Seek EPA Energy Star labeled windows.
Demand windows with an NFRC (National Fenestration Rating Council) sticker, and look for U-Factor values of 0.4 or less for windows, 0.6 or less for skylights. The U-Factor of the window summarizes its overall ability to keep heat inside a building.
Low-E glass and films help control heat gain and loss, keeping your home warmer in winter and cooler in summer. (Factory-applied Low-E coatings on internal glass surfaces are more durable and effective than films.)
Consider specifying fiberglass-framed windows for durability and very good insulating value.
Skylights result in substantial heat loss in winter, but if you choose
skylights, operable skylights can enhance ventilation of hot air in warm weather, improving comfort.
Look for recycled content windows.
Older windows are very unlikely to perform comparably with new windows, so windows should not be reused except in unheated structures such as greenhouses.
Windows provide ventilation, light, views, and a connection to the outside world. Drafty, old, poorly insulated, or simply poorly chosen windows can also compromise the energy efficiency of a building envelope. As with any manufactured product, the fabrication of windows requires energy, generating primarily air pollution. Energy efficiency is the primary consideration in reducing the environmental impacts of a window, followed by waste generated in manufacturing and general durability.
Residential window frames are usually wood, vinyl, aluminum, or fiberglass. Each has different cost, insulating ability, and durability:
- Wood requires maintenance to maintain its durability. Consider the source of the wood and seek FSC certified products.
- Fiberglass is energy intensive to manufacture, but is strong, durable, and has excellent insulating value.
- Aluminum and steel are poor insulators, and very energy intensive to manufacture. When using metal-framed windows, seek frames with ‘thermal breaks’ to limit the loss of heat to outdoors.
- Vinyl offers good insulation, but is highly toxic in its manufacture, and if burned.
For residential applications, seek windows with an NFRC (National Fenestration Rating Council) sticker. NFCR labels include five key pieces of information:
- (Most important) U-Factor summarizes the ability of a window to keep heat inside or outside a building. The lower the U-factor, the better its insulating value; look values of 0.4 or lower.
- The Solar Heat Gain Coefficient (SHGC) summarizes how well a window blocks heat from sunlight. Low SHGC values are best for our climate; look for SHGC values less than 0.4. (If one is designing for passive solar heating with the help of a knowledgeable professional, higher values may be desirable.)
- Visible Light Transmittance (VLT), a measure of how much light gets through a window. Desired VLT varies with taste and application.
- Low values for Air Leakage are best.
- The higher the Condensation Resistance, the better; values range from 0 to 100. Condensation can contribute to mold growth, but new high quality windows (with low U-Factor) will generally resist condensation far better than older windows. In Santa Cruz County’s mild climate, Condensation Resistance is generally the least important concern of these factors.
The EPA Energy Star label for windows can be a useful summary of these factors. For the California’s climate region, Energy Star qualified windows must have a U-factor of 0.40 or less and a SHGC of 0.40 or less. Energy Star skylights must have a U-factor of 0.60 or less and a SHGC of 0.40 or less. Skylights are the interface between the roof and ceiling, which is where convection brings the warmest air, so ideally skylights would be designed to have a lower U-factor than windows. However, skylight manufacturers have only relatively recently begun to offer highly insulated units.
Efficient windows are widely available. Any local retailer should be able to help select high-quality options. To make an informed choice, consider only windows that have NFRC ratings.
