Fogged windows can be a real... nuisance (no bad puns today). Fogged glass makes windows look constantly dirty, and some windows get so cloudy that you can't even see through them. These windows will also have a lower insulating value, but not by much. While replacing the window is usually most people's first instinct, this isn't always the best approach. A couple other options would to replace just the pane or have the window serviced to make it look clean again.