Sweating windows? They’re writing you a humidity report.
Sweating windows are your house’s built-in humidity gauge, and November is when they start talking. The first cold night drops the glass temperature, the furnace kicks on, and suddenly there are beads along the bottom of every sash.
Where those beads sit tells you what to do. Water on the room side means turn something down. Water on the outdoor side is usually just weather. Fog trapped between the panes means the glass itself needs attention.
Inside fog means the air is too moist for how cold the glass is. Outside fog on a cool morning is dew. Fog between the panes is a failed seal. Lower your humidity as the forecast drops, run the bath and kitchen fans, and treat water that pools or stains as a mold warning.
Read time · 8 minutes
Sweating Windows: Three Panes, Three Meanings
Before you treat sweating windows, find the wet surface. Drag a fingertip across the glass. If it streaks clear, the water is on your side. If it doesn’t, step outside and try again.
Too much moisture for the glass
Moist indoor air touched glass colder than its dew point. This is the common kind of sweating windows, and it’s a humidity signal, not a defect. It shows first along the bottom and edges, where glass runs coldest; Oregon State University Extension notes the edges can fog at lower humidity than the center.
Turn something downMorning dew on the outer pane
Outdoor-side sweating windows are dew, the same film you find on the car or the lawn. It forms on cool, calm mornings and burns off as the sun climbs. Insulated glass lets less house heat reach the outer pane, so that pane can run as cold as the air around it.
Usually just weatherA seal that let go
InterNACHI says a misty multiple-pane window means its seal has failed. NFRC adds that once moisture gets in, the insulating gas can escape. Unlike other sweating windows, no humidity setting will clear this one.
Have the glass checkedStreaks clear inside? Humidity. Streaks clear outside? Weather. Won’t wipe at all? Seal.
The Dew Point on Your Glass
Warm air holds more water vapor than cold air. Cool a pocket of room air far enough and it can’t carry what it’s holding, so the extra turns to liquid. That temperature is the dew point. In most winter rooms the glass is the coldest surface, so it reaches the dew point first. That’s where sweating windows begin.
Two things drive sweating windows: how much moisture is in the air and how cold the glass gets. Showers, cooking, laundry, houseplants and a full house of holiday guests add moisture. Cold snaps, wind and thin glass pull the surface temperature down. Change either side and the beads come or go.
Glass quality matters a lot. Oregon State Extension figures, for 20°F outside and 70°F inside:
That spread is why one house on the street has sweating windows and the next one doesn’t. For the numbers behind warmer glass, see our guide to window R-value and U-factor, and how gas-filled windows slow heat loss between the panes.
Humidity Dial: Will Your Glass Fog Tonight?
Will you wake up to sweating windows? Set an indoor humidity and an outdoor temperature. The test pane fogs or clears using two published tables: the maximum winter humidity the U.S. EPA lists from ASHRAE, and the humidity at which double windows began to fog in NRC Canada’s building digest.
At 20°F: EPA-listed max 35% · NRC double window fogged at 53% windy, 61% calm.At 0°F: EPA-listed max 25% · NRC double window fogged at 41% windy, 49% calm.At −20°F: EPA-listed max 15% · NRC double window fogged at 32% windy, 39% calm.
Clear. You’re at or below the maximum the EPA lists for this outdoor temperature. Typical double glazing should stay dry.
Borderline. You’re above the EPA-listed maximum but below where NRC saw double windows fog. Edges, windy nights and glass behind drapes may bead first.
Condensation likely. You’re at or above the humidity where NRC saw double windows fog at this temperature. Turn the humidifier down and run the exhaust fans.
A Winter Humidity Chart for Cold Snaps
The EPA’s moisture guide lists maximum indoor humidity by outdoor temperature, credited to ASHRAE. The NRC Canada digest shows where double windows actually began to fog with the room at 70°F. The gap between the columns is your margin.
| Outdoor temp | EPA-listed max indoor RH | NRC: double window fogs (wind) | NRC: double window fogs (calm) |
|---|---|---|---|
| +20°F | 35% | 53% | 61% |
| +10°F | 30% | — | — |
| 0°F | 25% | 41% | 49% |
| −10°F | 20% | — | — |
| −20°F | 15% | 32% | 39% |
Why so low? The EPA’s mold guidance suggests keeping indoor humidity below 60%, ideally 30 to 50%. On the coldest nights the glass sets a lower ceiling than that. To head off sweating windows, drop the humidifier a notch when the forecast drops, then bring it back up when the weather warms.
Six Fixes for Sweating Windows
Start at the top. Most room-side sweating windows clear up after the first three, and none of them cost much.
- Run the bath fan during and after showers.The EPA’s advice is to vent bath and kitchen moisture outside, not into the attic. Leave the fan on until the mirror clears.
- Use the range hood whenever you boil or simmer.A big pot of soup on a cold night is a steam machine. Lids help too.
- Match the humidifier to the forecast.Use the chart above and a simple humidity indicator, which the EPA recommends, so you know what the room is really reading.
- Open the blinds and pull furniture back.Drapes and couches trap cold air against the glass. The EPA suggests fans and moving furniture out of wall corners to keep air and heat moving.
- Seal the drafts around the frame.Cold air leaking at the trim and sash chills the glass edges. Fresh caulk and weatherstripping keep the edge warmer and drier.
- Upgrade the glass once the rest is handled.If sweating windows persist at sensible humidity, compare NFRC Condensation Resistance ratings: typically 1 to 100, and higher means less room-side fog. Our window performance ratings guide explains the label.
When Sweating Windows Become a Mold Risk
The EPA treats water on the glass as a sign of high indoor humidity that deserves a closer look. Sweating windows that dry by lunch are normal. Daily water that runs, pools and soaks into wood is how mold gets started. Watch for these:
- Water pooling on the sill most mornings, not just after a cold snap.
- Black or gray spots on caulk, the sill or the drywall below the window.
- Peeling paint or soft wood along the bottom rail and stool.
- A musty smell behind drapes that stay closed all day.
- Frost building on the room side of the glass on the coldest nights.
Wipe sills dry, lower the humidity and clean small spots promptly. If wood stays soft or stains keep spreading, have the window opening looked at.
The glass, not the air
When fog sits inside the sandwich, your humidity is not the problem. The seal around the insulated glass unit has failed. NFRC notes moisture can then degrade the low-E coating, and the gas fill can leak away.
The good news: InterNACHI reports the glazing assembly can be replaced about 75% of the time, so the whole window often stays. Defogging services remove the water, but InterNACHI points out that doesn’t fix the seal itself.
If several windows have failed at once, our replacement windows in Dayton guide walks through full replacement.
The Takeaway Worth Sharing
Sweating windows are a message, not a mess. Find the wet side, match humidity to the forecast, and save the glass budget for panes that won’t wipe clear.
Send this to the friend who wipes their windows with a towel every morning.
Glass That Won’t Wipe Clear?
Thrush & Son®: family-owned exterior pros since 1960.
If you’re in the Miami Valley and the fog lives between the panes, read the Thrush & Son® guide to severe condensation, then book a window visit.
➡️ Schedule in Dayton➡️ View All ServicesCall (937) 833-5136 · 115 Jefferson St, Brookville, OH 45309
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Further Reading
Official sources: the U.S. EPA, NRC Canada, NFRC, InterNACHI and Oregon State University Extension.
Sweating Windows FAQ
Why are my windows sweating on the inside in winter?
Indoor air is carrying more moisture than the cold glass can handle. When the glass drops below the dew point of the room air, the extra vapor turns to water on its surface.
Are sweating windows on the outside a problem?
Usually not. Fog on the outdoor side is dew that forms on cool, calm mornings and burns off as the sun comes up. It has nothing to do with your indoor humidity.
What indoor humidity should I keep in winter?
Lower it as it gets colder. The EPA lists ASHRAE maximums of 35% at 20°F outside, 25% at 0°F and 15% at −20°F. Year-round, the EPA suggests staying below 60%.
Can fog between the panes be fixed?
It means the seal on the insulated glass has failed. InterNACHI reports the glazing assembly can be replaced about 75% of the time, so the full window often doesn’t need to go.
When do sweating windows become a mold risk?
When water runs, pools on the sill or soaks into wood day after day. Dark spots, peeling paint, soft wood or a musty smell near the window are signs to lower humidity and act.
Written by Corey Thrush, Vice President and third generation of Thrush & Son®.
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