In our business this is called an IG or insulated glass failure. This happens when the rubber seal in a double paned window cracks. The crack will let in moisture from the air. The moisture then condensates in between the 2 panes of glass. Over time the condensation builds up and forms droplets of water inside. Then those droplets can and will eventually dry out leaving a foggy or hazy residue inside the window pane. Most likely the affected window is either an east or west facing window. Morning sun or afternoon sun. (The sun is what dries out and cracks the rubber seal over time). There is no remedy for this other than replacement. That’s the bad news. The good news is that most major window manufacturers offer lifetime warranties for such defects. So if you are experiencing fogging issues inside your windows call your window manufacturer and explain the issue. Good news could be just a phone call away.