Window Condensation: Inside, Outside or Between the Panes

Where the water is tells you what is wrong. Condensation on the room side of the glass is indoor humidity meeting cold glass, and it is about your air, not your window. Condensation on the outside surface is dew, and it usually means the window is doing its job. Fog or haze trapped between the panes is a failed seal in the insulated glass unit, and that is the only one of the three that needs a repair.

Check which surface the moisture is on before you do anything else. Touch it. If you can wipe it away, it is on a surface you can reach. If you cannot, it is sealed inside the unit.

Condensation on the inside of the glass

This is the one most homeowners see, usually on the coldest mornings of the year and usually first in bedrooms, bathrooms and the kitchen.

Warm indoor air holds moisture. When that air touches a cold surface, the moisture comes out of it as water. Your window glass is the coldest surface in most rooms, so it condenses first. The window is behaving like a thermometer for the humidity in your house.

Where the moisture is coming from

• Showers and baths, especially with a fan that is not vented outside or not run long enough.
• Cooking without a range hood that vents out.
• A dryer vented into a crawlspace, garage or attic instead of outdoors.
• Drying laundry indoors.
• Crawlspaces with no vapor barrier, which push moisture up into the house all winter.

What to do about it

1. Run bathroom fans during the shower and for a good while afterward, and confirm the fan terminates outside the house.
2. Use the range hood every time you cook, and check that it vents out rather than recirculating.
3. Make sure the dryer duct goes outside and is clear.
4. Open blinds and curtains on cold mornings. Heavy window coverings trap a pocket of still, cold air against the glass and make condensation worse.
5. Check the crawlspace for a vapor barrier and standing water.
6. Wipe the sills. Water sitting on a wood sill every morning is how a good window turns into a rotted one.

If the condensation is heavy on every window every morning and none of that helps, warmer glass is the real fix. A window with a lower U-factor keeps the interior surface of the glass warmer, so it stays above the point where water forms. That is one of the quieter benefits of replacing single-pane or aluminum-framed windows, and it is covered in signs your windows are costing you money.

Condensation on the outside of the glass

Some mornings you will walk out and find the exterior of your windows dewed over while the neighbor’s are clear. It looks like a defect. It is the opposite.

Dew forms outdoors when a surface drops below the dew point of the outside air overnight. On an efficient window, the exterior pane stays cold because so little heat is escaping through the glass to warm it. On a leaky old window, heat bleeds out and keeps the outer pane too warm for dew to form.

So exterior condensation means the window is holding your heat inside. It burns off as the sun comes up, it is most common in late summer and early fall on clear nights, and it needs no repair at all.

Fog between the panes

This is the failure. A double-pane window is two panes sealed around a spacer, with dry air or argon in the gap. When that seal fails, humid outdoor air gets into the space, and moisture condenses on surfaces you cannot reach.

How to recognize it:

• The fog or haze does not wipe off from either side.
• It comes and goes with the weather, heavy in the morning and thin by afternoon.
• Over time it leaves a permanent cloudy film or streaking on the inner glass surfaces, sometimes with a whitish mineral haze.
• It usually starts on one or two windows, often on the sunniest and most weather-exposed wall, not all at once.

There is no cleaning it, and there is no lasting repair to the unit itself. Drilling and venting a failed unit does not restore the seal or the insulating gas. What restores the window is a new insulated glass unit set into the frame you already have. We do that work every week, and the reasons a seal gives out in the first place are in why your window seal failed.

What we do

When you call us about a foggy window, the first thing Cory does on the visit is decide whether you have a glass problem or a window problem. If the frame is solid and square, the sash operates and the weatherstripping is intact, the fix is a new insulated glass unit in the existing frame, which costs a fraction of a new window. If the frame is rotted, the sash is warped or the window is a single-pane aluminum unit from the 1960s to 1980s, new glass in it fails again and we will say so rather than take the smaller job.

Glass-only replacement is not eligible for the Snohomish PUD or PSE rebates, and we tell you that before you decide, because it sometimes changes the math on a house with several failed units. Full window replacement can qualify when the windows meet the U-factor rule. Amounts and eligibility can change, and we confirm the current program with you before you order.

A quick way to tell them apart

1. Wipe the room side of the glass with your hand. Gone? Indoor humidity.
2. Step outside and wipe the exterior. Gone? Dew, and nothing is wrong.
3. Still there from both sides? A failed seal.
4. Look at whether it is one window or every window. One or two points to a seal. All of them, every cold morning, points to humidity or to old single-pane glass.
5. Check the sills for standing water and the wood for softness, whichever answer you got. Water sitting on wood is the thing that turns into an expensive repair.

Questions homeowners ask

Why do only some of my windows have condensation?

Rooms differ. Bedrooms hold the moisture from people breathing overnight, bathrooms and kitchens make their own, and north-facing glass stays coldest. Window coverings, furniture in front of a register and a closed door all make one room condense while the room next to it stays clear.

Can foggy double-pane windows be fixed?

Not by cleaning, and not by any lasting repair to the failed unit. The fix is a new insulated glass unit in the same frame, which is a common job for us and far cheaper than a new window. If the frame is sound, that is all your window needs.

Is condensation on the outside of my new windows a defect?

No. It means so little heat is escaping through the glass that the outer pane drops below the dew point overnight. It shows up on clear, calm mornings, usually in late summer and fall, and it clears as the day warms.

Not sure which one you have? Send us a photo

Take a picture of the window from inside and outside and tell us whether it wipes off. Cory measures, checks the frame and tells you whether it is glass, a window, or the air in the house. Free in-home estimate, written quote within 48 hours. Get in touch.