Dual-pane windows are common in houses and offices because of their improved insulation. They work by having two panes of glass separated by about a half-inch of space between the two panes, with the trapped air acting as an additional insulator. If the seal on any of the panes of glass fails, fog can build up between the windows, caused by the moist air. While normal fog moisture can be wiped off the surface, getting rid of fog within the windows requires some additional work.
Clean the outer surface of the glass windows to ensure that the foggy problem is not simply something on the surface due to moisture. Spray a light layer of glass cleaner on the outer panes of glass and wad up some old newspaper to work the mixture across the glass. Avoid using rags or paper towels, which will streak the glass. Replace the newspaper often to avoid letting the scrap pieces get too wet with glass cleaner, thus streaking the surface.
Remove the insulating pane of glass on your own. Cut out the seal or glaze around the edge of the outer panel of glass, known as the insulating panel, with the utility knife and putty knife, starting on the bottom and working your way up. Repeat the process for either panes of glass. Cut them free from the sash, remove them carefully and wipe down all sides of the two panes of glass with window cleaner and newspaper. Reinstall the glass pane and reseal the glass in place using silicone window caulking.
Replace the silica spacer bars that separate the two windows from each other. Over time, the silica soaks up moisture and becomes so saturated that it can no longer work, at which point it needs to be replaced. Check the silica spacer bars when removing the insulating pane of glass on the exterior of the window, and replace if necessary. The average spacer bar lasts for five years.