Ask Kevin: Window coatings and passive solar heating

Hi Kevin,

We're building new, and will be putting in vinyl windows (I know, bad environmental decision, but it's the wallet talking right now), and were wondering if the "Suncoat" on Milgard windows will actually interfere significantly with the passive solar heating that we're seeking here in Alaska? Our home will face south and, in the winter, could collect quite a lot of warmth through the windows. What would you recommend?
Jane

Thanks Jane,

The short answer is YES, Suncoat will NOT give you the passive solar gain that you're looking for in your new home. However, the solution is a bit more complex. There are basically two types of low-e glass—low solar heat gain glass, which keeps the energy from the sun out but also keeps longer wave energy within the home from escaping, and high solar heat gain glass, which keeps the longer wave energy in but also lets in the sun's energy for natural, free solar heating. The problem is that, due to the bias in favor of cooling found in our codes and with Energy Star, the U.S. window industry has all but abandoned offering high solar heat gain, low-e glass as an option. (To better understand this issue you might want to read this.)

Milgard is not really any different than the rest of the U.S. manufacturers in that regard. Here is what you'd be giving up: A dual-pane Suncoat window has a center of glass solar heat gain coefficient (SHGC) of 0.36 and a winter U-value with an argon fill of 0.29. That means that it is rejecting 64 percent of the solar energy. By comparison, a high solar heat gain low-e dual pane Energy Advantage system from Pilkington Glass has a center of glass SHGC of 0.70 and a winter U-value with argon fill of 0.29. So basically you can get the same insulation value but with the ability to let in TWICE the amount of free solar energy for your passive design.

So here's my recommendation: First, because of wallet issues, if you're going to use any vinyl in a home, windows are probably the best and only place to make that compromise. However, I recommend you find a manufacturer that provides foam filled vinyl frames. This will greatly improve the overall window performance. Second, you can ask the manufacturer to substitute a high solar heat gain glass for their standard offering; however, in talking with several manufacturers I have found that only Marvin has been willing to do that, and they only make wooden windows. If you can't find anyone to substitute the glass you need, have a window fabricator near you fabricate insulated glass units (IGU without frames) to your specification and then frame them directly into your south facing wall. Depending on the window size and wind loads in your area, a reasonable specification for the IGUs would be 1/8" float glass outer pane, 1/8" Pilkington Energy Advantage glass on the inner pane with the low-e coating on the third surface, 1/2" argon filled space between panes and a non-metallic warm edge spacer.

All the best with your project!

John Van Doren

image sxc.hu, carlos freitas