Doors & Windows guide
Condensation between window panes: why it happens and what fixes it
By the Home Doctor team · Reviewed
Quick answer
Fog or moisture between the two panes of a double-pane window means the edge seal of the insulated glass unit (IGU) has failed and humid air has gotten into the gap. It can't be wiped off or fixed from either side. It's mostly cosmetic: the window loses a little insulating value. Check the manufacturer's warranty first, since many cover seal failure for 10 to 20 years, or for life for the original owner. Otherwise, replacing just the glass unit typically costs $150 to $700 per window, well below the price of a new window.
- How urgent
- Cosmetic
Fog between the panes doesn't harm the house. The glass insulates a bit less, and the fog usually gets worse and can leave a permanent white haze. Moisture on the room side of the glass is different: if it runs down and soaks the sill, it can rot wood and grow mold.
- DIY or pro
- Call a pro
Figuring out which surface the moisture is on, lowering indoor humidity and clearing weep holes are DIY. A failed seal can't be fixed from either side: replacing the glass unit or the window is a job for a glass or window company.
Common causes and how to tell them apart
1. Failed seal in the glass unit
- How to tell
- Fog, droplets or a milky haze between the two panes that won't wipe off from either side. It comes and goes with sun and temperature, and older failures leave streaks or white spots.
- What fixes it
- The seal can't be repaired. Live with it, or replace the glass unit or the whole window. Check the warranty first.
2. High indoor humidity (room-side condensation)
- How to tell
- Water or frost on the room side of the glass, usually in cold weather and worst in the morning, in bedrooms, bathrooms and kitchens. It wipes off with a finger.
- What fixes it
- Lower the humidity: run bath and kitchen fans, use a dehumidifier, open blinds and curtains so warm air reaches the glass, and turn down any humidifier. Aim for about 30 to 50 percent, lower in very cold weather.
3. Normal outside condensation
- How to tell
- Dew on the outside of the glass on cool, clear, humid mornings that burns off once the sun hits it. Most common on newer energy-efficient windows.
- What fixes it
- Nothing to fix. The glass insulates well, so the outer pane stays cold and collects dew like a car windshield.
4. Moisture between the window and a storm window
- How to tell
- You have a separate storm window or insert, and the moisture is in the gap between it and the main window. You can open or remove one of them to reach it.
- What fixes it
- This isn't a sealed unit, so it can be wiped and dried. Warm, moist indoor air is leaking into the gap: weatherstrip the inner window, and keep the weep holes at the bottom of an exterior storm window open.
5. Water sitting against the edge of the glass
- How to tell
- The fog shows up first along the bottom of the glass. Rain collects in the bottom of the sash or track, and the small weep holes on the outside of the frame are clogged.
- What fixes it
- Clear the weep holes and keep the track clean. Standing water speeds up seal failure, so fix it before or when you replace the glass.
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What you can safely try first
- 1Wipe the room side of the glass with a dry cloth. If the moisture comes off, it's indoor humidity, not the seal.
- 2Wipe the outside of the glass. If it comes off there, it's normal dew.
- 3If it won't wipe off from either side, the seal has failed. Look for a name or date printed on the metal spacer between the panes or on a label on the frame, and check the manufacturer's warranty before paying for anything.
- 4For room-side moisture, get a hygrometer and keep indoor humidity around 30 to 50 percent: run bath and kitchen fans, use a dehumidifier and open the blinds.
- 5Clear the weep holes on the outside bottom of the frame with a thin wire so rain doesn't sit against the glass.
- 6Count the foggy windows and note rough sizes. Glass is priced per window, and a list speeds up quotes.
When to call a pro
- The fog bothers you and you want a clear view: a glass or window company can replace just the glass unit.
- The window may be under warranty: contact the manufacturer or installer before paying anyone else, since outside repairs like defogging can void it.
- Several windows have failed, or the frames are rotting, drafty or hard to open: get quotes on replacement windows.
- Room-side condensation keeps soaking sills or growing mold after you've lowered the humidity: have an HVAC contractor or energy auditor check the ventilation.
What it costs
| Job | DIY parts | Pro, installed |
|---|---|---|
| Hygrometer (humidity monitor) | $10–$25 | — |
| Portable dehumidifier | $150–$350 | — |
| Defogging service (per window) | — | $75–$200 |
| Replace the glass unit only (per window) | Not recommended | $150–$700 |
| Glass unit replaced under warranty (labor may be extra) | — | $0–$250 |
| Replace the whole window (per window, installed) | Not recommended | $500–$2,000+ |
Ballpark US ranges as of September 28, 2026. Your region, access and the specifics of your home can move these a lot. Get two or three quotes for anything over a few hundred dollars.
What to tell the pro
I have [number] [double-hung / casement / sliding / picture] windows with fog between the panes, roughly [width] by [height] inches. They're about [age] years old, made by [manufacturer / unknown], with [vinyl / wood / aluminum / fiberglass] frames in [good / poor] shape. I'd like a quote to replace [just the glass units / the whole windows]. Will the new glass match the original, including the low-E coating and gas fill, and what warranty comes with it?
Questions people ask
Why is there condensation between my window panes?
The seal around the edge of the double-pane glass has failed. The gap is meant to hold dry air or argon gas, and once the seal breaks, humid air leaks in and condenses on the inner surfaces of the glass as temperatures change. You can't wipe it off or reseal it from either side. It's mostly cosmetic, with a small loss of insulating value.
Can you fix a foggy double-pane window without replacing it?
Not reliably. Defogging services drill small holes, clean and dry the gap, then plug the holes or fit vents, and the view often improves, but the seal and any argon gas aren't restored and stains or etching can stay. Results are mixed, and it may void a remaining warranty. Replacing just the glass unit is the dependable fix, and the frame can stay.
How much does it cost to replace a foggy window pane?
Replacing just the sealed glass unit typically costs $150 to $700 per window, depending on size and glass type. A whole new window typically runs $500 to $2,000 or more installed. Check the warranty first: many manufacturers cover seal failure for 10 to 20 years, or for life for the original owner, though labor may not be included.
Why do my windows have condensation on the inside or outside?
Moisture on the room side of the glass means indoor humidity is high for how cold it is outside: run fans, use a dehumidifier and open the blinds. Dew on the outside on cool, humid mornings is normal, especially on energy-efficient windows, and burns off as the sun warms the glass. Only moisture you can't wipe off from either side means a failed seal.
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General guidance, not a substitute for a licensed professional who can see the problem in person. If you smell gas, see sparks or smoke, or anyone is hurt, leave the area and call 911.