Green Homes—Windows

Today’s advanced materials, better science, and new manufacturing processes make for high-performance window units. Those building new or wanting to replace inefficient windows with improved units have a lot of information to digest.

Window Components

A window’s performance is determined by three parts: the glazing, the sash and the spacer system. Measuring performance means looking at such factors as how the window conducts heat and transmits light. Follow a few simple steps to increase your windows' energy efficiency.

1. Look for an Energy Star label.
Energy Star is a joint program of the U.S. Environmental Protection Agency and Department of Energy. Its label confirms a window's energy efficiency based on its impact on heat gain and loss in cold weather and heat gain in warm weather for a given climate zone. The labels pertain to four climate zones: Northern,mostly heating; North/Central, heating and cooling; South/Central, cooling and heating; and Southern, mostly cooling.

2. Review the label from the National Fenestration Ratings Council (NFRC).
The NFRC is an independent organization that rates and certifies energy performance in all windows. Its label lists several performance rankings. Understanding the ratings is key to making the right choice.  While some rating scores are clearly beneficial, others have to be interpreted to your specific needs.

• U-factor. This measures how a window conducts non-solar heat flow. Ratings usually fall between 0.20 and 1.20. The lower the value, the greater the energy efficiency.
• Solar Heat Gain Coefficient or SHGC. This measures how much solar heat gain is admitted through a window and then released as heat into a home. The lower the SHGC, the less solar heat it transmits.
• Visible Transmittance or VT. This measures how much visible light comes through a window. Expressed as a number between 0 and 1, the higher the VT, the more light transmitted.
• Light-to-solar gain or LSG. This rating, not always provided, is the ratio between the SHGC and VT. It gauges the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light is transmitted without adding excessive heat.
• Air Leakage. This rating is optional so you may not find it on some products. The lower the number, the tighter the window and the less air that will pass through cracks in the assembly.
• Condensation Resistance. Another optional rating, it measures the ability of a product to resist the formation of condensation on its interior surface. Shown as a number between 0 and 100, the higher the CR, the better the product at resisting condensation formation.