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Visible transmittance (VT) measures how much light comes through a window. It too is expressed as a number between 0 and 1. The higher the number, the more visible light gets through the window.

Air leakage (AL) is a measure of infiltration through cracks in the window assembly. The rating is expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through.

Condensation resistance (CR) measures the ability of a window to resist the formation of condensation on the interior surface during cold weather. CR is expressed as a number between 0 and 100. A higher rating indicates that the product is better at resisting condensation formation.

Matching Windows to Climate

A companion to the NFRC label is the Energy Star label, which makes it easy to tell whether a given window is right for your climate. The Energy Star rating is based on minimum Department of Energy performance specifications by region. In the absence of an Energy Star label, follow the Energy Star guidelines.  The program groups window requirements according to four climate regions.

The Importance of Low-E

Of the many window technologies developed in recent years, none has had as great an effect on window energy performance as low-E (low-emittance) coatings. This microscopically thin, transparent metal layer is applied to one of the glass surfaces in the sealed space of the insulating glass unit (IGU). In an ordinary IGU (no coating), about two-thirds of the heat transfer across the gap happens via thermal radiation. Low-E coatings block most of this heat loss; double-pane glass with low-E insulates as well as uncoated triple- or quad-pane glass. This allows window manufacturers to offer high-performance windows using proven double-glazed window designs - particularly important for operable windows. All of the U-factors listed in the Energy Star criteria can be met using double glazing with low-E.

When the heat loss is reduced, the room-side glass surface temperature is warmer during cold weather. The infrared photographs provides a visual representation of comfort levels on a cold winter night with three different windows installed side by side; two have low-E coatings. A warmer surface on the inside of the glass also means less potential for condensation.

Spectrally Selective Low-E

Roughly half the energy in sunlight is invisible to the human eye. Low-E coating manufacturers have learned how to design coatings that let most of the visible light pass through, with little tint or coloration, while either transmitting or blocking most of the solar heat. The different glass designs can be grouped into generic categories of high, medium, and low solar-heat gain.

Spectrally selective low-E coatings do a good job of preventing winter heat loss and reducing summer heat gain while still allowing most of the visible light to enter the space.

Other Glass Technologies

Manufacturers have developed a number of technologies to improve window thermal performance in all climates. Some, like warm-edge spacers and gas fills, are widely used across the country. Others, such as tints and films, are used only for special applications.

Warm-edge IG spacer systems. The aluminum spacer bars traditionally used to separate the two panes in double glazing created a thermal short-circuit around the edge of the glass. Despite the warm center-of-glass temperatures achieved with low-E glass, the bottom edge of the glass was cold and had frequent condensation. Today more than 90 percent of the residential windows sold use some form of "warm-edge" system. Instead of aluminum, the new designs use low-conductance metals (like stainless steel), foam, or plastic for the spacers. The thermal-performance improvement from warm-edge technology is reflected in the total window U-factor rating found on the NFRC label. However, be sure to pay attention to window durability, too. Compare manufacturers' warranty provisions - the best performer may lack long-term warranty support because of uncertainty about the staying power of new materials and technologies.

Gas fills. Many manufacturers put low- conductance gases, such as argon or krypton, between the glazing layers to enhance the performance gains from low-E. Without low-E, the gas will have little effect. And even with the coating, the window U-factor will typically improve by less than 10 percent. Again, it's important to read the manufacturer's warranty to understand its provisions regarding gas retention.

Triple- and quad-pane glazing. Double-pane glass is optimized with a low-E coating and gas fill. To provide even better insulating values, some new window designs are incorporating triple- and quad-pane systems with multiple low-E coatings (one coating in each air space). Concerns about weight and thickness have some manufacturers replacing the internal layers with plastic or suspended films. Many remain skeptical of the durability of plastics encapsulated in the gap and exposed to sun and thermal cycles. Some newer window designs use thicker sash to accommodate all-glass triple or quad units. Refer to installation instructions to ensure that these products are compatible with your wall system.

Tinted glass. Tinted glass is sometimes used to reduce heat gain in hot climates. However, tinted glass gets hot in sunlight (from absorption) and suffers more loss of light transmission than spectrally selective low-E coatings. Although there are some spectral tinted glasses available today - usually light blue or green in color - residential windows tend to avoid tints, given the market preference for clear glazings. Also, these tints don't improve the U-factor, so a low-E coating is still required to meet code and Energy Star standards.

Aftermarket applied films. Tint films are often retrofitted onto windows in rooms that overheat due to direct sunlight. While they can effectively address overheating, the films can be problematic because the low visible-light transmission can excessively darken interiors. There can also be problems with film adhesion, and some window manufacturers will void their warranty if tint films have been applied. When buying new or replacement windows, look for products with a low SHGC, indicating that solar control is already built into the window.