A drawback of double-pane or insulated windows is that the seals that create the pocket of air or gas between panes can fail. When they do, moisture can seep in. When that moisture condenses, it fogs the glass – on the inside, where you can't wipe it off. This problem, which generally occurs in older windows and less expensive models, can't be repaired. However, there could be a more affordable fix than replacing the entire window.
Glass is a poor insulator, so in otherwise well-insulated homes windows can be the weakest links in terms of energy efficiency. Double-pane or insulated windows improve efficiency and can greatly increase your family's comfort. Newer, high-quality windows feature two perimeter seals – an inner seal designed to resist corrosion, aging and water infiltration and an outer one that improves overall window strength. Windows with just one seal are more likely to fail – or fail sooner. Double-pane windows also feature a spacer, usually a hollow aluminum tube filled with desiccant that takes up moisture and prevent fogging. But once that desiccant is saturated, the window will fog.
Older, low-quality or poorly installed double-pane windows are more likely to fail, according to The Family Handyman website. Most window seal failure – and window fogging – occurs because of seal breakdown and water puddling or other major moisture exposure. Intense heat, often because of too much direct sun, could be the actual cause, in many cases. Heat softens or weakens double-pane window seals, which leads to later moisture penetration and fogging. Age will do it too. Eventually, window seals lose their elasticity and leaks develop.
Replacing windows is the accepted wisdom once double-pane window seals give out. And it's true that rebuilding damaged windows -- removing, cleaning and resealing old panes – would be a painful task and not very cost-effective. In some cases, window glass has become etched and needs replacing. The Family Handyman recommends buying top-quality windows with a long – even lifetime – warranty to defend yourself against future seal failure and window fog.
A much less expensive option is to replace the glass panes, if there's a company in your area that offers this service. With this approach, your window frames remain in place, the old glass is removed and new, sealed glass and spacers are installed. You can even upgrade to Argon, low-E, grids or specialty glass. Accurate glass measurement is essential.