Insulated Storm Windows?

Can top energy performance be achieved by combining fairly standard windows with really good storm windows or even a second set of prime windows?

A low-e storm window at my colleague Peter Yost's house in Brattleboro. Click on image to enlarge.Photo Credit: Peter Yost

I’ve done a lot of digging into window options in the past few months—not only for a special report on windows that BuildingGreen published, but also for the renovation of the early-19^th-Century farmhouse that my wife and I recently purchased.

The state-of-the-art with windows in terms of energy performance and quality is clearly seen in the triple-glazed European windows that are certified by the Passivhaus Institut in Germany. You can now buy these wonderful windows with unit center-of-glass R-values above R-9 yet high-enough light transmission to work well for passive solar houses (solar heat gain coefficient above 0.60).

The problem is that these windows are incredibly expensive—some over $100 per square foot, which comes to $1,500 for a typical 3' x 5' window. You get a lot for that price in the way of top-quality materials, construction detailing, durability, thermal breaks, air tightness, and energy performance, but the windows are simply way above the budget range for most projects. Furthermore, replacing existing windows is often very hard to justify unless the existing windows are in very poor shape.

Insulated-glass storm windows?

As I’ve thought about ways to obtain top-performance windows more affordably, I keep coming back to the idea of installing (or keeping) fairly standard, double-glazed windows—with low-e coating and argon gas-fill—and then installing a much-better-than-usual storm windows.

I’m considering three different options. The first option would be to install a single-glazed, low-e storm window on the exterior of the prime window. The storm would have to have a durable (hard-coat) low-e coating, since it wouldn’t be protected in a sealed air space, as most low-e coatings are. With this configuration I would end up with triple glazing and two low-e coatings in the full assembly. I’m not sure whether these would be old-style storm windows that are installed and removed seasonally, or more sophisticated (and convenient) triple-track or double-track storms with screens.

A second option would involve an insulated (double-glazed) storm window, so that I would end up with quadruple glazing—two insulating glass units (IGUs) separated by an air space. Because no storm window manufacturer (yet) makes such a product, this would likely necessitate custom-made storm windows. I know those can be made, because I’ve seem insulated-glass, low-e storm windows that J.S. Benson Woodworking & Design has produced in the shop next-door to our BuildingGreen office in Brattleboro.

A third option would be to install two sets of insulated-glass prime windows. I’m still working through how this would work—and very much open to suggestions. I could install a double-hung window on the interior and a casement window on the exterior, or I could install two sets of double-hung windows.

I’ll have to think about cleanability of windows with these options; that might be a little challenging with the two prime windows. If I’m going to have two low-e coatings in the window assembly, I also need to figure out whether the temperature reached in the interior IGU might get too high. By trapping a lot of heat it’s possible that the temperature reached by the glazing seal could be higher than the sealants are rated for—in which case I might need to find a window made using silicone seals (as I believe Andersen uses).

If you have experience with such a detail (particularly options two and three), let me know what you did. Or let me know what your thoughts are even if you haven’t tried it. You can post a comment here and share your experience with others or, if you prefer, e-mail me directly.

Looking for new storm windows? Check out our GreenSpec guide to help you find storm windows.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.

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