a fogged-up double-hung window
Photo: Keller & Keller
Shown: Foggy condensation between the panes of a double-glazed window indicates that the seal around the edges of the panes has failed.
Q: Although we have double-hung, double-glazed windows, last winter frost formed on the inside surface of the metal-framed storm windows. Is there any way to prevent this?
Jim and Sandy Garvey, Bemidji, Minn.

A: Tom Silva replies: I'm glad you two are thinking ahead. The best time to solve weatherization problems is before the weather turns cold.

The reason that frost forms on the inside of your storm windows is that warm, moist air from the house is leaking past the double-hungs, then freezing on the storms' cold metal frames. Here's what to do to reduce or stop this problem:

First, make sure the weatherstripping on your double-hungs is in good condition. If it isn't, replace it so that you have a tight seal around each sash.

Next, when winter arrives, lock your windows securely. Sash locks aren't just for security; they also pull the sash together at the meeting rails and push the top sash up and the bottom sash down, closing any air gaps.

Also, take steps to reduce indoor moisture levels by using exhaust fans when bathing and cooking. And in your climate, I'd consider buying interior storm windows. The ones I've used, made by Innerglass Window Systems, are barely noticeable but do a great job of stopping air movement, not to mention reducing heating costs.

One last thing: Check the bases of the storms where they meet the windowsills. They should be free of caulk and have unobstructed weep holes so that any frost that forms has a way to drain out when it thaws. Without that drainage, your sills could rot.
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