Double-pane windows are constructed with a layer of gas—usually argon or plain air—between the two glass layers that helps reduce heat loss through the windows. When these windows appear foggy or have condensation between the layers, it’s an indication that the seal on the window assembly has failed.
Many double-pane windows have a substance called silica pellets inside an aluminum perimeter strip to absorb moisture from incoming air trying to enter the panes. Over time as the sealant begins to fail, these pellets will become saturated and unable to absorb additional water vapor, thus causing condensation to form. When this occurs, it is time to either replace or repair the window.
Windows with a southern or western exposure are more prone to condensation problems because of more frequent expansion and contraction brought about by temperature changes, thus causing seals to fail more quickly. If condensation is allowed to continue, it will lead to irreversible damage necessitating replacement. Industry experts estimate that if you catch the problem early enough, you’ll only have to replace glazing about 75 percent of the time. Sometimes the sashes must also be replaced, but it’s rare to have to replace the entire assembly. Although some contractors claim that they can remove condensation by a process known as defogging, some window industry experts remain unconvinced about the effectiveness of this method and continue to recommend replacement of the glazing.
Before starting the repair, make sure you have the following tools at your disposal: hammer, caulk gun, screwdriver, drill, pry bar, putty knife, utility knife, gloves goggles, dust mask, and heat gun. Make sure your insulating replacement glass is also the correct side for the sash. Depending on the original sealing method, you may also need neutral cure silicone or glass setting tape.
Remove the affected panes from the sash. Windows with gasket seals are usually held in place by screws at each corner. Be careful not to damage the gaskets so you can reuse them. Some windows have double-face adhesive setting tape, usually with removal stops on one side. Use a putty knife to carefully slice through the tape and remove it. For windows with caulk seals, carefully pry off the stops with a putty knife or thin pry bar.
Be careful with insulating glass that is caulked into place as you may have to virtually smash it and then pry out the window piece by piece. Wood frames often crack when you try to remove glass. A heat gun, set on low, can soften the adhesive and help avoid this problem. Set the new glass into place, replace stops, and reseal with the appropriate sealing method.