Condensation on windows in winter

You wake up on a cold morning and the glass is fogged or dripping. The sill is wet. This page explains why it happens in winter, where the water is (inside, outside or between the panes), and what actually fixes it.

First, find where the water is. It decides everything else:

Where the water is	What it usually means	Worry?
Inside, on the room side of the glass	Indoor air is humid and the glass is cold	Fix the humidity
Outside, on the outdoor side	Dew. A window manufacturer calls it a sign the window is insulating well [1]	No
Between the panes of a double-pane window	The seal between the panes has failed [1]	Not urgent, but it will not go away on its own

Possible causes

Condensation forms when a surface is colder than the dew point of the air touching it. ^[2] Window glass is usually the coldest surface in a room in winter, so it gets wet first.

Two things push it over the edge:

• Cold outside. “The colder the weather outside, the greater the chances of window condensation inside,” says North Dakota State University Extension. ^[3]
• Moist air inside. Everyday life adds water to the air: breathing, showers, cooking, laundry. NDSU Extension says each person adds about 3 pints of water vapor a day just from breathing. ^[3] A U.S. Department of Energy lab report puts it simply: the risk of condensation goes up as outdoor temperatures drop and indoor humidity rises. ^[2]

Things that make it worse:

• Closed curtains, blinds or shades. They keep warm room air away from the glass, so the glass gets colder. In DOE-funded lab tests, a fully closed cellular shade caused condensation on the window, and the report notes that shades are not the only covering that can do this. ^[2]
• Air leaks around the window. Iowa State University Extension notes that leaking cold air can cool the glass enough to cause condensation at higher outdoor temperatures. ^[4]
• Single-pane or older windows. Their inside surface gets colder, so they condense sooner. ^{[4] [2]}
• A humidifier set too high, or fans that do not vent outdoors. NDSU Extension says to make sure bathroom and kitchen fans vent to the outside. ^[3]

How to identify the cause

Check where the water is. Touch the glass on the inside. If it is wet on your side, it is inside condensation. If it is wet only when you look from outdoors, it is outside dew. If you cannot wipe it off from either side, it is between the panes.

Check the pattern:

• Worse in the morning, and in bedrooms? People breathe out moisture all night, often with the door and curtains closed. That fits the causes above. (Our reading, not a source’s finding.)
• Only on the coldest days? That points to cold glass more than very wet air.
• On double-pane windows when it is not very cold? That points to air that is too humid. NDSU Extension says condensation on double- or triple-pane windows most likely means too much humidity in the house. ^[3]
• Mainly in the bathroom or kitchen? Look at the exhaust fan first.

Measure the humidity (your own check). A small humidity gauge, called a hygrometer, shows the indoor relative humidity. Put it in the room with the wettest windows and note the reading in the morning for a few days. Compare it with the targets below.

How much condensation is normal?

There is no single official number. Here is what the sources we checked say about indoor humidity in winter:

Source	Winter indoor humidity it suggests
U.S. EPA (all year)	Below 60%, ideally 30 to 50% [5]
NDSU Extension	Around 40% [3]
Iowa State University Extension	30 to 40% [4]
Andersen (window maker)	30 to 35% [1]

The colder it gets outside, the lower indoor humidity has to be to keep the glass dry. Extension guidance gives a rough sense of it:

• Single-pane: at 70°F and 35% indoor humidity, condensation starts at about 30°F outdoors. ^[4]
• Double-pane: at about 40% humidity, condensation should start only when it is below zero outside. ^[3]
• Triple-pane: condensation should not show until it is close to −40°F outside. ^[3]

So as a rule of thumb: a little fog at the bottom of the glass on the coldest mornings is common. Daily dripping, especially on double-pane windows in ordinary winter weather, means the air is too humid.

How serious is it?

• NormalA thin fog or a few drops on the coldest mornings that clears during the day, with a dry sill.
• Fix the humidityCondensation most days, water running onto the sill, frost on the inside of the glass, or condensation on double-pane windows in mild weather. The EPA treats condensation on windows as a sign of high humidity and a reason to act. ^[5]
• Look for damageBlack spots or mold on the frame or sill, peeling paint, a soft or stained sill, or a damp wall below the window. See Is mold in my house dangerous? and Why is my wall damp?

Fog between the panes is a separate problem. It does not wet the room, but it means the seal has failed and the window will not clear by itself. ^[1]

What can you do?

How to stop condensation on windows inside

1. Wipe it up. Yes, wipe it. A DOE lab report recommends wiping condensation on the sill at least every few days in the heating season. ^[2] Daily is easier on paint and wood.
2. Open curtains and blinds during the day. That lets warm air and sun reach the glass and dry it. ^[2] At night, leave a gap rather than sealing coverings tight against the window. ^[4]
3. Run the fans. Use the bathroom fan when you shower and the kitchen fan when you cook. Check that they vent outdoors, not into the attic. ^{[5] [3]}
4. Vent the dryer outside. ^[5] If you run a humidifier, turn it down or off while the gauge reads high.
5. Keep air moving. Open interior doors and use fans where you can. ^[5]
6. Keep the house warm enough. The EPA lists raising the air temperature as one way to prevent condensation. ^[5]
7. Stop drafts around the window. Weatherstripping, caulk or a plastic window film kit can keep the glass warmer. ^[4]

A dehumidifier is often not the answer in winter. NDSU Extension says they are generally ineffective when humidity is already below 50%. ^[3] For a very airtight house that stays humid, the same source mentions an air-to-air exchanger, which brings in drier outside air and recovers up to 70% of the heat from the air it sends out. ^[3]

Condensation on the outside of windows

Leave it. It is dew on the outdoor glass. A window maker describes it as a sign the window is keeping heat in. ^[1]

Condensation between window panes

You cannot wipe it away. The seal has failed. ^[1] The usual fix is new glass in the existing frame, or a new window if matching glass is not available. ^[6] It is not an emergency, so you can wait for good weather and get more than one price.

Thinking about new windows?

Better windows help. The DOE lab report says the logical approach in cold climates is higher-performance windows, such as triple-pane, or secondary glazing like storm windows. ^[2] Window labels can include a condensation resistance rating from 0 to 100, where higher is better. ^[7] But new windows do not lower humidity. If the air is too wet, fix that first.

Typical cost

These are ranges reported by the sources cited, not quotes. “Scope and date” is the geography the source covers and when it was published or last updated. Your price depends on your home, your area and the contractor.

Most fixes for inside condensation cost little or nothing: using fans, opening curtains, adjusting a humidifier. The figures above are for fog between the panes, from one national cost guide. The same guide says glass-only replacement typically costs 30 to 70% less than a full new window when the frame is sound. ^[6] See how to read a cost estimate.

Does fixing it affect property value?

“Return” is more than resale price. Where we have no reliable figure we say so instead of guessing. See how we think about value.

Related tools

We do not have a calculator for this yet.

Deeper technical explanation

Dew point, in plain words. Air holds water as invisible vapor. Warm air can hold more than cold air. When air touches a surface cold enough, it cannot hold all its water and some turns to liquid. The temperature where that starts is the dew point. If the glass is below the dew point of the room air, water forms on it. If the glass is below freezing, you get frost instead. ^[2]

Why the bottom and edges go first. The edges of the glass and the bottom of the window are usually the coldest parts, so water shows there before the middle. (This is our plain-language summary, not a quote from a source.)

Why more panes help. Each extra layer of glass with a gap of air or gas between them keeps the inside pane warmer. That is why the extension figures above move from about 30°F (single-pane) to below zero (double-pane) to about −40°F (triple-pane). ^{[4] [3]}

The trade-off with insulating shades. A covering that insulates the window keeps the room warmer but makes the glass colder, which raises the chance of condensation. In DOE-funded tests, lifting the bottom of a cellular shade by about half an inch sharply reduced condensation but also cut the shade’s insulating value by 39%. ^[2] The report’s authors do not recommend lowering humidity as the main fix unless it is unusually high. ^[2]

Why outside condensation happens on good windows. A well-insulated window lets less heat reach the outer pane, so on a clear, cool night the outer glass can drop below the dew point of the outdoor air. That is our plain-language explanation of why a window maker calls outside condensation a sign of efficiency. ^[1]

For how moisture moves through a house more generally, see How moisture moves through a house.

Sources & methodology

This page uses the U.S. EPA’s moisture guidance, a DOE-funded laboratory report from Pacific Northwest National Laboratory, two university extension services (North Dakota State and Iowa State), a Penn State course on window labels, a window manufacturer’s explainer and a national cost guide. The humidity targets differ between sources, so we show them side by side instead of picking one. The outdoor temperatures at which condensation starts are approximate figures from extension guidance, not tests of your windows. The manufacturer is used only for what condensation outside and between the panes means. Its advice that inside condensation is not the window’s fault runs against its own sales interest. Costs are US national ranges, per window. The humidity gauge check and the morning-and-bedroom pattern are our own suggestions. See how we work out costs and all sources.

1. Window Condensation 101 — Andersen Windows Manufacturer / tradeWindow manufacturer. Used only for what condensation outside the glass (dew) and between the panes (seal failure) means, and its 30 to 35% winter humidity suggestion. It sells windows, so its advice on replacement is not used. Scope: General. Accessed October 15, 2026.
2. Evaluation of the Thermal and Moisture Performance of Insulating Shades (PNNL-33380) — U.S. Department of Energy / Pacific Northwest National Laboratory Government agencyDOE-funded lab report (with Oak Ridge National Laboratory, Hunter Douglas and AERC). Explains condensation via dew/frost point, finds closed cellular shades can cause window condensation, recommends opening shades by day, wiping sills at least every few days and higher-performance windows or secondary glazing in cold climates. Does not cover condensation between panes or outside. Scope: Laboratory tests representing cold-climate homes. Published or updated: Sep 2022. Accessed October 15, 2026.
3. Window Condensation a Common Problem — North Dakota State University Extension Government agencyExtension energy educator Carl Pedersen. About 40% winter indoor humidity, approximate outdoor temperatures at which double- and triple-pane windows condense, moisture from breathing, fans that vent outdoors, dehumidifiers often ineffective in winter, air-to-air exchangers. A 2011 news release, not a technical standard. Scope: Cold-climate US (North Dakota). Published or updated: Jan 2011. Accessed October 15, 2026.
4. Winter Window Condensation (Answerline) — Iowa State University Extension and Outreach Government agency30 to 40% winter indoor humidity and approximate condensation temperatures for single-, double- and triple-pane windows (attributed to NDSU's Ken Hellevang); air leakage cooling the glass; weatherstripping, caulk and plastic window covers; drapes. Short extension Q&A. Scope: Cold-climate US. Published or updated: Jan 2015. Accessed October 15, 2026.
5. A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency Government agencyHumidity targets, the "about 10 square feet" guideline, and the 24–48 hour drying window. Accessed October 4, 2026.
6. Window Glass Replacement Cost — HomeGuide Cost guide (crowd/contractor data)Per-window costs for double-pane glass replacement and whole-window replacement; glass-only typically 30 to 70% less than a full window when the frame is sound; fogging between panes points to seal failure. No separate defogging price. Scope: US national. Published or updated: Feb 2026. Accessed October 15, 2026.
7. Understanding Window Performance Labels (EGEE 102, section 10.10) — Penn State College of Earth and Mineral Sciences Building-science / engineeringUniversity course material. Condensation resistance (CR) on window labels runs from 0 to 100; higher means better resistance; typical residential windows about 20 to 80. Scope: US window labels. Accessed October 15, 2026.
8. About Mold and Your Health — U.S. Centers for Disease Control and Prevention Government agencyHealth effects, who is most at risk, why the CDC does not recommend mold testing, humidity and cleaning guidance. Accessed October 4, 2026.