MI Homesiding & Roofing
Fog between the panes is one specific failure: the sealed glass unit has lost its edge seal. It is not the same thing as condensation on the room side of the glass, it is not the same thing as a window that leaks water, and on its own it does not mean the window has failed. This page is about telling those apart, and about what your particular window actually allows once you know.
Related: window repair, window glass replacement, drafty window repair, and window leak repair.
If you can see haze, a cloudy film or beads of water between the panes of a window, and wiping both sides changes nothing, the sealed glass unit in that sash has lost its edge seal.
It is one of the most common window complaints in Knoxville and East Tennessee, and it turns up most often in double-pane windows that have been in service for a while.
At MI Homesiding & Roofing, the first job is working out which problem you actually have, because three different things get described as a foggy window and only one of them is a failed seal.
For more than 20 years, I’ve worked on homes where the owner had already been told the whole window needed replacing. Sometimes that is right. Often the frame and the sash are sound and the failure is confined to the glass unit.
What we will not do is name the fix before we have seen how the glass is held in. That is a decision the window makes, not us, and it is not visible from the ground.
Verified credentials
Who you are actually hiring, and who stands behind the work.
BBB Accredited Business — A+ Rating
- TN #13334Licensed & Insured
Licensed Home Improvement Contractor
Tennessee license #13334 · General Liability and Workers’ Compensation insured
Knoxville Chamber Investor
Investor member of the Knoxville Chamber
What Is Actually Failing
A modern double-pane window is not one piece of glass. It is an insulated glass unit: two panes, sometimes three, held a fixed distance apart by a spacer that runs around the perimeter, with the gap between them sealed at the edge.
That sealed cavity is the entire point of the product. Keep it dry and the window insulates and stays clear. Let outside air into it and both of those stop being true.
Four parts do the work:
01
The panes. Two sheets of glass, often with a Low-E coating on one of the surfaces facing into the cavity. That coating stays protected there — until the seal lets go.
02
The spacer. The perimeter bar that holds the panes apart and carries the edge seal. Materials vary by product; a metal spacer conducts more heat than a warm-edge design.
03
The desiccant. A drying agent held inside the spacer. It absorbs the small amount of moisture sealed in at the factory, and later anything that gets past a seal that has started to fail.
04
The cavity. The sealed gap itself. Depending on the product it holds air, or an insulating gas such as argon. Not every unit is gas-filled, and you cannot tell which yours is by looking at it.
When the edge seal fails, outside air carrying moisture works into the cavity. The desiccant absorbs it until it is saturated. After that, moisture condenses on the inner faces of the glass whenever conditions favor it, which is why fogging usually comes and goes with the weather for a while before it settles in permanently.
Knoxville’s humid summers and wide temperature swings give that cycle plenty to work with, which is a fair part of why failed units are so common here in older double-pane windows. Age, sun exposure on a west or south elevation, and how the window was installed all feed into it as well.
Three Different Problems Get Called a Foggy Window
From across the room they look much the same. They have almost nothing to do with each other, and only one of them is a failed seal.
There is a test, and it costs nothing. Wipe the inside surface. Then wipe the outside surface. If the haze clears from either one, the seal is not your problem.
01
On the room-side surface. Indoor humidity meeting cold glass. It wipes off. The glass is doing its job; the air in the house is simply carrying more moisture than that surface temperature can hold. Showers, cooking and unvented appliances all add to it. This is a humidity and ventilation question, not a seal failure.
02
On the outside face. Most often seen on cool mornings, and usually not a defect at all. It can appear on glass that is insulating well, because the outer pane stays closer to the outdoor air temperature instead of being warmed from inside. It normally clears as the day warms.
03
Between the panes. The one this page is about. It will not wipe off from either side, because the moisture is sealed inside an assembly you cannot reach. That is a failed edge seal, and no amount of cleaning will touch it.
And a fourth thing that does not belong on that list: water running in at the sill, staining the trim, or wetting the wall below the opening. That is not a glass problem at all. It is water getting past the flashing or the perimeter seal, and it is found and repaired a different way — see window leak repair.
The two get confused constantly, so it is worth being blunt about it: a fogged glass unit is not evidence that water is entering your wall, and a wall leak is not fixed by changing the glass. A window can have either one, both, or neither, and each is confirmed on its own evidence.
Signs of a Failed Window Seal
A failed edge seal rarely announces itself all at once. It works through a sequence, and where your window sits in that sequence tells you roughly how long it has been going on.
Roughly in the order homeowners notice them:
haze or condensation appearing between the panes
moisture that will not wipe away from either side
fogging that comes and goes with the weather, then stops clearing
a cloudy or milky cast across the glass in certain light
white or rainbow-colored mineral streaking on the inner surface
one sash that looks visibly different from the rest of the house
That last pair matters more than it looks. Once mineral deposits have etched the inner surface, cleaning the cavity would not help even if you could reach it, because by then the haze is in the glass rather than on it. A unit caught while it is still fogging intermittently has more options open to it than one that has been left for several seasons.
Worth saying plainly: none of these symptoms, on their own, tell you what the repair is. They tell you the seal has gone. What can be done about it is decided by the window itself.
What Can Be Done About It
A failed seal is not one repair with one price. There are several directions, and which of them your window supports is decided by the window, not by whoever is quoting it.
These are the options homeowners are generally choosing between:
01
Replace the glass unit only. When the frame and opening are still in good condition, MI can replace the failed glass or insulated glass assembly without replacing the entire window frame, when the window design allows it. Whether yours allows it comes down to how the glass is held in, the condition of the sash, and whether a unit matching that product and size can still be obtained. More on this under window glass replacement.
02
Replace the sash. Where the glass cannot be separated from the sash, or the sash itself is in poor condition, the sash is changed and the frame in the wall stays. Whether a replacement sash exists depends on the manufacturer and the age of the window.
03
Replace the whole window. The right call when the frame is compromised, when several units in the same window have gone, or when the product is old enough that nothing matching is still made. See window replacement and installation.
04
Schedule it rather than rush it. A failed seal is not an emergency. One fogged unit on a shaded elevation, in a sound frame, is a reasonable thing to put on a plan instead of treating as urgent.
You may also be offered a moisture-removal or defogging service, which works on the appearance by clearing the cavity rather than by replacing the unit. It is worth asking the company offering it exactly what the result is expected to be and for how long, because clearing the haze is not the same as rebuilding a sealed assembly, and it does not put back a gas fill the unit may originally have had. Where the inner surface has already etched, appearance may not come fully back either. We mention it because homeowners run into it and deserve the question in advance.
Which of these applies to your window depends on how the glass is held in the sash, whether a matching unit can still be sourced for that product, and the condition of the frame and sash around it. Those are things we establish on site. We do not name a method before we have looked at the window, and we do not quote one over the phone.
When Replacement Is the Sensible Route
Sometimes the glass is not the limiting factor. Replacement becomes the sensible route when the parts around the glass are what is wrong, or when the product has reached the point where nothing matching is still available.
It tends to be the recommendation when:
01
the frame or the sash is damaged beyond a straightforward repair
02
several units in the same window have failed
03
no matching glass unit or sash can still be obtained for that product
04
rot or structural damage is found in the opening once the trim is off
What a failed seal does not tell you is that the window has structurally failed. A fogged unit in a sound frame is a failed component inside a window that is otherwise working. The frame can be square, the sash can operate properly and the opening can be dry. Those are separate questions and they get checked separately — which is the whole reason the inspection comes before the recommendation rather than after it.
If replacement is the route, the choice of product is its own subject: see energy-efficient windows for how the rated numbers on a window work and what they do and do not promise.
Why a Fogged Unit Is Worth Dealing With
A fogged unit is easy to live with and easy to postpone, which is why they often sit for years. It is worth being accurate about what it does and does not cost you.
What actually changes:
the unit no longer performs the way its original rating describes
that rating was tested on an intact sealed assembly, so it no longer applies to this one
visibility through the glass is reduced, permanently once the surface etches
on a front elevation it is noticeable from the street
on wood sashes, persistent moisture in the glazing pocket can affect the surrounding material over time
What we will not do is attach a number to it. How much difference a failed unit makes to a given house depends on how many have gone, where they are, what the rest of the envelope is doing and what you pay for energy. We know none of those until we look, and anyone quoting you a percentage before they have looked has not measured your house either.
What the Inspection Looks At
Before anyone says what the repair is, four things have to be established. None of them can be done from a photograph.
01
Which surface the moisture is on. Both faces get wiped, in front of you, before anything else is discussed. This one step rules out two of the three problems above.
02
How the glass is held in the sash. This is what decides whether the unit can come out on its own, and it is the single most important thing we look at.
03
What identifies the product. Spacer stamps, hardware and sash markings, where they are present, help establish whether a matching unit or sash can still be sourced.
04
The frame and the opening. Whether the sash operates, whether the perimeter is sound, and whether anything around the window is wet — which is a different problem with different evidence.
Only then do we say which of the options above your window supports. If what we find changes the picture — rot in the opening, previous repair work, moisture behind the trim — the Hidden Damage Policy further down governs what happens next, and nothing moves forward without a written Change Order.
Why Homeowners in Knoxville Choose MI Homesiding & Roofing
At MI Homesiding & Roofing we would rather give you an accurate answer than a quick one. Homeowners work directly with me, and every project starts with a careful look at the window and the structure around it.
What that looks like in practice:
01
the symptom gets identified before the repair gets named
02
repair and replacement are both put on the table, with the reasons for each
03
what we find behind the trim is documented with photos before anything proceeds
04
more than 20 years of hands-on construction and exterior remodeling experience behind the recommendation
We have helped homeowners across Knoxville sort out window problems caused by age, moisture and poor installation — and, often enough, told them the window they were about to replace did not need replacing.
Serving Knoxville and Nearby Communities
MI Homesiding & Roofing provides foggy window repair in Knoxville TN and the surrounding communities, including:
Farragut TN
Powell TN
Karns TN
Hardin Valley TN
Oak Ridge TN
If a window in your house has gone cloudy between the panes, we can tell you which of the options on this page it actually supports — and if the answer is that it does not need replacing, that is the answer you will get.
What a Foggy Glass Repair Carries
Fog between the panes means the sealed unit has failed. What that costs depends on whether the frame and sash are still serviceable.
What the written estimate itemizes. The number of openings and their sizes, the window line and series by name, the installation method for each opening — insert, full-frame or new-construction — flashing and weather barrier at every opening, interior and exterior finish, removal and disposal of the old units, and an allowance for what is found once the opening is bare. Where two window quotes differ sharply, the gap is almost always in the method and the flashing rather than in the glass.
Two warranties, kept separate. The manufacturer warrants the window; we warrant the installation. Our workmanship warranty runs five years on a full window installation or replacement and one year on a repair scope. Manufacturer coverage varies by product line and comes from the manufacturer, not from us.
Licensed Home Improvement Contractor (TN #13334) · City of Knoxville Building Contractor (#BC13334) · BBB Accredited with an A+ rating · Workers’ Compensation and General Liability carried · family owned and operated, bilingual English and Spanish · 20+ years of hands-on construction and exterior remodeling experience · Google Reviews.
Foggy Window FAQs
What causes condensation between window panes?
Condensation between the panes means the sealed glass unit has lost its edge seal. Outside air carrying moisture has worked into the sealed cavity, and once the drying agent inside the spacer is saturated, that moisture condenses on the inner faces of the glass. It cannot be wiped away because it is inside an assembly you cannot reach.
Is fog between the panes the same as a leaking window?
No. They are different problems with different evidence. Fog between the panes is a failure of the sealed glass unit itself. A leaking window is water getting past the flashing or the perimeter seal and reaching the sill, the trim or the wall below. A fogged unit on its own is not evidence that water is entering your wall, and a wall leak is not fixed by changing the glass.
Can foggy windows be repaired?
Often, but not always by the same method. When the frame and opening are still in good condition, MI can replace the failed glass or insulated glass assembly without replacing the entire window frame, when the window design allows it. On other windows the sash is replaced, and sometimes the whole window is the sensible route. Which one applies depends on how the glass is held in, whether a matching unit can still be obtained for that product, and the condition of the frame and sash around it.
Does every double-pane window have argon between the panes?
No. Some sealed units are filled with an insulating gas such as argon and some are sealed with air. It depends on the product, and it is not something you can tell by looking at the window. Any claim about what is in your particular unit should come from the product information for that window rather than from an assumption.
Why does the fog come and go?
Early in a seal failure the amount of moisture in the cavity is still small, so it only condenses when conditions favor it, usually on cool mornings or in direct sun. As more moisture gets in, the fogging appears more often and eventually stops clearing at all. A unit that fogs intermittently is generally earlier in the same process, not a different problem.
Does a failed seal mean the whole window has failed?
Not by itself. A fogged unit is a failed component inside a window that may otherwise be sound. The frame can be square, the sash can operate normally and the opening can be dry. Whether the rest of the window is in good condition is a separate question, and it gets checked separately.
My windows fog up on the inside of the room. Is that the same problem?
No. Moisture on the room-side surface wipes off, and it comes from indoor humidity meeting cold glass rather than from a failed seal. Showers, cooking and unvented appliances all add to it, so it is a humidity and ventilation question. The test is simple: if wiping the surface clears it, the seal is not your problem.
My windows fog on the outside in the morning. Is something wrong?
Usually not. Condensation on the outer face is most often seen on cool mornings, and it can appear on glass that is insulating well, because the outer pane stays closer to the outdoor air temperature instead of being warmed from inside. It normally clears as the day warms.
Do foggy windows affect energy efficiency?
The unit no longer performs the way its original rating describes, because that rating was tested on an intact sealed assembly. How much difference that makes to a particular house depends on how many units have failed, where they are, what the rest of the house is doing and what you pay for energy. We do not attach a percentage to it, because a number like that would have to assume all four.
Should foggy windows be replaced immediately?
Not always. A failed seal is not a safety emergency on its own. If it is one window, the frame is sound and the glass is intact, it is reasonable to put the work on a schedule rather than treat it as urgent. Cracked or loose glass is a different matter and should be looked at sooner.
Do you charge to look at a foggy window?
On-site estimates are free for standard projects. For complex inspections, such as active leaks, previous repair work, or conditions we cannot verify from the ground, we charge a $350 Technical Assessment, credited back in full if you move forward.
Hidden Damage Policy
If rotted framing, failed flashing, or moisture behind the trim is discovered during the project, we stop and document the problem with photos. We explain what we found and why it matters, then provide a written Change Order before moving forward.
The opening is inspected before a new window goes into it.
Ready to Get Started?
Tell us what you are seeing and we will take a look. Free estimates, clear pricing, and a licensed, insured crew that does what it says.