Fog between the panes means the insulated glass unit's seal has failed. Your double-pane window is a sealed sandwich — two panes, a spacer, an argon or air fill, and a perimeter seal — and once that seal cracks, humid outside air leaks in and condenses on the inner glass surfaces where no cloth can ever reach. The haze isn't dirt, and it can't be cleaned; the unit has to be replaced.
Why the Seal Fails — and Why Idaho Speeds It Up
The spacer bar between the panes is packed with desiccant, a drying agent that quietly absorbs small amounts of moisture sneaking past the seal for years. Fog appears only after the desiccant saturates — so by the time you see it, the seal has usually been leaking for a while.
What kills seals is thermal pumping: glass expands in heat and contracts in cold, flexing the seal thousands of times. The Treasure Valley is hard duty for it — high-desert summers routinely swing 30 degrees or more between a 100-degree afternoon and a cool night, cycling the seal daily in a way milder climates never do. West- and south-facing units fail first.
Replace the Glass or the Whole Window?
If the frames are newer, square, and from a quality manufacturer, a glass-unit-only swap is the smart money — you keep the frame and hardware and replace just the sealed unit, often under the manufacturer's glass warranty. But on 15-plus-year-old builder-grade windows, one fogged unit is rarely the last: its siblings share the same age, materials, and sun exposure. Paying repeatedly to re-glaze failing frames costs more over five years than replacing the windows once, and the new units bring modern Low-E coatings and U-factors the old frames never had.
Wood Windows Inc has been making that glass-versus-window call honestly for 46 years, and our in-house installers handle both. Fogged glass? Call (208) 376-2200 and we'll come measure it free.
Frequently Asked Questions
Can I clean the fog out of a double-pane window?
No. The moisture is trapped inside the sealed unit, between the two panes of glass. There's no way to reach it without cutting the unit apart, and drill-and-vent defogging services can't restore the insulating gas fill — the unit is done.
How long should a window seal last?
Quality insulated glass units typically hold their seal 15 to 25 years, and good manufacturers warrant the glass for 20 years or more. Cheap builder-grade units can start failing inside 10, especially on sun-baked west elevations.
Does a fogged window still insulate?
Poorly. Once the seal fails, the argon fill escapes and moisture takes its place, so the unit performs closer to a single layer of glass. You lose most of the insulating benefit you paid for, even before the fog gets ugly.
Sources & Testing Data
Last reviewed 2026-07-05 by Wood Windows Inc — (208) 376-2200