· by Michael Blair, Nashville's Windows & Doors
Fog trapped between two panes is a failed insulated glass unit. The perimeter seal has broken, the inert gas has escaped, and moist air has taken its place. It cannot be cleaned, dried out or reversed.
Why do window seals fail in Middle Tennessee?
Middle Tennessee gives glass a large daily temperature swing, particularly on south and west elevations. The panes expand and contract against the seal every day, and after fifteen or twenty years it fatigues.
Units in direct sun fail first, which is why one elevation often goes while the rest of the house is fine.
What it actually costs you
The insulating value drops substantially once the gas fill is gone, so those windows conduct more heat in a Nashville August and more cold in January.
There is also a slow cosmetic decline. Mineral deposits from repeated condensation etch the inner surfaces, so a unit left for years develops permanent cloudiness even before you look at it in sunlight.
Should you replace the glass unit or the whole window?
If the frame is sound and the hardware works, replacing just the insulated glass unit is far cheaper than replacing the window. It is a standard job for a glass company and needs no exterior work.
Whole-window replacement makes sense when the frames are rotting, the sashes no longer seal, the hardware has failed, or enough units have failed that the labor of doing them individually approaches the cost of new windows.
- Frame sound and hardware good: replace the glass unit
- Rot at the sill or sash: replace the window
- Many units failed at once: price both ways
- Original single glazing: this is a different decision entirely
What to check before ordering
Whether the original manufacturer is still in business and whether the units carry a warranty. Insulated glass warranties of ten to twenty years are common and a surprising number of homeowners never claim.
Also confirm the glass specification. Replacing a low-E unit with plain glass saves a little and costs a great deal in a Middle Tennessee summer on a west-facing window.
FAQ
Why Your Double Glazing Has Gone Foggy — questions
Can foggy windows be repaired without replacing the glass?
There are defogging services that drill the unit, dry it and fit a vent. It removes the visible fog but does not restore the gas fill or the sealed cavity, so the insulating performance stays reduced and the unit remains vented to outside air. It is a cosmetic fix rather than a repair.
Is a foggy window an emergency?
No. It is an efficiency and appearance problem rather than a safety or weather-tightness one — the window still keeps water out. It is worth addressing before mineral etching becomes permanent, but there is no urgency measured in days.
How long should double glazing last in Tennessee?
Fifteen to twenty years for the sealed unit, at the shorter end on south and west elevations where the daily temperature swing is greatest. Quality of the original spacer and seal matters more than the frame material, which is why two windows in the same house can differ by a decade.
Does a window warranty cover fog between the panes?
Often, if it is still in term. Sealed glass unit warranties of ten to twenty years are common, and many homeowners never claim on them. Find out whether the original manufacturer is still in business, dig out the paperwork, and make sure any replacement unit matches the original glass, low-E coating included.