Window condensation?

Moisture on the room side of the glass means indoor humidity is meeting cold glass, and you can usually fix that yourself. Fog or haze trapped between the panes is different: the seal has failed and the glass unit needs replacing.

Not sure which one it is? Snap your window and we’ll narrow it down.

Is the window also drafty?

Drafts and condensation often share a cause: a poor seal. Drafty window

Likely causes

High indoor humidity
Most commonEasy DIY
Water beads or a film on the inside of the glass, worst in the morning or in cold weather, in kitchens, bathrooms, bedrooms and laundry areas. Ice on the inside of the glass is the same problem in colder weather.
Part: $30–250 (a dehumidifier or a bathroom fan timer)
Poor airflow around the window
CommonEasy DIY
Closed blinds, heavy curtains or furniture trap still air against cold glass. Condensation is worse behind them and clears when they're opened.
Cold glass from a single-pane or aging window
CommonModerate DIY
Condensation on single-pane glass, or on every window on the cold side of the house, and glass that feels very cold. It improves when indoor humidity drops, but returns in a cold snap.
Part: $10–40 (interior shrink film kit)
Failed seal between the panes
Less commonCall a pro
Fog, haze, water drops or mineral streaks between the two panes of a double-pane window that won't wipe off. It comes and goes with the weather and doesn't clear. This isn't a DIY fix.
Part: $75–200 to defog or reseal (cosmetic; it doesn't restore insulation); about $280 to replace the glass
Exhaust fans not venting outside, or a hidden moisture source
Less commonModerate DIY
Several windows fog at once, the bathroom fan vents into the attic, or there's a damp crawlspace, an unvented dryer, or a lot of houseplants. Musty smells point the same way.

Quick checks

Don't ignore standing water: wet wood trim and sills rot, and mold grows behind curtains and on the sill. Wipe it up and dry it, and wear a mask to clean visible mold. Older painted windows may have lead paint, so don't sand or scrape them. And don't caulk or seal shut a window needed as a bedroom fire exit.

Wipe the glass and see which side the moisture is on. If it wipes off, it's inside-room condensation; if it's between the panes and won't wipe, it's a failed seal.
Run the bathroom and kitchen fans during and after showers and cooking, and make sure they vent outside.
Open blinds and curtains, and keep furniture or vents from blocking the air against the glass.
Lower the indoor humidity: a hygrometer reading around 30–50% is a good target in winter, lower in a very cold snap.
A house that stays cold and damp may be losing heat rather than making too little; check the window for drafts before assuming the furnace is at fault.

Sharpen the read

Work out why your window is fogging, for free

No account, no phone number. Fixology’s AI reads your photo and tells you what’s likely wrong, how worried to be, and whether it’s a DIY fix.

Snap one of these:

the window up close where you see the moisture or fog
the whole window, so we can tell its type
the sill and trim below it, especially any staining or soft wood

Add a note or more photos next. Nobody calls you.

When to call a pro

Fog or water sits between the panes, the sill or trim is soft, stained or rotting, there's mold you can't clean off, or many windows fog even with humidity under control. For lifespan and upkeep, see the window guide.

Typical cost: $75–200 to defog or reseal a failed seal (cosmetic only), about $280 to replace the glass, and $300–1,200 to replace a whole window (about $750 on average). Rot repair on a sill or frame runs $250–800. Dehumidifier or fan fixes run $30–250 and need no pro.

Sources: HomeAdvisor: How Much Does Window Repair Cost?, Angi: How to Fix Foggy Windows

After a scan, Fixology can also help you find pros near you.