Foggy windows with condensation between glass panes means seal failure. Learn why it happens and why replacement is the only real fix.
If you see fog, moisture, or a milky haze between the glass panes of your double or triple-pane windows — not on the inside surface or outside surface, but BETWEEN the panes — you have seal failure. Here's what's happening and what to do about it.
Why It Happens
Double and triple-pane windows have an airtight seal around the edges that keeps argon gas (an insulating gas) trapped between the panes. Over time — typically 15-25 years — this seal breaks down from UV exposure, temperature cycling, and material fatigue. Once the seal fails, the argon gas escapes and humid outside air enters the space between the panes. When temperature changes cause that moisture to condense, you see the fog.
Can You Fix It Without Replacing the Window?
Some companies offer "defogging" services that drill small holes in the glass, flush out the moisture, and reseal. While this temporarily removes the visible fog, it doesn't restore the argon gas or the original insulating value, and the seal failure means moisture will return. It's a band-aid, not a fix. The window is still functioning as a single-pane window from an energy perspective.
The Real Solution
Replacement is the only permanent fix for seal failure. The good news: if you're seeing fog in some windows, the others are likely approaching the same age and will fail soon. This is often a good time to consider replacing all windows at once rather than one at a time — it's more cost-effective and ensures uniform performance across your home. Is it cheaper to replace all windows at once? →
Modern Windows Last Longer
Today's premium windows (like the Andersen products we install) use advanced seal technologies, better spacer systems, and UV-resistant materials that significantly extend seal life compared to windows manufactured 20+ years ago. Learn about our Andersen Certified window installation →
Repair or replace: how to decide
Some window problems are repairs. A failed seal in one otherwise sound window can sometimes be fixed by replacing the glass unit. Worn weatherstripping and a failed bead of exterior caulk are cheap to redo. Replacement makes sense when the frame or sill is soft, when several windows are failing at once, or when the windows are single-pane or early double-pane units with no Low-E coating. A short inspection tells you which you have.
Check these before blaming the window
Drafts and water near a window do not always come from the window. Air can leak through the gap between the frame and the wall framing, hidden behind the trim. Water can enter above, at missing head flashing or a siding joint, and run down inside the wall. High indoor humidity causes condensation on the room side of perfectly good glass. We look at all of these so you do not replace a window that was not the cause.
What a good estimate includes
A window estimate should name the manufacturer and product line, list every opening with its size and style, state the glass package, say whether each is an insert or full-frame installation, and cover trim, disposal and any permit. It should also say what happens if rot is found. If a quote gives only a total and a brand, ask for the detail before comparing it with another.
Common questions
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