Window Condensation: Causes and How to Stop It

Water beading on the inside of your glass on a cold morning is window condensation, and in almost every case it reports on the air in the room rather than a defect in the window. Glass is the coldest surface in a heated house, so indoor moisture lands there first. A pane fogging on the room side has not failed. It is telling you the indoor humidity is higher than that glass can carry at today’s outdoor temperature.

This post covers the three places condensation appears and how to tell them apart, what dew point means in plain terms, how much moisture a household produces daily, the exhaust airflow the code requires, and why plastic film and heavy curtains make things worse. The humidity figures come from the EPA and the University of Minnesota Extension Service, and the exhaust rates from the International Residential Code and ASHRAE 62.2.

Key takeaways

The EPA puts indoor relative humidity below 60 percent and ideally 30 to 50 percent, so fog on the room side means the air is wetter than that window can carry.
Condensation sealed between the panes is the only one of the three cases that is genuinely the window: the unit has lost its hermetic seal.
Condensation on the outdoor face is dew, and manufacturer guidance treats it as a sign the glazing insulates well.
The residential code asks for 50 cfm intermittent or 20 cfm continuous exhaust in a bathroom and 100 cfm or 25 cfm in a kitchen, discharged outdoors.
EPA guidance says wet material dried within 24 to 48 hours will usually not grow mould, which is the clock on a sill that stays wet.

The three places condensation shows up

Before anything else, establish which surface is wet. Touch it. If the water wipes off from inside the room, it came out of your indoor air. If the glass wipes clean and a haze stays behind, the water is sealed inside the unit. If the wet face is the outdoor one, that is dew.

The EPA’s moisture control guidance states the mechanism in one line: “If the dew point is above the temperature of the surface, water vapor will condense onto that cold surface.” It adds that “indoor window surfaces are often cooler than surrounding walls and are typically the first sites of condensation during cold weather.” Wet glass is an early warning about the room, and treating it as a product failure sends you to a showroom to solve a ventilation problem.

Where the water is	What it means	Is the window at fault
Room side of the inner pane	Indoor humidity is above the dew point of the glass surface	No, the air is too wet for that glass temperature
Sealed between the panes	The insulated glass unit has lost its hermetic seal	Yes, the one case that is the window
Outdoor face of the outer pane	Dew, because the outer glass radiated heat away and fell below the outdoor dew point	No, it usually indicates the unit insulates well
Frame, sill and the wall below stay wet	Sustained condensation, or liquid water entering from outside	Possibly, this needs a look at flashing and framing

Only the second row sends you to a glazier. The fourth is the one to take seriously, because it stops being about glass and starts being about the wall.

Dew point, without the physics lecture

The National Weather Service defines dew point as “the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%.” In household terms, your air carries a fixed amount of water, and there is a temperature at which it can no longer hold it. Colder surfaces get wet.

Relative humidity alone will not tell you whether your windows will fog, because it moves with air temperature and dew point does not. As the EPA puts it, “unlike RH, the dew point does not change with air temperature.” So 45 percent at 70°F is a different moisture load from 45 percent at 62°F.

This is why a humidity ceiling that works in October fails in January. The University of Minnesota Extension Service published a stepped table for it, and manufacturer condensation literature still reproduces the same steps. The dew point column below is calculated from the standard psychrometric relation at an indoor temperature of 70°F, so you can see what surface temperature each ceiling protects.

Outdoor temperature	Maximum indoor relative humidity	Indoor dew point at 70°F
20°F to 40°F	40 percent	45°F
10°F to 20°F	35 percent	41°F
0°F to 10°F	30 percent	37°F
minus 10°F to 0°F	25 percent	33°F
minus 20°F to minus 10°F	20 percent	27°F
Below minus 20°F	15 percent	20°F

Read it as a ceiling, not a target. North Dakota State University Extension puts the glazing effect concretely: held at 40 percent indoor humidity, a double pane window starts condensing below roughly 0°F outside, while triple glazing holds out to around minus 40°F. Better glass buys surface temperature margin, and removes no water from the house.

Where the moisture is coming from

Household moisture stops being mysterious once it is weighed. The University of Minnesota Extension Service fact sheet lists a family of four producing 8 to 12 pounds of water vapour per 24 hours by breathing alone, a gas range adding about 5 pounds a day, one shower about half a pound, and clothes drying roughly one pound of water per pound of dry laundry.

North Dakota State University Extension gives the same activities in pints. A ten minute shower releases about a pint. A family of four breathing out adds about 0.4 pint an hour. An unvented combustion heater releases roughly 7.5 pints per gallon of fuel burned. Materials releasing stored summer moisture in autumn contribute 6 to 17 pints a day, and a damp basement can give up as much as 100 pints a day.

That last figure is the one people miss. If windows fog upstairs and the crawl space has bare soil, the basement is very likely the source, and no bathroom fan upstairs will settle it. New and recently renovated houses are their own case: fresh concrete, plaster and lumber release water for months.

Work through your own windows in this order

Diagnosis is a sequence, and skipping to the end is how people buy glass they did not need. Stop at the first step that explains what you are seeing.

Wipe the room side with a dry cloth. If it comes clean, the moisture is indoor air. If a haze remains that you cannot reach, skip to the section on seal failure.
Check the outdoor face. Wet outside in spring or autumn after a clear, still night is dew. Nothing to do.
Put a hygrometer in the room with the worst window, read it after a few hours, and compare it against the ceiling in the table above for today’s outdoor temperature.
Walk the house for uncontrolled sources: a fan nobody runs, a tumble dryer venting indoors, an unvented gas or kerosene heater, exposed soil in a crawl space, a humidifier left on the heating system.
Check that every exhaust fan moves air and terminates outdoors. Hold a sheet of tissue to the grille. If it does not hold, the fan is not working whatever its label claims.
Look at what sits in front of the glass. Lined curtains, blinds closed tight to the frame and a blocked supply register all lower the glass temperature.
If humidity is inside its ceiling, every fan works, nothing vents indoors, and one window is still wet while the others are dry, the surface temperature at that opening is the problem.

Record the hygrometer reading and the outdoor temperature each time. Condensation is weather dependent, so one wet morning proves very little and two weeks of paired readings proves a great deal.

Bathroom and kitchen exhaust: the airflow the code expects

Showers and cooking release moisture faster than the rest of the house can absorb it, which is why both rooms get a dedicated fan. Table M1507.3 of the International Residential Code, as adopted in New Jersey, sets the minimum local exhaust rates: kitchens at 100 cfm intermittent or 25 cfm continuous, and bathrooms at 50 cfm intermittent or 20 cfm continuous.

ASHRAE 62.2 agrees. The 2013 edition calls for a tested installed airflow of 20 cfm continuous or 50 cfm intermittent in bathrooms, and 100 cfm intermittent in kitchens or a continuous measured flow of 5 air changes per hour. Where the air goes matters as much as how much moves. Section M1501.1 is blunt: “The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.”

Section M1506.2 adds that bathroom exhaust “shall not be recirculated within a residence” and must go directly outdoors. A fan ducted into an attic has not removed your moisture, it has moved it somewhere colder where it condenses out of sight. Run the bathroom fan during the shower and for 15 to 20 minutes afterwards, because tiles and towels keep evaporating after the water stops. Use the range hood every time you simmer, and prefer one that vents outdoors over a recirculating model.

Fog between the panes is the window’s own failure

Sealed glass units are two or more panes held apart by a spacer, with a desiccant inside the spacer to absorb the small amount of moisture present at assembly. A 2007 conference paper hosted by Oak Ridge National Laboratory defines the failure precisely: “a seal failure is said to exist when the sealants used in the manufacture of the IGU do not maintain a hermetic seal that prevents moisture in any quantity from entering the interior cavity.” Once the desiccant saturates, moisture condenses in the cavity and the unit fogs.

You can identify this in seconds. The haze comes and goes with sun and temperature rather than with your shower schedule, it is unreachable from both sides, and over time it leaves a permanent mineral tidemark on the inner glass. Lowering indoor humidity does nothing, because the water is sealed in and no longer part of your indoor air.

The repair is a new insulated glass unit in the existing frame, not necessarily a new window. Check warranty terms before any quote, because glass is frequently covered on different terms from frames. This post quotes no cost range, because no dated primary source range was found. When shopping, the NFRC label carries a Condensation Resistance figure, which ENERGY STAR describes as “scored on a scale from 0 to 100” where “the higher the condensation resistance factor, the less build-up the window allows.”

Plastic film, heavy drapes and the cold pocket at the glass

Interior shrink film and thick curtains both work by trapping still air. That reduces heat loss, and it also drops the temperature of the glass behind them, which is why condensation often gets worse after they go up. Graham Windows states the mechanism in its technical bulletin: “Window blinds or drapes will increase the possibility of condensation as they restrict the flow of warmer room air to the surface of the window, therefore creating a cool pocket of air immediately in front of the window.”

The same bulletin describes two glazing layers in series, such as an interior storm panel or taped film over an existing window. Warm moist room air migrates into the space between the layers and condenses on the colder outer surface inside that gap. The film created no moisture. It moved the condensation to a surface you can no longer wipe, often onto painted wood sash.

That does not make storm windows a mistake. The Department of Energy reports that low emissivity storm windows can save 10 to 30 percent on heating and cooling costs and cut overall home air leakage by 10 percent or more, at about one third the cost of full replacement. Adding layers raises the stakes on humidity control rather than replacing it.

The adjustments are cheap. Open blinds in cold weather or mount them further off the glass, and leave a gap at the top and bottom of lined curtains. Keep supply registers under windows clear, because that airflow exists to warm the coldest surface in the room. Running the system fan on circulate evens out temperature between rooms, and that only works if air can move, which is why how often to change a furnace filter matters for wet windows as well as running costs.

Getting the humidity down and keeping it there

Source control first, ventilation second, dehumidification third. In that order you remove water before it spreads, dilute what is left, then treat air that has already picked it up. Reversing the order is how people run a dehumidifier all winter against a crawl space they never covered.

Start with the unvented sources. Vent the tumble dryer outdoors. Cover exposed soil in crawl spaces with sealed polyethylene. Stop using unvented combustion heaters indoors, both for the 7.5 pints of water per gallon of fuel and for the combustion products. Turn off any humidifier on the heating system until the windows stay dry. Then get the fans right, using the code figures above as your baseline.

Air sealing and insulation belong in the same conversation, because they raise interior surface temperatures and give you headroom before anything condenses. If cost is the obstacle, the Department of Energy funds the Weatherization Assistance Program through state agencies for income eligible households, and moisture and ventilation checks are part of the standard health and safety scope. If you are already weighing equipment changes, the comparison of heat pump versus furnace matters here too, because how evenly a system moves air changes the surface temperature at the glass as much as the setpoint does.

Whatever you change, measure it. A cheap hygrometer in the worst room, read alongside the outdoor temperature, turns this from an argument into a reading. Wet glass at 30°F outside with 50 percent indoor humidity has an obvious cause. The same glass at 15 percent does not, and that is when the opening itself becomes the question.

When to stop and call a professional

Persistent wet framing, black growth spreading beyond the sill, or a musty smell with no visible source is a building envelope problem, not a wipe-it-down problem. At that point water is either entering the wall from outside or condensing inside it, and neither is fixed by a fan. Get a contractor or building inspector to open up and look.

The clock matters. EPA guidance is that “if wet or damp materials or areas are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow.” A sill wiped every morning and wet again by evening has been past that window the whole time it has been happening. Softness in the sill, flaking paint on the lower sash and staining on the wall underneath all say the same thing.

Call someone straight away for any of these: an unvented gas, kerosene or propane heater used as a regular heat source, which is a carbon monoxide risk as well as a moisture one; exhaust ducting that terminates in an attic or crawl space; wet sheathing or framing around an opening; visible mould over about ten square feet, which is where EPA guidance moves past homeowner cleanup; and any outlet or switch below a window that has been getting wet.

Frequently asked questions

Does condensation on the inside of the windows mean they need replacing?

Almost never. Water on the room side is indoor moisture reaching a surface colder than the dew point of the air, which is a humidity condition rather than a product defect. New windows raise the glass temperature and hide the symptom, but the same moisture keeps loading the air and finds the next cold surface, usually a wall corner or a closet on an outside wall.

What indoor humidity is right for winter?

The EPA asks for below 60 percent and ideally 30 to 50 percent. In cold weather you often have to sit at the low end, because colder outdoor air means colder glass. Extension and manufacturer guidance step the ceiling down: about 40 percent at 20°F to 40°F outside, about 30 percent near 0°F, and 15 to 20 percent in deep cold.

Is fog between the panes fixable without replacing the glass?

Services exist that drill the unit, dry the cavity and fit a vent, and a DOE conference paper describes one such pressure equalization approach. This post quotes no price or success rate for that work, because no dated primary source was found. The conventional repair is a new glass unit in the existing frame. Check the glass warranty first.

Why is only one window in the house wet?

That window is colder than the others, or the air in front of it is wetter, or both. Single glazing, a metal frame without a thermal break, a north face with no sun, a closed curtain and a blocked supply register all lower the surface temperature. Bedrooms also run wetter overnight, because sleeping adults add vapour to a closed room.

Will a dehumidifier stop window condensation?

It helps, and in a basement it may be the main tool, but it works on air you already have rather than on the source. Exhausting the bathroom and kitchen while moisture is being made removes that water first. Run it to a measured setpoint inside the 30 to 50 percent band rather than a vague comfort dial.

The glass is a thermometer, not the patient. It fogs because it is the coldest surface you own and the air touching it carries more water than that temperature allows. Fix the second half of that sentence, with fans that run and vent outdoors, sources cut off at the origin, and a hygrometer to tell you whether it worked.

Window Condensation Means Humidity, Not Failed Windows