Why do my windows fog up between the panes — and can it be fixed?

We are going to spend the rest of this page explaining why, because "you need new windows" is what everyone says and it is not always true.

What the fog actually is

A modern window is not a piece of glass. It is two panes held a fixed distance apart by a spacer, sealed at the edge, with the cavity between them filled with an inert gas — usually argon. Federal guidance describes the assembly plainly: the space between glazing layers, "usually about 1/2", is filled with argon or krypton gas; both are inert, non-toxic, clear, and odorless."

That gas is most of what you paid for. It conducts heat far worse than air does, which is the entire point.

When the edge seal fails, the gas leaves and ordinary humid air comes in. On the next cold night, the moisture in that air condenses on the inside face of the glass — where you cannot reach it. That is your fog. Some days it looks like haze, some days like a tidemark, and on a bad unit it eventually leaves a permanent mineral stain as the moisture cycles in and out.

There is no way to get to the inside of a sealed unit. That is what "sealed" means.

Why it happens faster here

This is a heat and sunlight problem, and Phoenix has an unusual amount of both.

Peer-reviewed testing on insulating glass units found that the edge sealants exhibit six to eight times more permeability at 60 °C than at 20 °C. The same research names the primary cause outright: *sunlight is "the most important aging factor on IGUs."* And it found that over seventy weeks of testing, aluminum spacers leaked more gas than non-metallic warm-edge spacers did.**

The Arizona State Climate Office publishes Phoenix normal maximum temperatures of 103.9 °F in June, 106.6 °F in July and 104.5 °F in August, with sunshine averaging "86 percent of possible" and a June maximum of 94 percent. (That period of record is 1971–2000, so treat it as a climate normal rather than a current reading.)

One honest caveat, and we are going to leave it in rather than paper over it. We have no field measurement showing that a Phoenix window's edge seal actually reaches 60 °C. The permeability finding is a laboratory result and the temperatures above are outdoor air normals. Those are two separate facts and we are not going to multiply them together to manufacture a scarier one.

What we will say is what everyone in this valley already knows from their own failed windows: around here, seals are the thing that goes.

The sequence, and why you get no warning

1. Nothing visible, for years. The seal degrades long before anything shows.
2. Gas escapes. The window still looks perfect and has quietly become a cheaper window. The measured penalty is real — the same research found gas loss can increase a unit's U-factor by up to 32%.
3. Moisture enters, and you see it. Usually first on the west and south elevations, because they take the most sun.
4. Staining. Repeated condensation cycles etch the glass. At that point even a perfect reseal, if one existed, would not give you a clear window.

By the time you can see fog, step two happened a while ago. That is why the honest answer to "how urgent is this" is: not an emergency, but the window stopped performing before you noticed, so there is no reason to wait for it to get worse.

About "defogging"

You will find companies that offer to clear a fogged window without replacing it. The method is to drill the unit, flush or vent it, and in some cases fit a small breather valve.

It does clear the appearance. It does not restore the window. The gas is gone and it is not coming back — the unit is now vented to outside air by design. You have bought back the view and kept a window performing like a cheaper one, in a climate where the gas fill was doing real work.

If you are selling the house in the spring and want it to look right, that is a legitimate reason to consider it, and we would rather say so than pretend otherwise. As a fix, it is not one.

So: glass only, or the whole window?

This is the question that actually has money in it, and a bidder who never raises it is not thinking about your house.

Glass-only replacement is a real option, and often the right one, when:

• the frame is square and sound
• the sash still operates and latches properly
• the weatherstripping is intact
• the frame is not a bare metal frame with no thermal break (see below)
• the unit is a standard size a supplier can still match

Replace the whole window when:

• the frame has moved, the sash drags, or the corners have opened up
• the frame is aluminum with no thermal break — you would be putting good glass into the actual problem
• several windows on the same elevation have failed, which usually means the rest of that elevation is close behind
• you were going to do it within a couple of years anyway, in which case doing it once is cheaper than doing it twice

On that last point, an honest note about how partial jobs price. Mobilization, permits, disposal and crew time are largely fixed. Spread across three openings they are punishing; across twelve they nearly disappear. Partial jobs cost more per window. Every time. That is arithmetic, not a penalty, and anyone who tells you otherwise is managing you.

Across 489 Phoenix-area homeowners who told us their project size, 69% were replacing six or more windows at once — 182 in the six-to-ten range and 157 doing the whole house. Only 54 were doing one or two. Most people here are not doing this one window at a time, and the fixed-cost maths is why — the full distribution is on how many windows people actually replace at once.

The frame question, which is really the Phoenix question

Across 472 Phoenix-area homeowners who told us what their window frames are made of, about 60% — 281 of them — have metal. Of those, 161 are twenty-plus years old and 75 are original to the house. 83 reported fogging between the panes.

That is not a coincidence. Federal guidance on metal frames is blunt: "metal or aluminum window frames conduct heat very rapidly, which makes metal a very poor insulating material", and such frames "should have a thermal break — an insulating plastic strip placed between the inside and outside of the frame and sash." A great many Valley windows do not have one.

If that is your situation, putting a new sealed unit into the old metal frame is treating the symptom. We go through how to identify what you have, and what to do about it, on metal window frames in Arizona.

What to ask whoever looks at it

1. Which elevations have failed units, and did you actually walk them?
2. Glass-only or full window — and why, for this specific opening?
3. What is the frame made of, and does it have a thermal break?
4. What spacer is in the replacement unit — non-metallic warm edge, or metal? The research above found the metal ones leak more.
5. U-factor and SHGC in writing for the exact unit and glass package quoted. Not "ENERGY STAR certified" — the numbers.
6. What the glass warranty covers, and what it is conditional on. This one catches people: on the product line we install, the lifetime coverage for insulated glass seal failure is conditional on the upgraded glazing package, dropping to 10 years without it. In a metro where seal failure is the dominant way windows die, that condition is the single most consequential line in the document — and every manufacturer has an equivalent. Ask for the PDF.
7. Is labor covered? Almost never by the manufacturer. That is the installer's separate warranty, with its own term.

Related guides

• When to replace your windows
• Single pane vs double pane windows
• Window replacement cost in Arizona