I have double-paned windows and some of them have a year-round fog inside. It lets the light in, but I can't see through the glass. I suspect there was a problem with the original installation or construction. Is it possible to clear up the fog? It's not weather-related and I can't clean it off because it's between the panes because the affected area is sealed shut. Only upstairs windows are affected, so it's not too much of a problem, but it sure is annoying!