Window Condensation: Inside, Outside or Between the Panes

Where the water sits tells you what condensation on a window means. Moisture on the inside of the glass comes from humid indoor air meeting a cold surface. Moisture on the outside is dew, and usually a sign the glass is insulating well. Fog trapped between the panes of a double- or triple-pane window is the one that points to a problem with the window itself: it usually means the seal around the glass unit has failed ³.

The first two are mostly about air and weather, and the fix is usually indoor humidity, not new windows. The third is a repair or replacement question. The rest of this article walks through each case and the numbers that matter.

Condensation on the inside of the glass

Condensation forms when the inside surface of the glass is colder than the room air and the air holds enough moisture to reach its dew point ¹⁵. Windows are typically the coolest part of an interior wall, Andersen notes, even with storm panels, low-E glass or triple-pane glass ¹. In cold climates, the moisture can freeze into frost ¹.

Andersen says interior condensation does not indicate a failure of the glass, and recommends that indoor humidity in winter months not exceed 30% to 35% ¹. The U.S. Environmental Protection Agency's general guidance is to keep indoor humidity below 60%, ideally between 30% and 50%, and it notes that condensation can be a sign of high humidity ⁴. The two figures answer different questions: EPA's is a whole-house target, while Andersen's lower winter figure is about keeping cold glass dry.

Ways to bring indoor humidity down, from Andersen's list ¹:

• Check ventilation, and run the bathroom exhaust fan when showering.
• Use a dehumidifier.
• Turn a furnace humidifier down or off.
• Open blinds and curtains during the day, and leave ceiling fans on to move air.

Persistent condensation is worth taking seriously. NFRC's consumer site notes that it can lead to mold and mildew growth inside the house ⁵, and EPA's guide on moisture covers what to do about mold ⁴.

Does condensation mean my house is too cold or my windows are bad?

Not necessarily. It usually means the air is humid for how cold it is outside. Better windows help: NFRC says one of the key benefits of efficient windows in cold climates is less condensation on the interior surface ⁵. DOE notes that insulating spacers between the panes help prevent condensation, and that poor installation can cause it even with efficient windows ⁹.

Condensation on the outside of the glass

Exterior condensation is most likely in spring and fall, or when a cool night follows a warm day, Andersen says ². It forms when the outer glass surface drops below the dew point of humid outdoor air, much like dew on grass or a car windshield ². Windows with low-E insulating glass keep more heat inside, so the outer pane can be a few degrees cooler than the inner one ². Andersen calls this an effective barrier between indoor and outdoor air; it usually evaporates during the day and does not affect the inside of the home ². Andersen lists an indoor temperature cooler than outside, as with air conditioning, among the conditions that come before it ².

No repair is needed. More on why low-E glass does this in low-E glass explained.

Condensation between the panes

Fog, droplets or a milky haze between the two layers of glass that you cannot wipe from either side usually means the seal of the insulating glass unit has failed, letting moist air in ³. Pella notes this is more common in older windows and often more visible on sunny sides of the house ³.

This one does not respond to humidity control. The options are:

• Replace the glass unit. If the frame and sash are sound, the sealed glass unit can often be replaced on its own. Fixr lists $150 to $600 for double-pane glass (its FAQ says the range includes labor; its price table labels the same range material only), and reports a national average of $375 for window glass replacement overall, in a range of $300 to $880 including labor (updated August 29, 2025) ⁷. More in window glass replacement cost.
• Replace the window. If the frame is rotting, the window is drafty, or several units have failed, a full replacement may make more sense. Repair or replace windows walks through that decision.
• Check the warranty first. Pella suggests reviewing what a manufacturer's warranty covers, since glass coverage terms vary ³.

Some companies advertise defogging or reseal services instead of a glass swap. Fixr reports $70 to $120 to replace a window seal ⁷. Ask what the service includes and whether it restores the gas fill between the panes.

NFRC's condensation resistance rating

When you shop for new windows, the NFRC label may show a condensation resistance rating. It is optional, so not every label has it ¹⁰. Higher numbers mean a window resists condensation better ⁶, on a scale of 1 to 100 ⁵. NFRC suggests choosing a rating well above what warmer regions need if you live in a cold climate ⁵. For the other numbers on the label, see energy-efficient windows explained.

A note on storm windows

Adding an interior storm window can cut condensation on the room side of an old window, according to DOE's Building America Solution Center, but if the storm is not airtight, moisture may collect between the storm and the original window ⁸. See storm windows before choosing that route.

When condensation is a reason to replace

• Inside the glass: usually not. Lower the humidity first.
• Outside the glass: no. It is a side effect of glass that insulates well.
• Between the panes: a repair at least, and possibly a replacement if other parts of the window are failing too.

If you decide the windows need work, you can compare quotes from window installers for glass replacement or new windows.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Andersen help article (August 20, 2026) on why interior condensation forms, that it is not a glass failure, the 30% to 35% winter humidity guideline and ways to lower humidity, checked October 22, 2026.
• ² Reported: Andersen help article (April 16, 2026) on exterior condensation, when it forms, the conditions before it (including air conditioning) and why low-E glass makes it more likely, checked October 22, 2026.
• ³ Reported: Pella release (November 16, 2017) on interior, exterior and between-pane condensation, seal failure and warranty review, checked October 22, 2026.
• ⁴ Official record: EPA guidance to keep indoor humidity below 60%, ideally 30% to 50%, and that condensation can be a sign of high humidity, checked October 22, 2026.
• ⁵ Official record: NFRC consumer site on how condensation forms, mold risk, the condensation-cutting benefit of efficient windows and the 1 to 100 condensation resistance scale, checked October 22, 2026.
• ⁶ Official record: NFRC label description of the condensation resistance rating, checked October 22, 2026.
• ⁷ Reported: Fixr double-pane glass replacement of $150 to $600 (labeled both with and without labor on the page), a $375 national average for window glass replacement within $300 to $880 including labor, and $70 to $120 to replace a window seal, from pricing databases, contractors and industry experts, updated August 29, 2025, checked October 22, 2026.
• ⁸ Official record: DOE Building America Solution Center guide on interior storm windows reducing room-side condensation and the risk of condensation between storm and window, checked October 22, 2026.
• ⁹ Official record: DOE consumer guide (2021 PDF) on spacers helping prevent condensation and installation affecting condensation, checked October 22, 2026.
• ¹⁰ Official record: NFRC consumer site listing condensation resistance as an optional label rating, checked October 22, 2026.