Fog Between Your Double-Pane Window Glass — What Failed and What to Do

If your window looks foggy or hazy and no amount of wiping either side clears it, the moisture is trapped between the two panes of glass. The short answer: the seal around the insulated glass unit has failed, letting humid air in and the insulating gas out, and the real fixes are replacing the glass unit or the window — not cleaning, since you cannot reach the inside surfaces.

What actually failed

A double-pane window is two sheets of glass sealed around the edges with a spacer, with the space between them filled with dry air or an insulating gas like argon. That sealed unit is what insulates. Over time — usually after 10 to 20 years, faster in windows that get strong sun or big temperature swings — the edge seal degrades, cracks, or pulls away. Humid outdoor air gets in, the insulating gas escapes, and moisture condenses on the inside faces of the glass whenever temperatures shift. That is the fog you see, and it comes and goes with the weather at first, then stays.

Fog between the panes is different from condensation on the room-side surface of the glass, which is a humidity and ventilation issue in the house, not a window failure. If wiping the inside surface clears it, the seal is fine. If the haze sits between the glass and cannot be wiped away from either side, the seal has failed.

What does not work

Defogging sprays, wiping, and ventilation changes will not fix a failed seal, because the moisture is sealed inside the unit. Some companies offer a "defogging" service that drills a small hole in the glass, vents the moisture, and installs a valve — this can clear the fog temporarily, but it does not restore the insulating gas or reseal the unit, so the window's energy performance stays reduced and the fog often returns. It is a cosmetic fix, not a repair of the seal.

Similarly, replacing just the outer glass or caulking around the frame does not help: the failed part is the sealed glass unit itself, not the frame or the exterior caulk.

The fix that works: replace the glass unit

In most cases you do not need a whole new window. A glazier or window company can measure the failed unit, order a replacement insulated glass unit (IGU), and install it in your existing frame. This restores the seal, the insulating gas, and the clarity, typically for roughly $150 to $400 per window depending on size and glass type — far less than full window replacement. It is the best option when the frame is in good shape and the window otherwise works well.

When full window replacement makes more sense

Replace the whole window when the frame itself is damaged, rotted, warped, or hard to operate; when several units in the same window have failed; or when the window is old enough that new, more efficient windows would pay back meaningfully in comfort and energy costs. Full replacement typically runs roughly $500 to $1,200 or more per window installed, depending on size, material, and style. If you are replacing one window for fog and the rest of the house has windows the same age, ask about pricing for the batch — the per-window cost usually drops.

What to check first

Confirm the fog is really between the panes, note how many windows are affected, and check whether any are still under warranty — many window manufacturers warrant the sealed unit for 10 to 20 years, and a warranty claim can cover the replacement glass. Also check the frame and operation: a window that sticks, leaks air, or has a damaged frame is a replacement candidate even if only one pane is fogged.

On ZIP Pros — "Pros in your ZIP" — describe the problem once and get free quotes from local pros, with real starting prices shown up front. Mention the window size, how many are fogged, and the window's age — that tells the pro whether a glass-unit swap or full replacement fits.