Extreme windows

Going low on windows
The most important measure to look at when choosing windows is the U-factor (their rate of heat loss)—the lower, the better. California Title 24 requires a U-factor of only 0.40 (Btu/hr-ft2-F), but windows with factors ranging between 0.28 and 0.34 are available from all major manufacturers. These windows typically have wood or vinyl frames (vinyl is cheaper), are double glazed with an argon gas gap, have low-e glass coatings, and are usually the best bang for the buck.

If your goal is maximum energy efficiency, you might choose windows with insulated fiberglass frames, krypton gas gaps, and additional coated glass panes. For example, triple-glazed, krypton gas, insulated-frame windows have U-factors as low as 0.14 and will dramatically reduce condensation. These windows cost about 30 percent more than double-glazed argon types and will not be cost-effective unless you are also super-insulating your walls to R-30 or better.

Cutting edge
Not yet satisfied and want more? If you’re considering an extreme window makeover, take a look at aerogel windows and skylights. Aerogel is a transparent super-insulator of porous silica that is sandwiched between glass panes and partially evacuated to achieve amazing properties.

They can be designed with either a small or large solar heat gain coefficient (SHGC) and U-factor as low as 0.05 (this equals R-20!). To put this into perspective, a single-pane, wood-framed window is about an R-1. While aerogels are generally used for commercial building daylighting, recent improvements in transparency will make them suitable for more window applications.

Perhaps the most innovative new window product for colder climates is heated glass from Thermique Technologies (www.thermiquetechnologies.com). A thin, transparent layer of metal oxide deposited on the glass transforms these windows into electrical conductors that radiate heat into the home at the flip of a switch.

In addition to eliminating cold drafts and condensation problems, the manufacturer claims that Thermique heated glass can lower a building’s overall energy use. I’m skeptical of this claim, but admit that there may be locations in a home where this technology is appropriate.

Believe it or not, there are even “smart” windows that can change their transparency in response to outside conditions. Better known as electrochromic windows, these windows change from a clear state to a tinted state when a small voltage is applied to special metal oxide films on the glass panes.

Usually, they are programmed to change from one state to another as the solar intensity varies and can be integrated with HVAC and lighting control systems. In this way, the window can be tailored to specific climate zones and building orientations. This feature avoids the typical trade-off between choosing windows with high SHGC to increase wintertime solar gain, or windows with low SHGC to reduce summer air-conditioning requirements. For more info, check out Sage Electrochromics at www.sage-ec.com.

For now …
Aerogel, heated glass and electrochromic windows are not quite ready for mainstream residential applications. In the near green future, however, they are certain to become more available and cost effective. For selecting conventional windows, check out this excellent resource from the Efficient Windows Collaborative: www.efficientwindows.org/index.cfm.