There are three kinds of window fog and only one of them is a window problem.

Fog on the inside surface, the one you can wipe, is a humidity issue in the room. Fog on the outside surface, also wipeable, is dew, and it usually means the window is doing its job well. Fog between the panes, the one you cannot wipe from either side, is the failure this article is about.

Window condensation between panes means the sealed insulating glass unit has lost its seal. It is not a cleaning problem, it is not caused by anything you did, and it does not get better on its own. What it does do is tell you something useful about the age and condition of the rest of your windows.

How a sealed insulating glass unit works

A modern window is not one piece of glass. It is an assembly.

Two or three lites of glass are held apart by a spacer around the perimeter. The cavity between them is filled with dry air or an inert gas, usually argon. The spacer contains a desiccant, a drying agent that absorbs any residual moisture. The whole perimeter is sealed, typically with two sealants doing different jobs, one for the moisture barrier and one for structural adhesion.

That sealed cavity is the insulation. It is also the entire reason low-e coatings work, since the coating lives on an interior glass surface where it is protected from cleaning and weather.

The assembly is called an insulating glass unit, or IGU. It sits inside the sash. The sash sits inside the frame. When people say a window has “failed,” they usually mean the IGU has failed while the sash and frame are still fine.

Why the seal fails

Seals do not fail because someone did something wrong. They fail because of what they are asked to do.

Thermal cycling. The gas in the cavity expands when the sun hits the glass and contracts when it cools. Every single day. That expansion and contraction pumps against the seal thousands of times a year. On a west facing Gulf Coast window in August, the daily swing is severe.

Ultraviolet exposure. Sealants are polymers, and ultraviolet light degrades polymers. Florida sun is not gentle about this.

Water at the glazing pocket. If water collects where the IGU sits in the sash and cannot drain, the seal sits in standing water. This is why blocked weep holes shorten window life.

Heat. Dark frames, dark exterior films applied after the fact, and interior blinds closed tight against the glass all raise glass temperature. Aftermarket film on a sealed unit is a known cause of premature seal failure and it commonly voids the glass warranty.

Installation stress. A unit installed in a frame that is racked or over-shimmed carries load it was not designed to carry.

Age. Sealants have a service life. Twenty years is a reasonable expectation for a quality unit in a moderate climate. Coastal Florida with full sun exposure is not moderate.

How to be sure it is a seal failure

Four confirmations, in order of ease.

• Wipe both surfaces. Interior and exterior. If the haze remains, it is between the panes.
• Look for the pattern. Seal failure fog typically appears at the bottom of the glass first and grows upward, because moisture settles. It often comes and goes with temperature, appearing on cool mornings and clearing by afternoon.
• Look for the residue. Over time, a failed unit develops a permanent mineral or chemical haze on the interior glass surfaces that does not clear even when the visible fog does. This is the point at which the glass is permanently marked.
• Check the neighbors. Walk the whole house. Seal failures cluster by age and by exposure. If one west facing window has failed, look hard at the rest of the west elevation.

That fourth step is the one that changes decisions. A single failed unit on an otherwise young window package is a one-off. Three failures on the same elevation in the same year is a fleet approaching end of life, and it changes the math entirely.

What it costs you beyond the view

The obvious cost is appearance. The less obvious costs matter more.

• Lost insulating value. The gas fill is gone and the cavity now exchanges with outside air. Depending on the original specification, that can mean a meaningful loss of thermal performance on that unit.
• Degraded low-e performance. The coating sits on an interior surface. Once moisture and contaminants reach it, its performance drops and the surface can etch permanently.
• Permanent glass etching. Repeated wetting and drying leaves mineral deposits on the interior surfaces. After enough cycles the glass is marked whether or not it is currently foggy.
• A signal about the rest of the window. The seal is often the first component to go. Balances, weatherstrip, and hardware are usually the same age.

The Department of Energy’s overview of windows, doors, and skylights is a useful reference for how much of a home’s heating and cooling load moves through fenestration and how energy performance ratings work, which is the framework for deciding whether a failed unit is worth replacing on its own or as part of a larger project.

Your three actual options

Option 1: Replace the insulating glass unit only.

A glazier removes the sash, takes out the failed IGU, and installs a new one. The frame and sash stay.

This works when the frame and sash are sound, the window is a common size and configuration, and the manufacturer or a local fabricator can produce a matching unit. It is the least expensive path and it is genuinely the right answer for a young window with one bad unit.

The limits are real. On impact rated windows, the IGU is part of the tested assembly, so a replacement unit must match the approved specification or the product approval no longer applies. On older or discontinued windows, matching the unit can be difficult. And it does nothing about aging hardware or weatherstrip.

Option 2: Replace the sash.

Some manufacturers sell complete replacement sashes for their own products. This gets you new glass, new seals, and often new weatherstrip while keeping the frame. It is a middle option, available mainly on newer windows from manufacturers who still support the line.

Option 3: Replace the window.

Right when the frame is compromised, when multiple units have failed, when the window is old enough that everything else in it is also aging, or when the existing window is not impact rated and you were going to address that eventually anyway.

The case for full replacement gets stronger the more of these are true. If you are looking at three failed units on a twenty year old non-impact window package on the Gulf Coast, replacing individual pieces of glass is spending money to preserve something you will replace within a few years regardless.

We can walk through that math honestly with you and show the replacement window options that make sense for your exposure and budget, including whether a partial approach buys you enough time to be worth it.

About defogging services

You will find companies that drill small holes in the glass, clean the cavity, and install a vent or valve. Be clear eyed about what this does and does not do.

It can improve appearance, at least for a while. It does not restore the seal. It does not restore the gas fill. It does not restore lost insulating performance. It does not undo etching that has already occurred. And it introduces a permanent opening in a unit that was designed to be sealed, which on an impact rated window means the assembly no longer matches its approval.

For a rental property with a cosmetic complaint and a short hold period, it can be a defensible economic decision. For a primary residence where the window is going to be replaced eventually, it usually just delays the decision at a cost.

What actually extends window life

Since seal failure is largely an environmental problem, the things that help are environmental.

• Keep the weep holes clear. Sand and pollen block them. Clear them each spring. This is the highest value five minute maintenance task on a window.
• Do not apply aftermarket film to sealed units. It raises glass temperature, is a documented cause of premature seal failure, and typically voids the glass warranty.
• Leave a gap behind blinds and drapes. Tightly closed treatments against the glass trap heat.
• Use exterior shading where possible. Overhangs, awnings, and planting reduce glass temperature far more effectively than anything applied to the glass.
• Manage interior humidity. This does not affect the seal, but it stops the other kind of condensation and it protects everything else in the house.
• Handle drainage. If water pools on a sill, find out why. Standing water at the glazing pocket shortens the life of the unit sitting in it.

Frequently Asked Questions

What causes window condensation between panes?

Window condensation between panes is caused by the failure of the perimeter seal on the insulating glass unit. Once the seal breaks, the inert gas escapes and humid outside air enters the cavity. The desiccant in the spacer absorbs moisture until it saturates, after which water vapor condenses on the interior glass surfaces where you cannot reach it.

Can foggy windows be fixed?

The fog can be reduced by a defogging service that drills the glass, cleans the cavity, and vents it. The seal cannot be restored, the gas fill cannot be replaced, and etching already on the glass is permanent. The durable fixes are replacing the insulating glass unit, replacing the sash, or replacing the window.

Is window condensation between panes covered by warranty?

Often, if the window is still within its glass warranty period and the failure was not caused by an excluded condition. Aftermarket window film is a common exclusion and a common cause. Find the original paperwork, note the manufacturer and approximate installation date, and call the manufacturer before spending money on a repair.

How long should a window seal last?

A quality insulating glass unit is commonly expected to last around twenty years in a moderate climate. Full sun exposure, dark frames, coastal heat, and daily thermal cycling all shorten that. On a west facing Gulf Coast elevation, earlier failures are normal rather than defective.

Does condensation between panes affect energy bills?

Yes, on that window. The unit has lost its gas fill and its cavity now exchanges with outside air, so its insulating value drops. Moisture and contamination on the low-e coating reduce its effectiveness as well. One failed unit will not show up on a bill. A whole elevation of them can.

Can I replace just the glass instead of the whole window?

Frequently yes. If the frame and sash are sound and a matching insulating glass unit is available, a glazier can swap the glass and keep everything else. On impact rated windows the replacement unit must match the approved assembly specification, so confirm that first. It is the most cost effective path for a young window with an isolated failure.

Why does the fog appear in the morning and disappear later?

Because it is temperature driven. Overnight the glass cools below the dew point of the humid air now trapped in the cavity and moisture condenses. As the sun warms the glass, the water evaporates back into the cavity and the fog clears. This cycle repeats daily and gradually deposits minerals on the glass.

Is condensation between panes a sign of a bad window?

Not necessarily. It is a sign of an aged or stressed seal, which happens to every insulating glass unit eventually. Where it does indicate a problem is when it appears early, which usually points to installation stress, blocked drainage, or aftermarket film. Early failures are worth investigating rather than just replacing.

Should I replace all my windows if one is fogged?

No, not on the strength of one failure. Walk the house first. If it is a single unit on an otherwise sound and reasonably young package, replace the unit. If several have failed within a year or two, particularly on the same elevation, you are looking at a fleet at end of life, and a whole house project is usually the better use of the money.

Does window film cause seal failure?

Aftermarket film applied to an existing sealed unit is a well known contributor. It absorbs solar energy and raises glass temperature, which increases thermal stress on the seal. Most glass manufacturers exclude film related failures from warranty coverage. Low-e coatings built into the unit at manufacture do not carry this risk, because they are engineered into the assembly.

Will cleaning the weep holes prevent foggy windows?

It helps, and it is worth doing regardless. Blocked weep holes let water collect in the glazing pocket where the insulating glass unit sits, and a seal sitting in standing water fails sooner. Clearing them each spring is a five minute task with a real effect on window life.

Are impact rated windows more or less prone to seal failure?

Neither inherently. Impact rated units use laminated glass, which is a different construction, but the perimeter seal faces the same thermal and ultraviolet stresses as any other insulating unit. The practical difference is on the repair side: replacement glass in an impact rated window must match the approved assembly specification, which limits your options.

Ready to Find Out Whether It Is One Window or All of Them?

Send us a photo of the fogged unit and roughly when the windows were installed. We will tell you honestly whether replacing the glass makes sense or whether you are better off putting that money toward the eventual project.

Visit the showroom at 17601 Ashley Dr, Bldg A-Suite 1, Panama City Beach, FL 32413, or call (850) 851-8249.