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A fogged window is a sealed double pane unit that has failed. The two panes have a gas between them for insulation, the seal keeps it in, and when the seal goes the haze forms on the inside surfaces where you cannot reach it.
You cannot clean it off. The haze is between the panes. Once a unit fogs, the glass has to be replaced, though the frame usually stays, which keeps the cost down.
If one window has fogged, the others of the same age are usually close behind. It is worth checking them all rather than doing them one at a time as each one goes.
