Whether replacing windows in an older house or choosing windows for a new house, your decisions on what type of windows to buy will be among the most important decisions you will make in terms of energy use. Because of the impact windows have on both heat loss and heat gain, proper selection of products can be confusing. To get the most up-to-date information on what to look for in energy-efficient windows for your home, visit the Efficient Windows Collaborative.
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, and be sure to have experienced contractors install your high-tech windows. 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 air tightness at a site.
- Certified by 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, southcentral, and southern. ENERGY STAR windows must carry the NFRC label (discussed below).
- 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.
- Efficient frame material and sash construction. Wood is still the most common material in use, and it insulates reasonably well. Unless a thermal break is incorporated into the design, aluminum frames conduct heat very rapidly and are therefore inefficient. Vinyl (PVC) windows, or vinyl frames insulated with fiberglass, are the most efficient and tend to insulate better than wood.
- Air-tightness. Examine air leakage specifications carefully when selecting windows, but in general, casement and awning windows are tighter than double-hung and other sliding windows.
- Glazing with low emissivity. 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 variety and placement of the low-e coating on the window varies for different climate zones and applications. ENERGY STAR offers purchasing tips to help you find which windows are appropriate for you.
- Multiple 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. Some of these windows use glass only; others use thin plastic films as the inner glazing layer(s).
- The right thickness of air space. With double-glazed windows the air space between the panes of glass has a big effect on energy performance.The wider the space, the less heat can be conducted through that space (as long as it stays under about 1″).
- 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.
- Proper 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. With new edge spacers, however, pay particular attention to warranties against seal failure.
Selecting New Windows for Your Home
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, southcentral, and southern. ENERGY STAR windows must carry the National Fenestration Rating Council (NFRC) label, allowing comparisons of ENERGY STAR-qualified products on specific performance characteristics.
U-value. U-factor measures how well a product prevents heat from escaping. It is the inverse of R-value, which is familiar to many people as a measure of insulation thermal performance. The lower the U-value rating, the better the overall insulating value of the window. 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) take into account heat loss through the glass, window edge, and window frame.
Solar heat gain coefficient (SHGC). The 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. Windows with high coefficients 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.
Air leakage. Air leakage is already listed by many window manufacturers, in terms of cubic feet of air per minute per foot of crack. An optional air leakage value is included on NFRC labels and in the NFRC Certified Products Directory.
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 can not predict actual condensation.