7 min read · Updated 2026-08-02
If you can see fog, haze, or water droplets between the panes of glass — and wiping both surfaces changes nothing — the insulated glass seal has failed. The window is not dirty. It is not going to dry out. And no spray, cloth, or dehumidifier will reach the space where the moisture is sitting.
This is one of the few window problems with a completely unambiguous diagnosis.
A modern window is not one piece of glass. It is an insulated glass unit (IGU): two or three panes held apart by a spacer bar, with the cavity filled with argon or krypton gas and sealed around the entire perimeter.
That seal does two jobs: it keeps the inert gas in, and it keeps water vapor out. A desiccant inside the spacer bar absorbs the small amount of moisture present at the factory.
Every day, the sealed unit heats up and cools down. The gas inside expands and contracts, and the seal flexes with it — thousands of cycles a year. Eventually the seal fatigues and develops a breach. Gas escapes. Outside air, carrying humidity, works its way in. The desiccant absorbs it until it saturates, and after that, every temperature drop condenses moisture on the inside of the glass where you can see it but not reach it.
In Michigan the cycle is aggressive: annual swings of 100°F between January nights and August afternoons work every seal in the house harder than a mild climate would.
Energy performance. This is the real loss. Argon gas has meaningfully lower thermal conductivity than air. Once it escapes, the unit’s insulating value drops substantially — a failed IGU performs closer to plain double glazing than to the rated product you bought.
Visibility. Mineral deposits from repeated condensation etch the interior glass surface permanently. Early on the fog comes and goes with temperature. Later, a permanent white haze remains even on dry days.
Not structural, not urgent. A failed seal does not let rain in. The window still keeps weather out and still locks. There is no reason to panic or to accept a same-day pressure quote. This is a problem you can plan around.
Condensation on the inside surface (the room side, wipeable) is not a seal failure. It means indoor humidity is high relative to the glass temperature. Common in kitchens, bathrooms, and any house in a Michigan winter. Fix it with ventilation, a bathroom fan that vents outside, or lower humidifier settings.
Condensation on the outside surface (exterior, wipeable) is actually a sign your windows are working well. It means the outer pane is staying close to outdoor temperature because so little heat is escaping through the unit. It appears on cool mornings and burns off.
Condensation between panes is the only one that is a defect — and the only one you cannot wipe.
If the frame is straight, the sill is solid, and the hardware works, the glass unit alone can be replaced. A glazier measures the unit, orders a matching IGU, removes the failed one, and installs the new one in the existing sash.
This is the right answer more often than the industry admits, because it is the smaller job. It typically costs a fraction of full-frame replacement and takes an hour or two per opening once the glass arrives.
Requirements: sash and frame in good condition, and a unit size the supplier can match. Unusual shapes, very large units, and obsolete sash profiles can make this impractical.
Justified when the seal failure comes with frame problems — rot, warping, split corner welds, or a frame that was never properly insulated. If you are already opening the wall, replacing the whole assembly is the better value.
Also worth considering if a large share of the windows in the house have failed at once. Units installed the same year fail in the same window of years; replacing them as a group costs less per opening than one at a time, and matching new glass to twenty-year-old frames rarely looks right.
Some companies drill small holes in the glass, flush the cavity, and install vents. It clears the visible fog.
Be clear about what it does not do: it does not restore the gas fill, does not restore the insulating value, and leaves the unit permanently vented to outside air — which means humidity keeps cycling through. It is a cosmetic fix for a performance problem. It can make sense if you are selling shortly and want clear glass, but it is not a repair.
Go to each fogged window and check three things:
Generally no. Seal failure is wear over time, and homeowners policies exclude gradual deterioration. Impact damage — a rock, a branch, hail — is a different matter and may be covered subject to your deductible.
Manufacturer warranties are the more likely path. Many IGU warranties run 10 to 20 years and specifically cover seal failure. If you have the original paperwork or can identify the manufacturer from a stamp in the spacer bar, it is worth checking before paying for anything.
Fogged glass is a solved problem with a clear price. What determines that price is whether the frame around it is still sound — which takes about a minute per window to establish.
If you are in Canton or the surrounding Wayne County communities, our free estimate tells you which of your openings are glass-only repairs and which need the full unit, in writing, with each priced separately. We quote the repair when the repair is the right answer.
Related reading: How long do windows last · Window glass and seal repair
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