Humidity · stack · 6 quick cards
Condensation on the Inside of Windows
7 cards · By the Air in the House editors, from EPA, NDSU Extension, Home Ventilating Institute · how stacks are made · Updated
Why are your windows wet on the inside?
Condensation on windows forms when warm, moist room air touches glass colder than its dew point; in winter it usually means indoor humidity is above about 40% for the cold outside.
- EPA calls condensation on windows, walls or pipes a possible sign of high humidity and says to dry the surface and cut the moisture source.
- NDSU Extension: on double-pane glass at 40% indoor humidity, fog should start once it is below 0°F outside; triple-pane holds out to near −40°F.
- NDSU Extension's fix has two parts: warm the glass (for example with a plastic cover) and keep winter humidity under 40%.
- NDSU Extension estimates 3 pints of moisture from each occupant's breath daily, before any showers or cooking.
- Dehumidifiers usually only bring air down to about 50%, so NDSU Extension says they won't stop winter window fog.
- EPA: dry anything water-damaged within 24–48 hours, and treat mold patches larger than about 10 sq ft as a bigger job.
Why does condensation form on the inside of windows?
Condensation forms on the inside of windows because the glass is the coldest surface in the room, and air that cools against it can no longer hold all of its water vapor. NDSU Extension's energy educator compares it to a chilled drink sweating on a summer day: warmer air has room for more water, colder air has less, so the extra settles on the glass.
Video from the Pella Windows and Doors channel on YouTube: What causes condensation on windows?. That channel has no tie to Air in the House; nothing loads from YouTube except the still picture until you press play. Turn over for the detail
The temperature at which that happens is the dew point. The more water vapor in the room, the higher the dew point, and the sooner a cold pane reaches it. That is why the lower corners and edges of a window fog first: they are the coldest parts of the glass.
Where does the water come from? Breath is a steady source, roughly 3 pints per occupant daily by NDSU Extension's estimate, before showers, cooking and laundry.
Is it the window or the house?
It is usually both: a cold window sets the stage, and indoor humidity decides how much water lands on it. NDSU Extension says that if you see condensation on double- or triple-pane windows, you most likely have too much humidity in the house.
Use the glass and a hygrometer together:
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- One old single-pane or metal-framed window sweats while modern double-pane windows stay clear: the window is the weak spot.
- Most windows in the house sweat every morning: humidity is too high for the weather. Check the reading; above 40% in a cold spell is the usual cause.
- Only windows behind heavy curtains or blinds sweat: the covering keeps room heat off the glass. NDSU suggests leaving drapes open at night, while noting the energy loss that comes with it.
- Only bathroom or kitchen windows sweat: the moisture is local, and exhaust fans are the fix.
The page on ideal indoor humidity in winter explains why the target drops as the weather gets colder.
Source the documents listed on this stack's sources card
Next How do you reduce window condensation?↓How do you reduce window condensation?
Reduce window condensation by lowering indoor humidity and keeping the glass warmer, the two steps NDSU Extension gives in its guide to eliminating condensation on windows. Start with the moisture, because that also protects walls and attics you can't see.
- Turn humidifiers down or off. EPA's humidifier advice is direct: if water condenses on windows, relocate the humidifier, lower its setting or use it less.
- Hold humidity in the right band. Keep indoor humidity between 30% and 50% (EPA), never letting it sit above 60%. In cold weather, aim lower, around 30–40%, while you see water on the glass.
- Use the bath fan properly. Run the fan during the shower and for about 20 minutes afterward (the Home Ventilating Institute's guidance); a timer or humidity-sensing control does it for you. Timer options are covered on bath fan run time.
- Make sure fans vent outdoors. NDSU Extension warns that fans venting into the attic can cause unseen moisture and mold problems. See bathroom fan venting.
- Cook under a hood ducted outside, on the back burners where you can.
- Warm the glass. Interior plastic film or storm panels keep the inner surface warmer; open curtains at night help too.
- Skip the winter dehumidifier. NDSU Extension notes these generally only reach about 50% humidity, which is not low enough for cold-weather glass.
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In a very airtight house, the long-term answer is steady fresh air. NDSU Extension mentions air-to-air exchangers for tight homes, and HVI says most heat- or energy-recovery units keep roughly 70–80% of the outgoing air's energy in the house. The comparison is on ERV vs HRV.
When does window condensation turn into a mold problem?
It turns into a mold problem when the water stays: on sills, in frame joints and on the drywall below, morning after morning. EPA's mold guidance says water-damaged areas and items should be dried within 24–48 hours to prevent mold growth, and NDSU Extension adds that ice on the glass can damage the window and the surrounding wall.
A thin film that clears by mid-morning is a nuisance, not damage. Water that runs down onto the sill, ice that builds up in the corners, black spots on caulk or wood, and soft or stained paint are the signs to act on.
Turn over for the detail
If mold has already started, EPA says a patch under about 10 sq ft (roughly 3 ft by 3 ft) is usually a job you can handle yourself following its cleanup guidelines. A larger area, or a lot of water damage, calls for EPA's more detailed remediation guidance or a professional. Either way, the cleanup won't last unless the humidity comes down.
What about condensation between the panes?
Fog between the two panes of a sealed double- or triple-pane unit is a different problem from fog on the room side. The water is inside the glass unit, where room humidity, fans and wiping cannot reach it, so it points to a sealed unit that has let moist air in.
That kind of fog does not mean the house is too damp. It is a glazing repair: the sealed glass unit or the sash usually has to be replaced, and a window installer can tell you which. Older windows with a separate storm window are another case: fog on the inner face of the storm pane usually means warm room air is slipping past the inside sash, so weatherstripping the inner window helps.
Where the water shows up, and what it points to Where you see water Likely cause First step Room side of most windows Indoor humidity too high for the cold Lower humidity toward 30–40% and run exhaust fans Room side of one old window That window's glass or frame runs cold Add interior film or a storm panel Windows behind curtains only Covering keeps room heat off the glass Open the curtains at night Bathroom or kitchen window only Local moisture from showers or cooking Run the bath fan or range hood longer Between the panes of a sealed unit Sealed glass unit has let moist air in Ask a window installer about replacing the glass unit Inner face of an older storm window Room air leaking past the inside sash Weatherstrip the inner window Source the documents listed on this stack's sources card
Next Window condensation numbers↓Window condensation numbers
- 40%NDSU's winter humidity aimNDSU Extension
- below 0°Foutdoor temperature at which double-pane glass at 40% starts to fogNDSU Extension
- about 50%the lowest a typical dehumidifier reachesNDSU Extension
- 24–48 hourstime to dry wet areas before mold growsEPA
- 10 sq ftmold patch size EPA says most people can clean themselvesEPA
Chart drawn by Air in the House from figures cited in this stack. Source NDSU Extension · EPA
Next Quick cards↓
Quick cards
Short questions people ask next, one answer each.
Why do windows sweat more in the morning?
Overnight the outdoor temperature bottoms out, the glass gets coldest and people have been breathing moisture into closed bedrooms for hours. By late morning the sun and the heating system warm the glass again.
Is a little condensation on new windows normal?
A light film on the coldest mornings can happen even with good double-pane glass, which NDSU Extension says should start fogging below 0°F at 40% indoor humidity. Fog on milder days means the house is too damp.
Can window condensation damage the walls?
Yes. NDSU Extension notes that as ice forms it can damage the window and the surrounding wall, and running water can soak the sill and drywall below.
Will a new set of windows stop the fog?
Warmer glass helps, but NDSU Extension's point stands: if double- or triple-pane windows sweat, the house most likely has too much humidity. Fix the moisture first.
Does window condensation in summer mean the same thing?
Fog on the outside of the glass on a cool summer morning is outdoor dew forming on a window cooled by air conditioning. It is harmless and clears as the sun warms the glass.
Why does only the bedroom window fog up?
A closed bedroom with sleepers in it gains moisture all night and gets less air from the rest of the house. Leaving the door open, or running a whole-house ventilation system, usually clears it.
Where these cards come from
Built from North Dakota State University Extension's window-condensation guidance, EPA's mold and humidifier pages and the Home Ventilating Institute's ventilation pages, read in October 2026. Outdoor-temperature humidity charts are left out because their source tables could not be confirmed, and no window, film or dehumidifier brands are recommended.
- EPA a possible sign of high humidity
- NDSU Extension a chilled drink sweating on a summer day
- NDSU Extension eliminating condensation on windows
- EPA humidifier advice
- Home Ventilating Institute keep roughly 70–80% of the outgoing air's energy
Cards are written by the Air in the House editors from the documents above and dated when they were last checked. How stacks are made