Thermal windows are constructed of multiple panes of glass held in place by a metal, vinyl or wood frame. The area where the glass meets the frame is sealed to prevent air and moisture from seeping into the spaces between the panes. When this seal fails, the window loses much of its insulating value and may develop leaks that allow warm air to escape and cold air to seep in.
Windows are greatly affected by all types of weather, from the drying effects of a hot summer to the alternate freezing and thawing that occurs during the winter months. Unlike other building features, windows are the surest exit route for the heat you're trying to keep inside. The rate at which this occurs is called the level of emissivity, which is how quickly a window radiates heat outwardly, and up to 90 percent of a room's heat can be lost through a window. Add to that a thermal seal that has separated from the glass, and you might as well leave the window wide open during the winter.
To offset this, many windows are now composed of several layers of glass that significantly reduce emissivity. Gas pumped in between them creates an extra barrier between the glass and the elements. The window's framing material also plays an important role in keeping the glass panes tightly bound and preventing air from entering or escaping. A broken seal allows air in, and that trapped air results in condensation that is visible between the panes. No thermal window is completely condensation-proof, but the fact that the steam and water droplets are visible may mean that your windows should be repaired or replaced. Silica gel is added to the outside metal frames of some thermal windows. It absorbs moisture that eventually accumulates in the bottom of the window. As the seal deteriorates, the gel becomes saturated with moisture that translates to a thick fog or snowflake-like deposits on the inside of the glass.
What to Look For
Inspect your thermal windows often and particularly before and after the winter heating season. Check the framework as well as the caulk or putty used to create the seal between it and the glass for peeling, warping, flaking or chipping. You should not be able to slip even a piece of paper into a crack in the seal, as that small a gap translates into a lot of wasted heat. Inspect the window frame carefully for loose sections and note if the glass wiggles in the frame. Carefully holding a lighted candle at different areas around a thermal window frame will show you where the problems are.
Energy Loss and Repairs
Condensation is annoying and unsightly, but heat loss is a more serious problem when it comes to compromised thermal window seals. For the do-it-yourselfer, there are several ways to repair a damaged window seal without having to replace the whole unit. You can easily remove many of today's newer windows and replace a single section. You can also drill small holes in the glass, wash the inside, replace it, and install vents in the holes to keep the window from fogging up again. This process is inexpensive, but it raises the window's emissivity rating. On older double-paned windows, trim off the old caulking and remove one of the panes. Clean both panes, apply new caulking to the inside of the outer pane and replace the inside pane, applying new caulk to it as well.
- Window Defoggers: Why Do Thermal Pane Windows Fail?
- Kansas State University Engineering Extension: Windows and Doors
- Window Medics: Why Windows Fail -- Window Condensation
- Thermal Windows: What is a Thermal Barrier?
- National Fenestration Rating Council: The Facts About Windows & Heat Loss
- International Association of Certified Home Inspectors: Condensation in Double-Paned Windows
- Structure Tech: Foggy Glass? You Don't Need a New Window
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