Window replacement can be an easy weekend do-it-yourself project if you do your homework and research what types of windows are best for your home. One aspect of window replacement that often confuses consumers is the jargon of energy efficiency. Read below about what the various numbers and terminology mean in order to make a more informed decision when you visit your local hardware store.

NFRC/Energy Star Certification

You should first look for the National Fenestration Rating Council (NFRC) certification symbol on any window. This means that the window is Energy Star certified, and thus uses 20-30% less energy than stipulated by federal law. 


Next, look for the U-Factor, or U-Value. This is usually labeled as (U.S./I-P), and is typically a number between the values of 0.15 and 1.25. The U-Factor measures the extent of the window's insulating properties, and more specifically the rate of heat transfer. Windows with double or even triple panes will usually register at a U-Factor below 0.30.

Visible Transmittance (VT)

The number that signifies visible transmittance is a measure of how much light the window lets in. It is measured on a scale of 0 to 1, with most windows falling in the middle fifty percent of that range. Windows that have a higher visible transmittance figure will let in more light, so consider which room you're planning to install the windows in. In rooms with fewer windows, you may want to install windows that have a lower U-Factor but a higher VT number.

Solar Heat Gain Coefficient (SHGC)

This figure is always between 0 and 1, but usually in the range of 0.25 to 0.80. It measures heat caused and transmitted through the window by sunlight only. If you live in a warm climate but want to keep your home cool, it may be wise to invest in windows with a lower SHGC, so as to block more solar heat from entering the house.

Air Leakage (AL)

Heat--solar and otherwise--and light are just two factors to consider when purchasing windows. Another is of course the amount of air that windows let in to the house. Unlike the other figures discussed above, air leakage isn't measured as a number on a fixed scale, but as the amount of cubic feet of air that leak into the home per minute. All windows should have figures of at least less than 0.3 cf m/ft squared. 

For more information about choosing the best windows, contact a company like Ken Caryl Glass, Inc.