condensation between window panes

https://aged-wiki.win/index.php/How_Argon-Filled_Windows_Improve_Home_Comfort_and_Energy_Bills

That foggy buildup trapped between the layers of a double-pane window happens when the seal breaks and warm, moist indoor air seeps in, then condenses on the cooler glass as the seasons change. Once that moisture collects, it usually blurs the view and can lead to mildew or mineral stains that are nearly impossible to wipe away from the inside.