Understanding Efficient Windows

Selecting the right windows is a critical aspect of designing an energy-efficient building. Windows provide important solar heat gain, but they are also the least insulating part of the building envelope. For example, a highly insulated wall may have an R-value of 30, while a typical low-E window may only be R4. It is therefore important to understand the ‘net’ energy flows through a window, and how best to select the right window for the application.

Six elements of a window are important for high performance buildings:

1. Low-e coatings
2. Argon, Krypton, or Xenon gas fills
3. Multi-pane insulated glass
4. Insulated frames
5. Warm edge spacers
6. Casement style windows (or fixed)

Low-E Glass
Low-e coatings are not all the same! Low-e coatings make glass spectrally selective, allowing in much of the visible light and solar radiation, but reflecting long-wave heat. These coatings can also be tuned to let in more or less solar or visible light, but typically there is a tradeoff between a low U-value and high solar gain. Unfortunately most low-e products, in an attempt to get the most marketable U-value, also have a low solar heat gain co-efficient (SHGC).

In general, pyrolytic low-e or a “hard coating” provides more solar gain though somewhat less insulation, whereas soft-coatings provide more insulation but less solar gain.

Another unique product, Heat Mirror used by SeriousWindows, is a film with a ceramic low-E coating that is suspended between two panes of glass, not only providing low-e but performing like a triple-pane without the extra weight of glass. More importantly because heat mirror is thin and light weight, windows can be build with multiple Heat Mirror films allowing windows with 1, 2 or 3 suspended films. A heat mirror window with 3 films, is the energy performance equivalent of a 5 pane window with up to R-21 insulation value, all with the weight and look of a double pane.

Low-e coating position
The glass surface to which the low-E coating is applied affects the SHGC of the window assembly. If the low-E coating is applied to the inside glass surface of the indoor pane of glass, the window will have a higher SHGC than if it is placed on the outside pane of glass. It is important to get windows with a correct low-e coating placement, remember to specify your desired placement with your window supplier.

The glass surfaces are numbered starting from the outdoor surface (#1), counting to the indoor surface. In a double pane unit, the low-e coating can be applied to surface #2 or #3. Surface number #3 provides a higher SHGC, whereas surface #2 provides a lower SHGC. Similarly, with a triple pane surfaces you’ll want low-e on #3 and #5 for higher SHGC, and #2 and #4 for lower SHGC.

Insulating Gas
Argon gas increases a windows R value by up to 23%. It is an important part of an efficient window. Argon is an inert, non-toxic gas that that makes up about 1% of the atmospheric air we breathe. It is very heavy, and so doesn’t convect as readily, making the window more energy efficient. Argon is readily available and cheap, it should always be used in windows. Krypton and Xenon gases, other inert gases, provide even more insulation value that argon in a thinner window unit, but at somewhat higher cost.

About Capillary tubes
If windows are manufactured at substantially different altitude than the point of use, a manufacturer must put a capillary tube in the glass unit to equalize the pressure in the glass. If there is a capillary tube, all of the argon will leak out. If the manufacturing facility is more than 2000-4000 feet different elevation from the installation, it is likely a capillary tube will be mandatory and the window cannot include argon gas. Even if the manufacture is willing to seal the unit with argon, if the window is installed at a higher elevation than the point of manufacture, it will bow outward to compensate for the pressure difference, which may be architecturally unacceptable or put the window at risk for stress fracture.

The only company who can sell window with insulating regardless of elevation is SeriousWindow which uses a equalization balloon to ship their windows. If a salesperson for a company that ships windows in from out of state claims they don’t use capillary tubes, ask lots of questions to insure they are really sealing the windows without capillary tubes.

Will argon leak out?
No. A national lab study has shown that only 10% of argon leaks out of windows over a 20 year period in a well manufactured window. If a company doesn’t warrantee their argon be cautious, it may be an indication they use capillary tubes.

Troy Harvey
Heliocentric
Salt Lake City, Utah
(801) 453-4934