High-Performance Windows: Looking Through the Options: Page 4 of 6

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High-performance windows
High-performance windows are multipurpose—including providing an inside view of the outside world, while contributing to a home’s overall energy efficiency.
riple-glazed, low-e window from Unilux
This triple-glazed, low-e window from German company Unilux offers excellent insulation performance, yet transmits enough sunlight to be effective for passive solar heating.
Well-placed windows
Well-placed windows can provide ample daylighting and ventilation to interior spaces.
Old single-pane window
Old single-pane windows are a constant household energy drain and compromise a home’s overall thermal comfort.
Alpen HPP windows
Alpen HPP can manufacture windows that have up to five glazing layers (three suspended films between two layers of glass), resulting in lightweight high-performance units that weigh only a little more than standard double-pane windows.
Gas-filled window
Some gas-filled windows, such as this one from Alpen HPP, are shipped with bladders to equalize pressure fluctuations in transit. They are removed during installation.
wood with aluminum cladding
Frame options include wood with aluminum cladding.
vinyl with polyurethane foam fill
Frame options include vinyl with polyurethane foam fill.
uninsulated vinyl
Frame options include uninsulated vinyl.
NFRC window label
NFRC window labels include U-factor, solar gain, transmittance, and optionally, air leakage and condensation resistance.
High-performance windows
riple-glazed, low-e window from Unilux
Well-placed windows
Old single-pane window
Alpen HPP windows
Gas-filled window
wood with aluminum cladding
vinyl with polyurethane foam fill
uninsulated vinyl
NFRC window label

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.

Comments (1)

Jim and Elaine Stack's picture

another very ECO choice is to modify the windows already in place. Instead of ripping out the old and adding to the landfill you can add Inflectors on the inside of the window opening. They have a reflective side for summer and dark side to absorb IR heat in winter. They are also air tight and take makes a big difference.
Since they are held in place with magnetic strips you can flip them over for heat rejection or gain in a minute. You can even take them off and enjoy a fresh breeze when it's temperate out.
I put them on all my windows and it makes a big difference. Our little 4 Kw system runs our 100% electric home and vehicle with energy to spare. .

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