Condensation on the Inside of Windows: Causes and Fixes

Water droplets or frost on the room side of a window mean that the indoor air is carrying more moisture than it can hold once it cools against the glass. The cause is high indoor humidity, a window whose inner surface runs cold, or both at once. Which one dominates decides whether the answer is ventilation, humidity control or different windows.

Fog trapped between the two panes of a sealed glass unit is a separate issue and is not what this page covers.

Why the inside of the glass gets wet

Condensation forms when room air cools against a surface to its dew point, the temperature at which the air is saturated and the extra water vapor turns to liquid. Cornell Cooperative Extension names three factors that usually act together: high indoor humidity, air movement that carries the vapor to the window, and a cold glass surface.

The glass is cold because it insulates poorly. Cornell’s fact sheet puts a single-glazed window at roughly R-1 and a double-glazed window at roughly R-2, against R-11 to R-20 for a typical wall around it.

The moisture comes from ordinary living:

Breathing. North Dakota State University (NDSU) Extension notes that each person adds about 3 pints of water vapor a day just by breathing.
Showers, cooking and laundry add more on top of that, NDSU says.
The whole household. Montana State University Extension estimates that a family of four can add more than six gallons of moisture to the air each day.

When the glass is cold enough, the same moisture collects as frost or ice instead of water, Montana State notes.

Humidity problem or window problem?

If nearly every window fogs, the air in the house is too humid; if only some do, those windows are the cold spots. A 1982 University of Minnesota Agricultural Extension Service piece put it the same way: when all the windows fog or drip for long periods, reduce the humidity; when only a few do, those windows may need improving. Cornell treats light condensation on single-glazed windows in kitchens, baths and laundry rooms, while those rooms are in use, as a nuisance that is unlikely to cause damage. It treats moderate condensation on double-glazed windows away from wet rooms, or heavy condensation on single-glazed windows in wet rooms, as a real concern.

Patterns worth noting before anyone visits:

Nearly every window fogs on cold mornings. When windows of different ages and on different sides of the house all fog, the common factor is the air.
Only a few windows fog. Those windows have colder inner surfaces, or they sit in rooms that produce moisture.
It is worse behind closed curtains. Cornell explains that shades and drapes separate the glass from warm room air, which lets the glass cool further below the dew point.
Water collects on the inside face of a storm window. Cornell traces this to moist room air leaking through gaps in the main window and condensing on the colder outer layer.

What indoor humidity to aim for in winter

EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. In cold weather the practical ceiling is lower, because the colder it is outside, the colder the glass gets.

NDSU Extension recommends keeping winter humidity under 40 percent.
Montana State suggests about 30 to 45 percent during the heating season.
NDSU’s benchmark: with double-pane windows and 40 percent indoor humidity, condensation should begin to appear when the outdoor temperature drops below zero. With triple glazing it should not appear until outdoor temperatures approach 40 below zero.

A hygrometer, a small humidity meter that EPA notes is sold at many hardware stores, turns guesswork into a number.

Ways to lower humidity that the extension services list:

Turn off humidifiers in cold weather (NDSU, Cornell).
Run kitchen and bath fans that exhaust outdoors while cooking and bathing, and make sure they do not discharge into an attic or crawl space (NDSU, Montana State).
Vent the clothes dryer outdoors (Cornell, Montana State).
Cover a dirt crawl space floor with polyethylene sheeting (Montana State, Cornell).

One limit is worth knowing: NDSU notes that dehumidifiers are generally ineffective once indoor humidity is below 50 percent, which is common in winter. NDSU and Cornell both point to whole-house ventilation with an air-to-air heat exchanger, which removes moist air while recovering part of its heat.

What the window itself contributes

The warmer the room-side surface of the glass stays, the more humidity a window tolerates before it sweats. Layers of glazing, coatings, spacers and frames all affect that temperature.

Condensation Resistance rating. The National Fenestration Rating Council (NFRC) rates how well a window resists condensation on its interior surface, on a scale of 1 to 100. A higher number means condensation is less likely. NFRC recommends a significantly higher rating in cold climates than warmer regions need.
The rating is optional. NFRC notes that manufacturers can choose whether to include it, so it may not appear on the label.
Triple glazing. The Department of Energy’s Building America Solution Center suggests considering triple-pane windows in cold climates, both for efficiency and to minimize condensation on window interiors.
Spacers. The same guide describes the spacers in ENERGY STAR windows as non-metal insulating materials that also insulate the edges of the glass.
Storm windows. Montana State notes that adding a plastic or glass storm window over a single-pane window raises the surface temperature and reduces condensation, provided it is sealed at the edges.

Why persistent condensation matters

Condensation that forms and melts every day can do real damage. Cornell warns that daily melting and dripping onto the sill may cause paint and structural damage. It adds that window condensation of that degree can signal hidden condensation in the attic or inside the walls, and suggests checking behind furniture, inside closets and on other cold surfaces.

EPA’s guidance is to dry wet surfaces and reduce the moisture source. Materials dried within 24 to 48 hours usually will not grow mold. For a moldy patch under about 10 square feet, EPA says homeowners can usually handle the cleanup themselves. Above that size, or after heavy water damage, EPA refers readers to its remediation guidance for larger jobs, and says a hired contractor should have experience cleaning up mold.

On health, EPA notes that mold can cause allergic reactions in sensitive people and advises consulting a health professional before cleanup if there are health concerns. Anyone with symptoms they connect to mold or damp should talk to a physician; this page does not give medical advice.

When to call a professional, and what to ask

Call for help when humidity is in range but windows still stream, when humidity will not come down, or when there is damage:

Humidity is already in range, but some windows still stream on cold nights. Those windows are the cold surface. A window contractor or a home energy auditor can assess them.
Humidity will not come down, even with exhaust fans running. That is a ventilation question for a heating and cooling contractor.
There are soft sills, stained walls or mold over about 10 square feet. EPA’s thresholds apply, and the moisture source has to be found before cosmetic repairs.

Questions for a window contractor:

Which of my windows are single-pane or metal-framed, and which have worn weatherstripping?
What Condensation Resistance rating do the proposed windows carry, and if it is not on the label, will the manufacturer provide it?
Do the proposed units use insulating, non-metal spacers?
Would storm windows or new sashes solve the problem here, or has the frame itself failed?

Questions for a heating and cooling contractor:

Do the bath, kitchen and dryer exhausts actually discharge outdoors?
Is a humidifier on the furnace adding moisture in winter?
Would balanced ventilation with heat recovery suit this house?

Bring a week of hygrometer readings and a list of which windows fog and when. It turns a general complaint into a specific diagnosis.