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.
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.