Feb10th

High Performance Windows

Many ENERGY STAR qualified new homes feature high-performance windows. High-performance, energy-efficient windows can improve the energy efficiency of your home by reducing heat loss in cooler climates and heat gain in warmer climates.

image map of house with links to ducts, envelope, windows, insulation, and equipment

Window technologies have advanced dramatically and prices for these windows have dropped significantly. Look for windows with the ENERGY STAR label. Heat gain and loss through windows accounts for up to 50% of a home’s heating and cooling needs. Many technological improvements have been made in recent years that have advanced the insulating quality of windows including:

Improved Window Materials

Advances in window technology such as double glazing and low-e coatings substantially reduce heat loss and gains. Look for ENERGY STAR or National Fenestration Rating Council (NFRC) labels to be sure you are getting high-efficiency windows.

Improved Framing Materials

Low conductance materials, such as wood, vinyl, and fiberglass perform better than aluminum. Look for “thermal breaks” where aluminum frames are used in heating-dominated climates to avoid condensation. Insulated frames, including insulating spacers between glazings, also perform better than uninsulated frames.

Air Tightness

High-performance or advanced windows need to be sealed around framing and other gaps that may exist. Caulks, foams, and weather-stripping work well to keep drafts out.

High-performance, energy-efficient windows can offer you:

  • Quieter home interior — multiple panes and insulated frames block outside noise.
  • Reduced fading of curtains, furniture, and flooring — low-emissivity (solar window) coatings can block up to 98% of UV rays.
  • Reduced utility bills — houses lose less heat in winter and absorb less heat in summer.
  • Improved quality windows are made from better-quality materials easier to operate and carry extended warranties.

Dig Deeper

Windows typically comprise 10 to 25 percent of the exterior wall area of new homes. Research studies report that windows in heating-dominated climates account for up to 25 percent of a typical house’s heating load and that in cooling-dominated climates, windows account for up to 50 percent of the cooling load.

In recent years, many technological advances have been made that significantly enhance the thermal performance of windows. As shown in Figure 1, these technologies include improved framing materials, low-emissivity and solar control coatings, low-conductance gas fills, improved thermal breaks and edge spacers, and better edge sealing techniques. These technologies can be used independently or in combination, but must be selected based on climate to optimize performance.

Windows can improve the thermal performance of homes by minimizing heat loss in heating-dominated climates and by minimizing solar heat gain in cooling-dominated climates. Thus windows with lower U-factors or higher R-values perform better in heating-dominated climates and windows with lower solar heat gain coefficients (SHGC) perform better in cooling-dominated climates (see Figure 2). SHGC is a measure of the amount of solar energy that a glazing material allows to pass.

The materials and design of the frame also influence thermal performance. Low conductance materials, such as wood, vinyl, and fiberglass, perform better than high conductance materials such as aluminum. Look for “thermal breaks” where aluminum frames are used in heating-dominated climates to avoid condensation. And finally, insulated frames perform better than uninsulated.

Air tightness is another important consideration. Windows are now being tested and rated for air tightness. A rating of 0.2 cfm/ft (cubic feet per minute of air leakage per linear foot of window edge) or lower is considered good. The best windows have a rating of 0.1 cfm/ft or lower.

An effective building envelope is a key element for an energy-efficient home. ENERGY STAR labeled homes are often constructed with high-performance windows that can improve the effectiveness of the building envelope and improve comfort.

Benefits

High-performance windows can provide many benefits including:

Improved comfort. High-performance windows reduce conductive heat losses and gains resulting in warmer interior surfaces during the winter and cooler interior surfaces during the summer. In homes, approximately 40 percent of our physical comfort is due to the radiant heat exchange between our bodies and the surrounding interior surfaces. Thus, high-performance windows improve comfort by reducing this radiant heat exchange. In addition, improved frames reduce drafts and provide more consistent temperatures throughout the house.

Quieter home. High-performance windows often utilize multiple glazing and insulated frames. These features reduce unwanted noise from the outside.

Increased quality. High-performance windows are often constructed with better quality materials that can result in stronger, easier to operate, and longer lasting windows. As a result of these improvements, manufacturers frequently offer extended warranties on these products.

Improved indoor air quality. High-performance windows often have air tightness ratings of 0.2 cfm/ft or less which reduce the amount of unconditioned air leakage into a house. This air leakage can bring in dirt, dust, and other impurities that can negatively affect indoor air quality.

Lower utility bills. High-performance windows are better insulated and more air-tight. These features reduce energy consumption for heating and cooling which result in lower utility bills, making homes less expensive to operate.

Reduced obsolescence. Based on recent trends for improved efficiency, high-performance windows are expected to become standard practice for the building industry. Since it is both difficult and costly to replace windows after a house is built, it is best to install high-performance products during the original construction. ENERGY STAR labeled homes constructed with high-performance windows are, therefore, expected to be less vulnerable to obsolescence.

Fewer condensation problems. High-performance windows stay warmer in the winter resulting in drier windows with fewer condensation-related problems. Condensation can stain fabrics, lead to mold and mildew build-up, and in cold climates cause damage due to the freeze/thaw cycle.

Reduced wear on home furnishings. Low-E coatings can block up to 98 percent of the ultraviolet radiation emitted by the sun. This radiation causes curtains, window treatments, carpeting, and furniture to fade and wear faster.

Improved resale position. High-performance windows can provide the many impressive benefits listed above resulting in a more comfortable, quieter, and higher quality home with lower utility bills and fewer condensation and fading problems. These benefits can translate into higher resale value.

Source: EPA

Links:

State by State Window Recommendations

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