Condensation Between Window Panes: Causes & Fixes

Why fog appears between the panes

A double- or triple-pane window isn't one piece of glass — it's two or three panes bonded around the edges into a sealed unit, with dry air or an inert gas (usually argon or krypton) trapped between them for insulation. That edge seal is what fails. Once it does, outside humidity works its way into the gap, and because there's no way to clean the inside of a sealed unit, any moisture that gets in just sits there — fogging up in humid or temperature-swinging weather and sometimes clearing on dry days, which is a telltale sign it's seal failure and not a surface smudge.

Seal failure usually comes down to age (most insulated glass units are only warrantied for 10–20 years), UV and thermal cycling breaking down the sealant over time, or a manufacturing defect that shows up early. It has nothing to do with how clean you keep the window — no amount of washing the outside will fix fog that's trapped inside the unit.

What you can actually do about it

• Check your warranty first. Many window manufacturers warranty the insulated glass unit itself for 10–20 years (sometimes longer) separately from the whole-window warranty — a failed seal within that window can mean a free or discounted glass replacement.
• Glass-only replacement. If the frame and hardware are still in good shape, a qualified installer can often replace just the failed insulated glass unit rather than the whole window — usually cheaper than full replacement, when it's an option for your window's brand and age.
• Full window replacement. For older windows, obsolete glass sizes, or frames that are failing along with the seal, replacing the whole window is often the more practical (and sometimes cheaper) option than sourcing a discontinued glass unit.
• "Defogging" services. Some companies offer to drill the glass, vacuum out the moisture, and reseal it. It's cheaper than replacement, but it's a temporary fix — the original seal is still compromised, and fog commonly returns within a year or two.

Pro tip from our network contractors

Before paying for anything, find your original purchase paperwork or the manufacturer sticker on the window frame — a failed seal well within a 20-year IGU warranty is often a straightforward warranty claim, not an out-of-pocket repair.

Is it worth fixing, or time to replace?

A single fogged window in an otherwise healthy set is usually worth repairing or glass-swapping. But if you're seeing seal failures on multiple windows around the same age, that's a sign the whole batch is reaching end of life — and it may be more cost-effective to plan a full replacement project than to keep fixing one window at a time. See signs you need new windows and are new windows worth it for the bigger-picture math.