Because of the impact windows have on both heat loss and heat gain, proper selection of products can be confusing. To add to the complexity, window glazing technology has changed tremendously in recent years. The best window glazings today insulate almost four times as well as the best commonly available windows from just fifteen years ago. Because of the rapid pace of change, even skilled designers are often not fully aware of the potential these new glazings offer for energy-efficient building design. The National Fenestration Rating Council (NFRC) is the principle organization responsible for evaluating and labeling windows based on their energy performance. How to interpret their label and the specific performance properties to look for are discussed below. First, let’s walk through some of the basic features to look for in a new window.
Reliability and good installation. Choose windows with good warranties against the loss of the air seal. If the glazing seal is lost, not only will fogging occur, but also any low-conductivity gas between the layers of glass will immediately be lost. Consumers should recognize that the manufacturer’s quality control at the factory and care during shipping can have a big impact on the window’s airtightness at a site.
Furthermore, it is senseless to invest thousands of dollars in new windows only to have amateurs install them in your home. The high-rated performance of a window is meaningless if it is installed improperly with gaps and air leaks around the window. Be sure to have experienced contractors install your high-tech windows.
Proper dimensions. To maximize energy performance, choose windows with larger unbroken glazing areas instead of multi-pane or true-divided-light windows. Applied grills that simulate true-divided- light windows are fine; they do not reduce energy efficiency.
Frame material and sash construction. Window frame and sash construction has a big impact on energy performance. Wood is still the most common material in use, and it insulates reasonably well.
Aluminum has been used extensively, particularly in the western part of the U.S., but unless a thermal break is incorporated into the design, aluminum frames conduct heat very rapidly and are therefore inefficient. Vinyl (PVC) windows are gaining in popularity, especially in the replacement market, and some vinyl frames insulated with fiberglass insulate better than wood. “Pultruded” fiberglass frames and wood-polymer composite frames are other options becoming more common in high-efficiency window products. They insulate well and their cost generally falls between that of wood and vinyl.
Airtightness. Windows vary dramatically in how effectively they block infiltration. In general, casement and awning windows are tighter than double-hung and other sliding windows. This is because when a casement or awning window is closed, it is pulled in against a compression-type gasket. Sliding windows have to use seals that permit the sash to slide, so they are rarely as airtight. You will find, though, that double-hung windows from a few manufacturers are tighter than casement windows from others, so it makes a lot of sense to examine air leakage specifications carefully when selecting windows.
Multiple layers of glazing. Until the 1980s, the primary way manufacturers improved the energy performance of windows was to add additional layers of glazing. Double glazing insulates almost twice as well as single glazing. Adding a third or fourth layer of glazing results in further improvement. In the 1970s, with rising concern over energy, triple-glazed and even quadruple-glazed windows entered the market. Some of these windows use glass only; others use thin plastic films as the inner glazing layer(s).
Thickness of air space. With double-glazed windows, the air space between the panes of glass has a big effect on energy performance. A very thin air space does not insulate as well as a thicker air space because of heat conducted through that small space. During the 1970s, a lot of window manufacturers increased the thickness of the air space in double-glazed windows from 1⁄4” to 1⁄2” or more. If the air space is too wide, however, convection loops between the layers of glazing occur. Beyond about 1”, you do not get any further gain in energy performance.
Low-conductivity gas fill. By substituting a denser, lower-conductivity gas such as argon for the air in a sealed insulated glass window, heat loss can be reduced significantly. The largest window manufacturer in the country today, Andersen Windows, uses argon gas-fill in all of its insulated glass windows, and most major manufacturers offer argon gas-fill as an option. Other gases that are being used in windows include carbon dioxide, krypton, and argon-krypton mixtures.
Edge spacers. The edge spacer is what holds the panes of glass apart and provides the airtight seal in an insulated glass window. Avoid traditional hollow aluminum spacers because they have extremely high conductivity. Instead, choose edge spacers that are thin-walled steel, silicone foam, or butyl rubber. Generally, better edge seals are a low-cost option when ordering windows, and worth considering. The net effect of improved edge spacers can be a 2–10% improvement in window energy performance, depending on the other performance characteristics of the window. With new edge spacers, however, pay particular attention to warranties against seal failure, which results in fogging and loss of any low-conductivity gas-fill.
Glazing with low emissivity. Tinted glass and tinted window films have long been used in commercial buildings to reduce heat gain through windows. Today, low-e coatings made of a thin transparent layer of silver or tin oxide are used on high-performance windows to reduce the solar heat gain without reducing visibility as much as older tinted glass. The coatings permit visible light to pass through, but they effectively reflect infrared heat radiation back into the room, reducing heat loss in winter. The variety and placement of the low-e coating on the window varies for different climate zones and applications. Low-e windows with high solar heat gain coefficients are appropriate for northern climates where passive solar heating is advantageous, while “southern low-e” windows with low heat gain coefficients are appropriate in milder climates where summer cooling is more necessary.
Match the application. The different properties of low-e glazings allow you to choose different types of glazing for different sides of your house. For example, if you want to benefit from passive solar heating for the south side, choose windows containing a top-performing low-e glass with a high solar heat gain coefficient. On the north, install the lowest U-value windows you can afford. Or to keep things simpler, you can order the same glazings for the east-, west-, and north-facing windows.
Some window manufacturers now produce both “Northern” and “Southern” climate low-e products. But other window manufacturers still offer just one type of low-e glazing as their standard, and charge extra for substitutions—if they provide options at all. So a decision to choose different glazings for windows of different orientations may require some extra shopping around. If you do order different glazings for your different windows, be sure to keep track of which windows have which type of glazing, because they will probably all look identical!
Look for ENERGY STAR. Windows, doors, and skylights qualifying for the ENERGY STAR label must meet requirements tailored for the country’s four broad climate regions: northern, north-central, south-central, and southern. ENERGY STAR windows must carry the NFRC label (discussed below), allowing comparisons of ENERGY STAR-qualified products on specific performance characteristics such as solar heat gain, insulating value, and infiltration.
ENERGY STAR offers guidance for the kinds of low-e coatings appropriate for cold, moderate, and warm climates.Some manufacturers make windows for all climates, while some only specialize in one. Search under “Products” for “Windows, doors and skylights.” www.energystar.gov
Window Properties and NFRC Ratings
Before 1993, when NFRC developed standardized rating procedures for heat loss through windows, there was little consistency in how manufacturers listed energy performance. Today, energy-efficient windows are evaluated on five standardized performance measurements. These are labeled clearly on each window so you can compare one window to the next. NFRC labels describe the whole window U-factor (U-value), solar heat gain coefficient (SHGC), visible light transmittance, air leakage, and condensation resistance, all described briefly below. Because window dimensions and frame-to-glass ratio have a big impact on the total energy performance of a window, and yet it is difficult to account for all sizes, NFRC-rated windows fall into two representative sizes.
When a window manufacturer has its products certified, a great many different types and configurations of windows may be included. As defined by NFRC, a product line is a group of windows that have similar frame and operating characteristics (e.g., “Andersen Perma-Shield Casement windows”). Within each product line, there may be various products. Differences among products include the number of layers of glazing, the use of or type of low-e coating, the type of gas-fill, edge spacer, etc.
U-value. U-value is the measure of the amount of heat (in Btus) that moves through a square foot of window in an hour for every degree Fahrenheit difference in temperature across the window. U-value is the inverse of R-value, which is familiar to many people as a measure of insulation thermal performance. Because it is the inverse, the lower the U-value rating, the better the overall insulating value of the window.
The lower the U-value, hte less heat escapes through a window. U-value quantifies thermal conductance in Btus per hour per square foot of window per degree Farenheit (temparature difference between indoors and outside).
Typical U-values range from 0.20 to 1.20. The U-factor ratings listed on NFRC labels (and in the NFRC Certified Products Directory) are whole-window U-values. That is, they take into account heat loss through the glass, window edge, and window frame.
Solar heat gain coefficient (SHGC). SHGC describes how much solar energy is transmitted through a window. Solar heat gain can be beneficial — providing free passive solar heat during the winter months— or it can be a problem, resulting in overheating during the summer. An SHGC of 0.8 means that 80% of the solar energy hitting the window gets through. As noted above, major window manufacturers make low-e windows with different solar heat gain coefficients. Windows with high coefficients (above 0.60) are designed for colder climates, while windows with low coefficients are designed for hotter climates.
Visible light transmittance. While SHGC describes the relative amount of solar energy that can pass through a window, the visible light transmittance is simply the relative amount of sunlight that can pass through, measured on a scale between 0 and 1. The higher the number, the greater the amount of light that can pass through. A tinted window or a window with a large ratio of frame to glass will have a lower visible transmittance value.
Air leakage. Another important energy property of windows is air leakage or air infiltration. Air leakage is already listed by many window manufacturers in terms of cubic feet of air per minute per foot of crack, with typical values ranging from 0.1 to 0.3. An optional air leakage value is included on NFRC labels and in the NFRC Certified Products Directory. The NFRC has adopted the same basic procedures for measuring air leakage that have been used by the industry in the past.
Condensation resistance. Finally, the ability of a window to resist the formation of condensation on the interior surface is very important in evaluating the relative durability of a window. The NFRC measures condensation resistance on a 0–100 scale. The higher the rating, the better that product is at resisting condensation formation. This rating is optional for new products, and it cannot predict actual condensation.