Operability & window seals. Operable windows work either by sliding in a track (double-hung, single-hung, or horizontal-sliding) or by being hinge-mounted (casement and awning windows). The latter tends to have less air leakage, because compression gaskets can be used—as the window is closed, a gasket pushes against the frame for an excellent air seal.
Double-hung windows have a lower sash that can be raised with an upper sash that can be lowered, and are the most popular windows in the United States. Single-hung windows typically have a fixed upper sash and an operable lower sash—with roughly half the crack, airtightness is improved. Improvements in design and better weatherstripping make today’s double- and single-hung windows much better than they used to be—and, in some cases, almost as good as the best casement or awning windows.
Casement windows manufactured in the United States typically open outward, while European casement windows more commonly open inward. The “tilt-turn” designs on high-end European windows (and some U.S.-made products) can open inward with side-mounted hinges or can tilt down with bottom-mounted hinges. Most have multiple closure catches for excellent air sealing. Whether casement windows open inward or outward affects the type of window accessories that can be used, such as interior insulating blinds or exterior roller shades.
Tinted glass. For hot climates, tinted glass can help block solar heat gain to reduce air conditioning loads. Tinted glass, though, can significantly darken the view, and in some cases block out as much as 90% of the sunlight. My preference for hot climates is to specify relatively clear glass (high visible transmittance), but with a solar heat gain coefficient (SHGC; the amount of solar radiation that passes through a window) of less than 0.3.
Dynamic glazing. A high-tech (and higher-cost) option to tinted glass is variable-tint or dynamic glass. This glass can be changed from clear to tinted with the flip of a switch to reduce heat gain or glare, and some products offer variable levels of tinting. Sage Electrochromics and View (formerly Soladigm) manufacture dynamic glass, though this option is only available with certain window lines—and costs a lot (on the order of $50 to $100 per square foot). These products are used primarily in commercial applications. Both require a small amount of current to maintain the tinting, but the energy use is minimal compared to the energy saved by reducing air-conditioning demands.
Vacuum glazing. While not commercially available yet, vacuum glazing may improve energy performance dramatically, and with very thin windows. Prototypes have been produced by Guardian Industries and several other manufacturers. The space between the layers of glass can be as thin as 250 microns (a quarter-millimeter), and the panes of glass have to be held apart with tiny beads or pillars (which are barely visible).
Heat conduction and convection in vacuum glazing are largely eliminated because most of the air molecules have been eliminated from the airspace. A moderate vacuum is created in these glazings (about 10–4 torr—compare this to a thermos bottle, which has a much “harder” vacuum of about 10–6 torr). This leaves radiant heat transfer as the primary mechanism for heat loss through glazing.
However, without a low-e coating, there is relatively little to be gained by the vacuum, because radiant flow is not effectively blocked. With standard hard-coat low-e, Guardian’s prototype vacuum window would only achieve R-2 to R-3, while a standard sputtered soft-coat low-e double-glazed unit with the vacuum glass would yield about R-7. The best triple-layer (low-e3) could achieve as much as R-12. (Note: Values are center-of-glass measurements.) The greatest challenge with vacuum glazing is how to seal the edges and maintain the seals over many years.