Updated September 2026. How we research this
If the haze is between the panes and a cloth will not touch it, the sealed glass unit has failed. Moisture is now inside a cavity that was built to stay dry, and no amount of cleaning reaches it. The useful question is not how to clean it, but which of three fixes your window actually needs.
Worth knowing.
Condensation on the room-side or the exterior of the pane is normal weather. Moisture trapped between the panes means the sealed glass unit is finished, and the frame around it is still perfectly usable. Which fix you need turns on that difference and on the age of the window.
First, work out where the moisture is
This is the step most homeowners skip, and it changes everything that follows. Wipe both faces of the glass, inside and out.
| Where the moisture sits | What it means | What fixes it |
|---|---|---|
| On the room side, wipes away | Indoor humidity condensing on cold glass. A ventilation and moisture problem, not a window defect | Reduce indoor humidity, run extractor fans, improve air movement. The window is doing its job |
| On the outside face, wipes away | Dew forming on a cold exterior pane on a humid morning. Generally a sign the glass is insulating well | Nothing. It clears with the day |
| Between the panes, will not wipe away | The perimeter seal of the insulated glass unit has failed and moisture has entered the sealed cavity | Replace the glass unit, or the sash or window. Cleaning is impossible |
What has actually happened inside the window
An insulated glass unit is two or more panes held apart by a spacer that contains a desiccant, with a primary seal to keep water vapour out and a secondary seal to hold the assembly together. The cavity is dry, and on most modern units it is filled with argon or another inert gas to slow heat transfer.
Thermal cycling, UV exposure and frame movement work on that perimeter seal for years. Once it breaks down, humid air migrates in faster than the desiccant can absorb it, and the haze appears. Glazing sources describe the same sequence consistently, and two consequences follow that matter more than the appearance.
First, the gas fill is gone. A failed unit still looks like a double-pane window, but the thermal barrier it was sold with is not there any more. Second, the failure is one-way: the cavity does not dry itself out, which is why the fog comes and goes with the weather but never leaves.
Your three real options, and what each one restores
1. Replace the insulated glass unit
The sash and frame stay; a new sealed unit goes in. This is the right answer when the frame is square and dry, the hardware works and the sash is not damaged. You get back a sealed cavity, a fresh desiccant and a factory gas fill, which is the whole point.
Glass shops usually price this work more keenly than replacement-window companies, because it is what they do all day. Angi puts glass-unit replacement at $250 to $1,000 in our patio door research; for a standard window it is generally the low end of that band, but it varies with size, coating and whether the unit is tempered or laminated. Get local quotes: there is no standardized national price for this work.
One thing to ask, which almost nobody does: what the replacement unit’s specification is. NFRC ratings apply to a whole window as an assembly, not to a pane of glass, so the label your window carried when it was new does not automatically describe it once a different glass unit is installed. Ask the shop what coating and gas fill the new unit has, in writing.
2. Replace the sash, or the whole window
Correct when the frame is rotted, out of square or leaking, when the sash itself is damaged, when balances and hardware have failed as well, or when a large share of the windows in the house have failed seals at once. At that point you are not buying clarity, you are buying a working window.
If you are heading that way, price it properly first with the window replacement cost calculator, and read insert vs. full-frame, because that decision moves the total more than the brand does.
3. Cosmetic defogging
A hole is drilled, the cavity is cleaned or vented, and a valve may be fitted. The haze often clears. What does not come back is the sealed construction: the spent desiccant is not replaced, the cavity is not refilled with gas under factory conditions, and the unit is now deliberately vented to the outside air.
It can be a reasonable choice if you are selling soon, if the window is in a garage or outbuilding, or if the units are an odd size with a long lead time. Treat it as cosmetic. Two questions before agreeing: does it affect any remaining glass warranty, and what happens when it refogs?
| What it restores | Glass unit replacement | Sash or window replacement | Cosmetic defogging |
|---|---|---|---|
| Clear view | Yes | Yes | Usually, for a time |
| Sealed, dry cavity | Yes | Yes | No |
| Factory gas fill | Yes | Yes | No |
| Fresh desiccant | Yes | Yes | No |
| Frame, sash and hardware condition | No | Yes | No |
| A documented product rating | Only if the shop specifies the new unit | Yes, on the new product | No |
Check the warranty before you pay for anything
Insulated glass is one of the few components most manufacturers cover explicitly, and the terms are often longer than people assume. From documents we have already read for our product library:
- Marvin warrants standard insulating glass with stainless-steel spacers against seal failure for 20 years in units under 60 square feet, and 10 years at 60 square feet or more, while stating that removal, installation and disposal are not included. See our Marvin Essential review.
- Andersen publishes a transferable Owner-to-Owner warranty; dealers describe 20 years on glass for the 400 Series. See our Andersen 400 review.
- ProVia warrants insulated glass using its Super Spacer for as long as the original purchaser owns and lives in the home, transferable once. See our ProVia Endure review.
Find the original paperwork, the serial number etched in a corner of the glass, or the installer’s invoice. A covered claim usually supplies the glass and leaves you paying the labour, which still changes the arithmetic considerably.
What a fogged-window quote should tell you
- Which windows are being treated, and whether each one gets a new glass unit or a new sash.
- The specification of the replacement unit: coating, gas fill, spacer type, tempered or laminated where required.
- Whether the existing frame was inspected, and what was found.
- The warranty on the new glass unit, and on the labour, stated separately.
- Lead time, since units are made to size.
- What happens if the frame turns out to be damaged once the sash is out.
If you have a glass-shop quote and a replacement-company quote in front of you, they are not describing the same job, and comparing the totals tells you nothing. Put them side by side in Compare Two Window Quotes, and use what a quote should include to fill the gaps.
Replace the glass unit when
- The frame is square, dry and sound
- Sashes operate and lock properly
- One or a few windows are affected
- The windows are otherwise recent enough to be worth keeping
- Matching units are available in your size
Replace the window when
- There is rot, water damage or movement in the frame
- Hardware and balances have failed as well
- Seals have failed across many windows at once
- The units are single-glazed or early aluminium and the room is uncomfortable
- You are already replacing siding or trim on that elevation
How we researched this
The mechanism described here, spacer, desiccant, primary and secondary seals, gas fill and one-way moisture ingress, is described consistently across glazing industry and glass-trade sources. We have not tested insulated glass units and we publish no laboratory findings. The glass-unit price band is market context from Angi’s 2026 guides, not a standardized figure; there is no national price for this work, which is why we say get local quotes. Warranty terms are quoted from each manufacturer’s published documents and attributed. The point about NFRC reflects how certification works: ratings apply to complete fenestration products, so a glass-only change is not automatically described by the original label. Full methodology.
FAQ
Can you clean between double-pane windows?
No. The cavity is sealed at the factory and there is no access without cutting into the unit. If the haze does not wipe off either face, the seal has failed.
Does a fogged window still insulate?
Not as designed. The gas fill escapes when the seal fails, so the unit performs closer to plain double glazing, and worse as moisture accumulates.
Is defogging worth it?
It can clear the view, and that is all it reliably does. It does not restore the sealed cavity, the desiccant or the gas fill, and the unit is vented afterwards. Reasonable before a sale or on a garage window; not a performance fix.
Can I replace just the glass in a vinyl window?
Often yes, if the sash and frame are sound and the manufacturer or a glass shop can supply a matching unit. Some sash designs make it impractical, in which case a new sash is the next step up rather than a whole window.
Is the fog mould?
The haze itself is condensation and mineral deposits from evaporated water. Prolonged moisture at the edges can lead to mould around the seal or on the frame, which is a reason to deal with it rather than live with it.
Will my new glass have the same rating as the old window?
Not automatically. NFRC ratings apply to whole products, so ask the glass shop to state the coating and gas fill of the replacement unit in writing.
Planning note: This is general guidance for U.S. homeowners, not a diagnosis of your window. Water appearing on the frame, the sill or the wall below is a different problem from a failed glass unit and is worth a professional inspection. See our methodology and disclaimer.