Foggy Window Between Panes: Repair Options

Fog or droplets trapped between the two panes of a double-pane window, where you cannot wipe them from either side, usually mean the seal around the insulated glass unit has failed and let moist air into the gap ¹. Cardinal, a glass maker, says glass units with condensation in the airspace should be replaced ¹. There are three ways to deal with it: replace just the sealed glass unit, pay a defogging service to clear it, or replace the whole window.

When the frame and sash are sound, a new glass unit is the narrower repair. Fixr lists $150 to $600 for double-pane glass in its material-only table and says labor adds about $50 to $125 (updated August 29, 2025) ⁴. Defogging is cheaper but described as temporary ³, and a whole new window makes sense when the frame or sash is failing too. Before paying for any of them, check the glass warranty: the three manufacturer warranties compared below cover seal failure for up to 20 years ^{6 7 8}.

First, confirm it is between the panes

Cardinal suggests cleaning both the outdoor and indoor glass surfaces to make sure the moisture is really in the airspace and not on either face ¹. Moisture on the room side usually comes from indoor humidity, and moisture on the outside is dew; neither is a seal problem, and both are covered in window condensation. Cardinal adds that airspace fog shows up first in winter, when cold outdoor air chills the glass, and that a small crack that is easy to miss can cause it too ¹.

Why seals fail

A double-pane glass unit is two sheets of glass held apart by a spacer around the edge, with the gap sealed and often filled with argon or krypton ⁹. Andersen's warranty calls the edge seal an organic seal ⁶, and like any sealant it ages. Pella says that, given enough time, the seals between the panes break down and let moisture in, that this usually happens in older windows, and that the sun's heat can speed it up, which is why fog tends to show on the sunny sides of a house ².

Bob Vila's guide describes the mechanism as "solar pumping": the glass expands in the heat and presses on the seal, which is why west- and south-facing windows often fail first ³. It lists other causes as well: faulty installation, the house settling, heat guns, high winds and chemical strippers ³.

Once the seal is broken, the gas fill is no longer held in, and the guide notes that any extra insulating value from an inert gas is gone ³. That is the difference from the slow, normal leakage through an intact seal, which NFRC puts at about 0.5% of the fill per year for an efficient window ⁹. More on fills is in argon vs krypton window fill.

Option 1: Replace the glass unit

The sealed unit comes out of the sash and a new one, made to the same size, goes in. The frame, sash and hardware stay. Fixr's material-only figures are $150 to $600 for double-pane glass and $375 to $850 for argon-filled glass, with labor adding about $50 to $125 ⁴. The same page lists $70 to $120 for a window seal repair, which it describes as removing the pane, finding the break, cleaning inside, replacing the gas and the seal, and reinstalling the glass; in its section on glass versus whole-window replacement, it says a window is usually not worth saving if the seal is broken ⁴.

This route fits when the window still opens, closes and locks well and the frame shows no rot. The price detail, including how glass type and size move it, is in window glass replacement cost; this article does not repeat it.

Option 2: A defogging service

Defogging companies clear the view without replacing the glass. Bob Vila's guide describes the process as drilling a small hole in the exterior pane, injecting a chemical between the panes to dry the condensation, drawing out as much air as possible with a vacuum, and sealing the hole ³. It calls this a temporary fix, because the seal is still broken and the window may fog again, and puts the service at about $60 to $100 (guide updated September 7, 2021) ³.

What to weigh before choosing it:

• The seal itself. The guide says the seal stays broken after defogging ³, and once the seal has failed, any insulating benefit of a gas fill is gone ³. Ask the company what its process does about both.
• The warranty. Andersen's window warranty excludes failures related to product modifications ⁶. Pella's vinyl warranty excludes products with modifications it has not approved ⁷, and Marvin's excludes modification and alteration ⁸. If the windows are still under warranty, ask the manufacturer before anyone drills the glass.
• What the company promises. Ask how long the clear view is expected to last and what happens if the fog returns.

Option 3: Replace the whole window

A new window replaces the frame, sash, seals and hardware along with the glass. Fixr reports a national average of about $800 per window, in a typical range of $450 to $1,200 (updated January 21, 2026) ⁵. It costs more, but it is the route to consider when:

• the frame or sill is soft, rotted or warped,
• the sash sticks, drafts or will not lock, or
• several windows of the same age are fogging.

Repair or replace windows walks through that decision, and how long do windows last covers age.

What the warranties say about seal failure

In each of these three warranties, the glass has its own section and its own term:

Read the exclusions too. Andersen's list of exclusions includes condensation ⁶, so confirm the moisture really is between the panes before you call. Pella excludes damage from high in-home humidity, glass breakage that is not its fault and then leads to seal failure, and after-market window films ⁷. Marvin excludes third-party products such as films and tinting, and says it does not warrant the amount of argon or other gas in the glass at any time after manufacture ⁸. Pella's 2017 note says most of its warranties give at least 20 years of coverage on glass ².

Before a claim, gather the brand, the series and the purchase date. The full comparison of terms across brands is in window warranties explained.

Questions to ask before you pay

1. Is the window still under the glass warranty, and does the manufacturer cover labor?
2. Is the quote for a new glass unit installed, or for the unit only?
3. Will the new unit match the original glass package, including low-E and gas fill?
4. If it is a defogging service, how long is the result expected to last, and is it compatible with the warranty?
5. Is the installer licensed where you live? How to choose a window installer lists what else to check.

If the window needs more than glass, you can compare quotes from window installers.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Cardinal IG technical bulletin IG01 (November 2021) on airspace condensation from seal failure or a small crack, replacing such units, and cleaning both surfaces to confirm, checked October 26, 2026.
• ² Reported: Pella release (November 16, 2017) on seals breaking down with time and sun, fog in older windows and on sunny sides, and at least 20 years of glass coverage in most of its warranties, checked October 26, 2026.
• ³ Reported: Bob Vila guide (updated September 7, 2021) on solar pumping and other causes of seal failure, loss of the inert gas's insulating value, and defogging as a temporary fix that leaves the seal broken, at about $60 to $100, checked October 26, 2026.
• ⁴ Reported: Fixr material-only double-pane glass ($150 to $600) and argon glass ($375 to $850), labor of $50 to $125, window seal repair ($70 to $120) and its note that a window with a broken seal is usually not worth saving, from pricing databases, contractors and industry experts, updated August 29, 2025, checked October 26, 2026.
• ⁵ Reported: Fixr window replacement national average of about $800 per window and typical range of $450 to $1,200, updated January 21, 2026, checked October 26, 2026.
• ⁶ Reported: Andersen 400 Series and 200 Series limited warranty (October 2023) on the organic seal, 20-year glass and seal coverage, labor, and exclusions for modifications and condensation, checked October 26, 2026.
• ⁷ Reported: Pella vinyl limited warranty on 20-year nonlaminated glass seal coverage, labor within 2 years, and exclusions for high in-home humidity, unapproved modifications, after-market films and breakage-related seal failure, checked October 26, 2026.
• ⁸ Reported: Marvin limited warranty effective January 12, 2026, on 20-year and 10-year seal coverage by size, removal and installation costs, no warranty on gas content after manufacture, and exclusions for modification and third-party films, checked October 26, 2026.
• ⁹ Official record: NFRC consumer page on argon and krypton fills and putting gas leakage from an efficient window at about 0.5% of the fill per year, checked October 26, 2026.