High-Performance Windows: Looking Through the Options: Page 5 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

Measuring Window Performance

The most important measures of window performance are insulating value, visible light transmission, and solar heat gain.

U-factor. Insulating performance is expressed as U-factor, the number of Btu moving through a square foot of window in an hour with a 1°F difference in temperature across the window. U-factor is the inverse of R-value (U = 1/R); a better-insulating window has a lower U-factor and a higher R-value.

This insulating performance can be for the center of glass or the entire unit, which factors in the effect of the IGU’s edges and the window frame. The National Fenestration Rating Council (NFRC) U-factors are for the whole unit, rather than center-of-glass. As IGUs have gotten better (lower U-factors), the effect of window edges and frames has become relatively more significant, so the difference between center-of-glass and unit values has increased. The high-performance windows on my house have a center-of-glass insulating value of R-12.2 (U-0.082) and a whole-unit value of R-8.2 (U-0.12).

Visible light transmittance (VT) is a value between 0 and 1 that corresponds to how much total visible light can pass through the glazing relative to a 1/8-inch-thick piece of clear glass. The higher the value, the more light is admitted. This is the light you want for daylighting, and it determines how clear the windows appear. Most people favor windows with high VT.

The solar heat gain coefficient (SHGC) is also a value between 0 and 1, and measures how much solar heat makes it through the window. The SHGC includes portions of the electromagnetic spectrum that are invisible—infrared and ultraviolet. These spectra don’t provide visible light, but they contribute to heat gain. A window can have a fairly high VT and low SHGC because of the selective transmittance properties of advanced low-e coatings. It’s part of the “magic” of modern windows.

Air leakage measures how tightly windows close and the effectiveness of weatherstripping or gaskets. In the United States, air leakage is typically measured in cubic feet per minute (cfm) per square foot of window area, assuming a 75 pascal difference in pressure across the window (based on a standard test method).

Finally, condensation resistance measures how likely condensation is to occur at the window perimeter. Even in high-performance windows, condensation may occur at the window edges, because greater heat loss occurs there and the glass temperature is lower—causing condensation to form on the inner surface. NFRC has a test method for determining condensation resistance, with the resultant number between 1 and 100—the greatest resistance.

Successful Window Shopping

Choose durable frame materials. Rotting window frames and sashes is a common cause of window failure. To maximize the life of windows, avoid exposed wood on the exterior. If wood windows are being used, select clad windows, which have protective exterior cladding of aluminum, vinyl, or fiberglass. With non-wood windows, avoid metal (steel or aluminum) because of their high conductive heat loss. Windows with frames made from extruded vinyl or pultruded fiberglass allow insulation (usually polyurethane) in the hollow frames, though not all vinyl or fiberglass windows are insulated—you’ll need to verify. In general, the most durable, high-performance windows have fiberglass frames.

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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