Low-emissivity coatings were a quantum leap in window technology. Thanks to this innovation, the average double-pane window is now twice as resistant to cold or heat as it was in 1980. Even so, windows remain gaping energy wounds. For small projects, the ticket is to upgrade with windows that are tuned to climate and orientation — and install them properly.
Tuning windows takes effort. It means selecting different windows for different orientations. When a remodel involves a change in footprint, the prescription is two-fold: optimize windows for passive heating, cooling and lighting, and — here's the hard part — minimize their use for other purposes.A look into the crystal ball
Robert Clarke founded Boulder-based Alpen Energy in 1981, on the heels of Low-E glass. In 2007, BuildingGreen.com  selected his windows as a "Top-10 Product." In the table below, Clarke describes windows of today and tomorrow.
|Window Component||State-of-the-Art||The Future|
|Glazing||Ultra-thin metallic coatings on glass/films via vacuum sputtering||Dynamic coatings that change properties in response to light, heat or the turn of a dial|
|Glazing package||Sandwich of 2 Low-E panes and clear film filled with argon, krypton or zenon gas; 99.5% UV blockage; R-14||Incremental improvements to achieve R-20 (same as 2 by 6 wall)|
|Window frame||Pultrusion fiberglass frame with hollow or foam core; R-4 to R-8||R-10 performance via deeper windows for 2 by 6 framing, better seal between frame and sash and aerogel core|
|Flange to wall connection||Enhanced connection geometry, molded returns and zero expansion foams||½-inch R.O. gives way to computer matching of windows to SIP panels and tight, pre-engineered Z flanges|
Colorado green architect John Van Doren notes, "Most 'high-quality' windows in the U.S. are fashion statements, falling far short of the performance of European and Canadian windows." Recognizing this, the Department of Energy is revising its Energy Star  ratings. Among the changes that could go into effect in 2009:
Meanwhile, Van Doren says, get the lowest U-factor possible. His selection process starts with these rules of thumb on SHGC:
Good overall window sites: