Foggy Dual-Pane Windows: What Causes It, and Can It Be Fixed?

Haze or moisture between the two panes means the hermetic seal on the insulating glass unit has failed. Once that happens the unit cannot be restored to its original condition — it has to be replaced. The good news is that you almost always replace the sealed glass unit, not the whole window, which is a far smaller job.

This is one of the most common calls we get, and it is also one of the most commonly oversold. A fogged window is frequently quoted as a full window replacement when the frame, the hardware and the operation are all perfectly sound.

Here is what is actually happening inside the glass, and what your real options are.

What is an insulating glass unit?

A dual-pane window is not simply two sheets of glass. The assembly — the insulating glass unit, or IGU — is made of:

• Two panes of glass, usually with a low-emissivity (low-E) coating on one internal surface.
• A spacer around the perimeter holding them apart at a fixed distance.
• Desiccant inside that spacer — a drying agent that absorbs the small amount of moisture sealed in during manufacture.
• A hermetic edge seal bonding the whole assembly and keeping the cavity closed.
• Often an inert gas fill, typically argon, which insulates better than air.

The cavity is a closed system. Everything about the unit's insulating performance depends on it staying closed.

Why does the glass fog?

The edge seal fails, water vapour begins entering the cavity, and the desiccant absorbs it — until the desiccant is saturated. After that, there is nowhere for the moisture to go, so it condenses on the internal glass surfaces when the temperature drops. The fog you see is the symptom; the seal failure happened earlier.

That delay is why fogging often seems to appear suddenly in a window that was fine for years. The seal may have been compromised for a long time before the desiccant gave out.

Seals fail for ordinary reasons: thermal cycling flexing the assembly day after day, water sitting in the glazing pocket because of poor drainage, solar heat gain in units that trap heat, physical stress during installation, or simply age. Coastal exposure and large south-facing units both accelerate it.

The Insulating Glass Manufacturers Alliance defined unit failure in Technical Bulletin 1205-89 as permanent obstruction of vision through the unit caused by accumulation of dust, moisture or film on the internal glass surfaces. That is the industry definition, and it is worth knowing because it is the language a warranty claim will use.

Three kinds of condensation — only one is a failure

This is the single most useful thing in this article. Before assuming your window has failed, work out which surface the moisture is on.

Where the moisture is	What it means	What to do
Between the panes — you cannot wipe it off from either side	Seal failure. The unit has failed.	Replace the insulating glass unit
Inside surface — wipes off from indoors	Indoor humidity meeting a cold surface. Not a window fault.	Ventilation, extraction, humidity control
Outside surface — wipes off from outdoors	Dew on the exterior pane, usually early morning	Nothing. This is normal.

The third one surprises people. Exterior condensation on a dual-pane window is generally a sign the window is working well: the outer pane is staying close to outdoor temperature because so little heat is escaping through the unit. Homeowners occasionally call this in as a defect on brand-new high-performance glass.

The test is simple. Try to wipe it. If it wipes off from either side, the seal is fine.

Can a fogged window be "defogged"?

There are services that drill a small hole in the glass, clean or flush the cavity, and fit a vent. This can clear the visible haze. It does not restore the sealed unit — the cavity is now permanently vented to outside air, the argon fill is gone, and the desiccant is not renewed. It is a cosmetic treatment, not a repair.

We are not going to tell you nobody should ever do it. If the priority is purely to see through a window in a rarely used room, and the alternative is spending nothing, some homeowners are satisfied. But be clear about the trade:

• You lose the insulating gas fill permanently, so the unit's U-factor gets worse.
• The cavity now breathes outdoor air, so moisture can re-enter and staining can recur.
• Drilling the glass will normally void any remaining glass warranty.
• Mineral deposits and low-E coating damage already etched onto the internal surfaces do not come off.

If the window is one you look through every day, replacing the unit is the answer.

Do you need a whole new window?

Usually not. Replacing the IGU keeps the existing frame, sash, hardware and installation intact, and there are three practical reasons this matters in California:

1. It is a repair, not a replacement. Swapping glass inside an existing frame is generally treated differently from replacing the window assembly, so you avoid the permit and compliance path that a full replacement triggers. See our guide to Title 24 window requirements for San Diego.
2. It is far less disruptive. No stucco, no trim, no interior finish work.
3. It costs a fraction of a replacement window, installed.

Full replacement becomes the right call when the frame is corroded, rotted or racked, when the sash will no longer hold a seal, when the hardware is obsolete, or when you want to change the size or configuration of the opening.

Check the warranty before you pay

Most manufacturers warrant IGUs against seal failure for a defined period, often ten to twenty years, sometimes longer on premium lines. Warranties are frequently transferable to a subsequent owner, and homeowners routinely pay for glass that was still covered.

Find the manufacturer stamp — usually etched in a corner of the glass or printed on the spacer bar, visible through the pane. Photograph it. That stamp, plus the approximate age of the window, is enough to start a warranty check.

Does it matter how many windows are fogged?

Yes, diagnostically. One failed unit in a house is ordinary bad luck. Several failing in the same period usually points at a common cause: the same production batch, a shared exposure such as a hot west elevation, or a glazing detail that is holding water at the sill. Worth identifying, because otherwise you replace units and the replacements fail the same way.

The short version

• Fog between the panes means the seal has failed. It cannot be restored.
• Wipe test first — interior and exterior condensation are different, and exterior condensation is normal.
• Replace the glass unit, not the window, unless the frame or hardware is also gone.
• Drill-and-vent "defogging" is cosmetic, costs you the gas fill, and voids the glass warranty.
• Check the manufacturer stamp for a live warranty before paying for glass.
• Several units failing at once means look for a shared cause.

We handle glass replacement and window service across San Diego County. If you are unsure which kind of condensation you have, send a photo and say whether it wipes off — that alone usually settles it.