Why is there condensation on my windows?
Condensation forms on the inside of your windows when warm, damp indoor air meets glass cold enough to wring the water out of it. The glass sits below the dew point, the temperature where air has to give its water back, which means indoor humidity is too high for that surface. EPA's target: below 60 percent, ideally 30 to 50. Fog trapped between the panes is different, a failed seal.
The window is a humidity meter you already own
On a cold morning, the inside face of your window glass is usually the coldest surface in the house, and the water beaded on it came out of your own indoor air, which the US Environmental Protection Agency (EPA) says should sit below 60 percent relative humidity, ideally between 30 and 50. Relative humidity is the amount of water in the air compared with the most it could hold at that temperature. The event on the glass has a name: the surface dropped below the dew point of the room's air, the temperature at which air can no longer carry all its water and starts giving some back as liquid.
Nothing leaked, and nothing came in through the frame. The water was already in the room, invisible, until the glass got cold enough to collect it.
Read that way, a fogged pane is useful. It is a humidity meter you already own, telling you the air is holding more water than that surface temperature can tolerate. EPA treats it exactly so: moisture collecting on windows and other cold surfaces is its named sign that indoor humidity is running too high.
This page stays on what the window is telling you and what to do about it.
Where does all that water in your air come from?
Breathing alone adds about 3 pints of water vapor per person to a house's air every day, by North Dakota State University Extension's estimate, before a single shower runs.
The rest of an ordinary day stacks on top of that baseline. EPA's list of what pushes indoor humidity up includes humidifiers, steam radiators, moisture-generating appliances such as clothes dryers, and combustion appliances such as stoves, with cooking and showering adding more. Its moisture-control guidance names the quieter contributors too: long showers, running water, boiling or steaming while cooking, houseplants, and drying clothes indoors.
Two of those are worth singling out. A humidifier can manufacture this problem outright. EPA's wording is plain: your humidistat, the dial that tells a humidifier what humidity to hold, is set too high if excessive moisture collects on windows and other cold surfaces. And any fuel-burning heater that is not vented outdoors, a kerosene heater being EPA's example, releases water vapor into the room as it burns, which is why EPA says to vent moisture-producing appliances, dryers and stoves included, to the outside.
None of these sources is a defect. They are life happening inside a sealed box, and in winter the coldest visible wall of that box is the glass, so the glass is where the evidence shows up first.
Inside the glass, between the panes, or on the outside?
Where the water sits on a fogged window decides whether you have a humidity problem, a dead window, or nothing at all.
Fog sealed between the two panes of a double-pane window is the case that fools people: ENERGY STAR, the federal efficiency program run by EPA and the Department of Energy, lists moisture between the panes as a sign the window's insulating seal has failed and the unit is a candidate for replacement. No wiping reaches it and no dehumidifier touches it, because the water is trapped in the gap that existed to slow heat flow. A failed seal is a window-replacement decision, and no change to your indoor air reaches it. The window still keeps rain out, it has just stopped insulating as designed.
Water on the room-side face, the kind a finger smears, is the indoor-humidity signal the rest of this page is about. North Dakota State University Extension is blunt about it on modern glass: condensation on double- or triple-pane windows most likely means too much humidity in the house.
Dew on the outside of the window on a warm morning is the opposite event and means nothing is wrong; ENERGY STAR notes that on a hot, humid day the cool air inside your house can pull the exterior surface below the outdoor dew point. That is weather, and it burns off.
The same physics explains why one house fogs while the neighbor's does not. Anything that keeps the interior glass warmer suppresses condensation: EPA names storm windows and caulking, and notes an interior storm window works better than an exterior one. The Department of Energy's window guide adds warm-edge spacers, the strips that hold the panes the correct distance apart, as a feature that cuts heat flow and helps prevent condensation.
Warmer glass is resistance, not immunity. ENERGY STAR says even its certified windows can fog in cold weather.
Mold on the sill: why ventilation comes before a dehumidifier
EPA's instruction for moisture collecting on windows is unusually direct: act quickly to dry the wet surface and reduce the moisture source.
The urgency is about mold. Mold begins growing indoors when spores land on surfaces that are wet, and a sill that gets re-wetted every cold night is a surface that never fully dries. ENERGY STAR describes the end state on an inefficient window: water condenses on the interior surface, pools on the sill, and over time chronic condensation damages sills, cracks paint, and encourages mold growth. EPA's mold guide carries the same failure as a photo caption, a leaky window whose wooden frame and windowsill are beginning to rot.
So the first move is a towel, this morning. The second is taking moisture out of the room, which costs nothing. Run the bathroom fan or open a window when showering, and use exhaust fans or open windows when cooking or running the dishwasher, which is EPA's baseline advice. Vent the clothes dryer outside. Turn a humidifier down and watch whether the glass clears over the next few cold nights.
A dehumidifier is the last rung, not the first. It works: if venting and source control have not cleared the glass, a dehumidifier will pull the room back under EPA's 60 percent line. But it is a running appliance removing water you could often have kept out of the air for free, which is why it sits second. Our humidity-control method page covers when the machine earns its place and how to size it.
Where do I start?
- Wipe the glass, sill, and frame dry today. A surface that gets wet every night and never fully dries is where mold starts.
- Check where the water actually sits. If fog is trapped between the panes and neither side wipes clean, the seal has failed and no humidity fix applies.
- Run the bathroom fan or open a window when you shower, and use the exhaust fan whenever you cook or run the dishwasher.
- If you run a humidifier, turn it down. Moisture collecting on the windows means it is set higher than the coldest glass in the room can tolerate.
- Confirm the clothes dryer vents outside, and skip drying laundry on indoor racks during cold weather.
What actually works?
Common questions
Should I run a dehumidifier for window condensation?
Yes, if free fixes have not cleared the glass first. Vent the shower and the stove, get the dryer exhausting outside, turn any humidifier down, and give it a few cold nights. If windows still fog, a dehumidifier is the right tool and will hold the room under EPA's 60 percent humidity line. Source control comes first because the machine spends electricity removing water you could often have kept out of the air. Our humidity-control page covers sizing.
Is condensation between window panes bad?
It means the window's insulating seal has failed. ENERGY STAR lists moisture or cloudiness between the panes of a double-pane window among the signs it is time to replace the unit. Nothing about your indoor air causes it or cures it: the fog sits inside the sealed gap, so no dehumidifier, fan, or humidity change will clear it. The window still keeps weather out, it has just stopped doing the insulating job it was built for, so replacing it is the fix.
Can condensation on windows cause mold?
Yes, on the sill and frame rather than the glass itself. Mold starts when spores land on surfaces that stay wet, and a sill re-wetted every cold night qualifies. EPA's advice for moisture collecting on windows is to act quickly: dry the surface and cut the moisture source. Chronic condensation can pool on the sill, crack paint, rot wood, and let mold take hold, which is why the towel matters as much as the humidity fix does.
What humidity should my house be in winter?
EPA's band applies year round: below 60 percent relative humidity, ideally 30 to 50. Cold-climate extension services run lower in winter because colder glass fogs at lower indoor humidity. North Dakota State University Extension suggests around 40 percent, and the University of Minnesota Extension's comfort band starts at 25 percent in winter. In a mild climate, 30 to 50 is usually enough to keep windows clear.
Sources cited on this page
Every number on this guide page traces to one of the 10 sources below: 8 government or regulatory.
Government & regulatory
- US EPA, What are the main ways to control moisture in your home? (moisture condenses on window glass because the glass is cold; humidistat set too high if excessive moisture collects on windows; moisture-source list; storm windows and caulking keep the interior glass warmer)
- US EPA, A Brief Guide to Mold, Moisture and Your Home, web version (keep indoor humidity below 60 percent, ideally between 30 and 50; vent dryers, stoves and kerosene heaters to the outside; run the bathroom fan or open the window when showering)
- US EPA, A Brief Guide to Mold, Moisture, and Your Home, EPA 402-K-02-003, 2012 printing (act quickly to dry moisture collecting on windows, p.11; mold begins when spores land on wet surfaces, p.2; leaky-window frame and sill rot, photo caption p.4)
- US EPA, Mold Course Chapter 2: Why and Where Mold Grows (condensation forms when warm, humid air contacts a cold surface; indoor humidity sources include humidifiers, steam radiators, dryers, stoves, cooking and showering)
- ENERGY STAR, Residential Windows, Doors & Skylights (water condenses on interior window surfaces when the surface temperature is below the dew point of indoor air; certified windows are more resistant to condensation but not immune; exterior condensation in warm weather)
- ENERGY STAR, Qualified Windows Protect the Home and Increase Comfort, 2006 PDF (condensation on an inefficient window pools on the sill; chronic condensation damages sills, cracks paint, and encourages mold growth)
- ENERGY STAR, Ask the Experts: Is it Time to Replace Your Windows? (moisture or cloudiness between the panes of a double-pane window signals a failed seal and is a replacement trigger)
- US Department of Energy, Guide to Energy-Efficient Windows (spacers that keep panes the correct distance apart reduce heat flow and help prevent condensation)
Institutional & standards
- North Dakota State University Extension, Window Condensation Common Problem, Jan 2011 (condensation on double- or triple-pane windows most likely means too much indoor humidity; about 3 pints of water vapor per person per day from breathing; around 40 percent winter humidity as cold-climate guidance)
- University of Minnesota Extension, Do you have too much moisture in your home? (winter window condensation as a symptom of excess moisture; 25 percent winter to 50 percent summer comfort band, cold-climate guidance)
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