Do Fogged Double-Paned Windows Indicate Heat Loss?

How They Work

• Double-pane windows use the properties of gases to reduce energy loss. Glass is a poor insulator, while gases are much better. A double-pane window is a sort of sandwich: two layers of glass with about half an inch of gas in the "dead space" between them. High-end windows have noble gases like argon between the panes; less expensive windows usually fill the space with air. The gas in the space retards heat radiating through the first glass pane, allowing the second pane to remain cooler. In winter, this reduces heat loss through windows; in summer it reduces solar heating through the glass. Some double-pane windows use coatings to reduce heat transfer further, and many companies now offer even more energy-efficient triple-pane windows.

Window Construction

• For the "sandwich" to work, the gas must be tightly sealed between the glass panes. The two sheets sit in a metal or vinyl frame with strips of sealant both inside and out. The seal between the panes contains tiny beads of silica gel desiccant, the same moisture-absorbent material found in small bags in many consumer products.

Seal Failure

• Over time, daily warm-cool cycles cause microscopic cracks to form in the seals. These cracks allow water vapor-bearing outside air to contaminate the sealed airspace. The silica gel beads in the inner seal adsorb moisture that penetrates the outer seals, keeping the air space dry until they become saturated. When the desiccant can no longer perform, water vapor can condense on the glass within the window.

Condensation

• If a window seems fogged, determine which glass surface is wet. A window with condensation or fogging between the glass panes has failed. Not only can moisture within the air space block light, but moist air also transmits heat more readily than dry air. This means that the window has lost some of its insulating capability. Condensation on the room side of a double-pane window, especially on cold days, does not indicate failure of the seals; it simply means the humidity inside the house is high. Condensation on this surface could eventually damage the seal on the inside of the window, so it should be avoided as well. Reducing humidity can be accomplished with a dehumidifier or by proper venting of inside air.

Replace or Repair

• Homeowners have three options: Replace failed panes, replace an entire window or repair a failed pane by "defogging" it. Window glass companies can replace a failed pane for a fraction of the cost of a replacement window, but if seal failure was caused by damage to the frame, you may have to replace the entire window. Defogging sounds like a bargain, at a cost of one-third to one-half the price of replacement glass. Window defogging companies drill small holes in the glass, treat the interior of the glass to remove moisture and install one-way valves to keep air out. The process may be only temporary, however, and homeowners who have used it seem equally divided between happy and unsatisfied.