in the morning?
Bedrooms collect moisture overnight: Missouri Extension lists breathing and perspiring as about 16.8 pounds of water a day for a family of four, and the door is usually closed. University of Minnesota Extension adds that a night-setback thermostat can leave the furnace off for hours, so the air cools and stops circulating and condensation is most likely in sleeping rooms; running the furnace fan several times an hour, or leaving the heat on at night, should solve it. It also says condensation that clears quickly once the house warms up in the morning, with no damage to the windows, may be a reasonable trade-off for the energy a setback saves. Wipe the sills dry and check the bedroom's humidity with a hygrometer.
How do I clean mold off a window sill?
Wear gloves and goggles, scrub the mold off with detergent and water, and dry the sill completely, as EPA advises for hard surfaces. Bleach isn't needed for routine cleanup; if you use it, ventilate and never mix it with ammonia. Don't paint or caulk over mold. Soft or rotted wood, or mold that keeps coming back, needs the moisture problem fixed and possibly the wood repaired.
Is black mold on window sills dangerous?
EPA says molds come in many colors, and the color doesn't tell you how harmful it is. Mold can cause allergic reactions such as sneezing, runny nose, red eyes and skin rash in sensitive people, and can irritate the eyes, skin, nose, throat and lungs. Clean small spots promptly, protect yourself while you do it, and talk to a doctor if anyone in the house has symptoms or a weakened immune system.
Does Wrenly Home clean window condensation or mold?
We wipe window sills and vacuum window tracks in every deep clean and move-out clean, and a move-out clean also does the inside of the glass. We don't do mold remediation, replace caulk or repair rotted sills. A few small mildew spots on a hard sill are the kind of thing you can clean yourself with the EPA steps above; for anything bigger, fix the moisture source and call a remediation professional.
Sources
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home (last updated Feb. 18, 2026). https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home. Accessed 2026-11-03.
- U.S. EPA, Mold Cleanup in Your Home. https://www.epa.gov/mold/mold-cleanup-your-home. Accessed 2026-11-03.
- U.S. EPA, Use and Care of Home Humidifiers. https://www.epa.gov/indoor-air-quality-iaq/use-and-care-home-humidifiers. Accessed 2026-11-03.
- North Dakota State University Extension: Eliminate Condensation on Windows. https://www.ndsu.edu/agriculture/extension/eliminate-condensation-windows. Accessed 2026-11-03.
- Center for Energy and Environment (Minneapolis nonprofit): Managing indoor humidity levels, all year round (Clara Higgins, July 9, 2026). https://www.mncee.org/managing-humidity-levels. Accessed 2026-11-03.
- University of Minnesota Extension Service: Humidity and Condensation Control in Cold Climate Housing, CD-FO-3567 (Roger A. Peterson and Lewis T. Hendricks, 1988). https://conservancy.umn.edu/server/api/core/bitstreams/a017b07e-cff8-40ec-8a5a-2221401ad9a4/content. Accessed 2026-11-03.
- University of Missouri Extension, E3A Home Energy series: Condensation (EM0112). https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pub/pdf/energymgmt/em0112.pdf. Accessed 2026-11-03.
- NOAA NCEI, U.S. Climate Normals 1991-2020, monthly normals for Peoria Greater Peoria Airport (station USW00014842). https://www.ncei.noaa.gov/access/services/data/v1?dataset=normals-monthly-1991-2020&stations=USW00014842&format=json&dataTypes=MLY-TAVG-NORMAL,MLY-TMIN-NORMAL,MLY-TMAX-NORMAL. Accessed 2026-11-03.
General home-care information drawn from the sources above, not medical, mold-remediation or contractor advice. If anything here is out of date, email [email protected] and we'll correct it.