Fog Between Double-Pane Windows: A Failed Seal

• What’s happening inside the glass
• Inside the gap or on the room side? A 30-second test
• What fixes it (and what doesn’t)
• Does it matter for the rest of the house?

Most of this site is about water on the room side of the glass — a humidity problem you can manage. Fog between the two panes is a different thing entirely, and no amount of fan time or dehumidifying will touch it.

§What’s happening inside the glass

A double-pane window is an insulated glass unit (IGU): two panes held apart by a spacer and sealed around the edge, with air or argon in the gap. InterNACHI explains that the spacer holds silica desiccant to soak up any stray moisture. As the edge seal fails over time, moist air gets in, the desiccant saturates, and condensation starts forming inside the gap. Glazing firms describe the same sequence, adding that the insulating gas escapes once the seal is broken.

§Inside the gap or on the room side? A 30-second test

• Wipe both faces. Room-side condensation wipes off; seal-failure fog sits in the sealed gap and can’t be reached.
• Watch it over a day. Room-side condensation changes quickly with temperature and humidity. Seal-failure haze may come and go but never fully clears.
• Look at the edges. Failure often starts near the spacer before it spreads, and older failures leave mineral streaks or a milky film.

§What fixes it (and what doesn’t)

A failed IGU can’t be resealed in place. The usual remedy is replacing the glass unit only; if the frame and sash are sound, the whole window rarely needs replacing. A glazier measures the unit and orders a replacement to fit the existing frame.

“Defogging” services drill the unit, dry it, and plug the holes. InterNACHI notes there’s debate over whether this is a real repair: it removes the visible water, but it doesn’t fix the seal and may not restore any insulating value. It’s a cosmetic improvement.

§Does it matter for the rest of the house?

A blown unit insulates worse, so its room-side surface can run colder. That can mean more ordinary condensation on that pane on cold mornings. If one window streams while others stay clear, check it for between-pane fog before blaming household humidity. If the windows are fairly new, check the manufacturer’s warranty on the sealed units before paying for replacements.

Sources: InterNACHI, Condensation in Double-Paned Windows; Gloria Glass, Insulated glass unit failure; KJM Group, Misted double glazing.

Originally published at https://dry-glass-notes.surge.sh/articles/fog-between-double-pane-windows-failed-seal.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.