Why Is My Window Fogging Up Between the Panes?

What it means (and how it’s usually fixed) 

Fog or condensation between panes is usually a sign of a failed window seal in your insulated glass unit (IGU). Once that seal breaks, moisture enters the airspace, and the insulating gas can leak out, reducing efficiency.

Source: Tom's Guide+1 

Why the fog sometimes “disappears” 

Fogging can come and go with temperature and humidity changes, but the underlying seal failure remains.

Source: Tom's Guide+1 

What fixes it? 

In most cases, the proper fix is replacing the IGU (the sealed glass unit), not replacing “just one side” of the glass.

Source: Glass Doctor 

If you’re in Chelan County, Wenatchee Valley Glass can inspect the window, confirm the cause, and recommend the most cost-effective repair approach.