Q: We're about to spend a lot of money on replacement windows. The existing aluminum-framed, double-glazed windows are only about 15 years old, but the seals on two of them have failed and the glass has fogged up. We're considering wood-framed, double-glazed units with low-e glass. But is this plan worth the expense if the seals are going to fail in another 15 years? Our heating costs aren't really that bad.

– Barbara, Longbranch, Washington

A: Tom Silva replies: Moisture between the panes of a double-glazed window is a sure sign that the seal around the perimeter has broken. Fogging alone doesn't hurt the performance of the window substantially (unless it has a soft low-e coating), but it doesn't look good. Unfortunately, the only way to get rid of the fog is to replace the sash.

The question is: Do you really need double-glazed replacements? There's no way to predict how long a glass seal will last because so many things can make it fail, including poor installation, strain caused by building movement, high winds, and the careless use of a heat gun when stripping paint. Some manufacturers even warn of damage from pressure washers, window-cleaning chemicals, and sun-reflective window films. The best warranties on window seals these days are for 20 years. Considering your relatively mild climate, double-glazed low-e glass probably isn't worth the premium you'd pay.

If I were you, I'd buy single-glazed wood windows and protect them with good-quality wood or metal storm windows. (If you're anxious to keep your drapes and furniture from fading in the sun, both windows and storms can be fitted with single-pane, low-e glass, which will also give additional energy savings.) Together, single-glazed windows and storms, or single-glazed units with storm panels attached to the sash, provide energy performance comparable to a double-glazed unit at much less cost. And there's no chance of seal failures.
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