Window Fogging

Window fogging is the common term homeowners use for the milky, hazy, or dirty-looking condition between the glass panes of a double- or triple-pane window. Unlike surface condensation (which wipes off), fogging is trapped inside the sealed cavity and cannot be removed by cleaning. It is always a sign that the window’s insulating glass unit seal has failed.

What Causes the Fog?

After the seal fails, outdoor air enters the cavity. That air contains water vapor. When the window cools (at night or in cold weather), the dew point inside the cavity is reached and water condenses on the glass interior surfaces. Initially, the desiccant in the spacer bar absorbs this moisture. Once the desiccant is saturated, moisture remains in vapor or liquid form inside the unit — creating the visible fog. The minerals and dissolved solids left behind by repeatedly condensed and evaporated water can also leave a permanent white haze or scale on the interior glass surfaces.

Fogging vs. Surface Condensation

Does Fogging Affect Energy Bills?

Yes. When the seal fails, the argon or krypton insulating gas escapes and is replaced by regular air. The window’s U-factor worsens (more heat loss in winter). A fogged window with a failed seal may have a U-factor 30–50% higher than when new — increasing your heating costs noticeably if multiple windows are affected.

The Only Real Fix

IGU replacement — removing the failed glass pack and installing a new factory-sealed unit — is the only way to permanently restore a fogged window’s appearance and thermal performance. Cost: $150–$350 per unit for most residential windows.