Energy-Efficient Windows

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Want to reduce your utility bills? Consider the impact of windows. Especially here in our hot humid climate in central Texas. The right windows help keep you comfortable in your home.

The good news: Window manufacturers have stepped up to the plate with new, high-performance windows that help reduce cooling costs in homes.

But have you shopped for windows? Whew, there's a lot of technical lingo and numbers to sort through. This brief blog post is a little primer on window technology and what to look for.

What do those numbers mean?

U-factor. How fast a window lets non-solar heat pass through it. In technical terms, a measurement of the escape of BTUs per square foot per hours, per degree Fahrenheit. The lower the U-factor, the more energy efficient the window. Austin's 2010 residential energy code requires a U-factor of 0.51 or lower.

SHGC (solar heat-gain co-efficient). How much solar energy the windows lets through. Indicates how well it blocks heat caused by sunlight. The lower the SHGC, the less solar heat it transmits, which is what we want in central Texas. Austin's 2010 residential energy code requires a SHGC of 0.30 or lower.

Air infiltration. How much does that window leak? It's measured as the amount of air that passes between a window sash and frame, measured in cubic feet per minute per square foot of frame area. The lower the rating, the slower the air leaks, which is what we want.

Visible transmittance (VT). How much visible-spectrum light transmits through a window. The higher the VT, the more visible light.

Light-to-solar gain (LSG). The ratio between the SHGC and VT.

What to look for in windows

Dual- or triple-pane glass units. The panes are separated by a hermetically sealed space that's often filled with argon gas.

Low emissivity coating (low-E), which rejects unwanted solar heat and blocks UV rays, while still providing a clear view.

Window frames built of durable materials and designed to block the transfer of heat.

Planning ahead

If you're planning a new home, you have the opportunity to place and shade windows properly -- important for day-lighting and for cross-breezes, for example. But that's a blog topic for another day.

Want more details about energy-efficient windows?

You can find useful information at the Efficient Windows Collaborative web site.