E-Scales. E-zines. U-factors. U-571. R-values. AL. What does all that even mean?
They say it’s easy to get lost in the numbers, but when you look at some of those energy-rating acronyms, it’s even easier to get lost in the letters. But never fear. We at AHS — yes, another acronym — have the lowdown on how to decipher it all.
Know the energy efficiency score
A home energy rating tells you what you need to know about a home’s energy efficiency — or how much bang you’re getting for your energy buck. There are a number of factors considered in these scores, including levels of insulation, amount of air leakage, equipment efficiency, size of home, materials used and number of rooms.
The Home Energy Rating System, or HERS Index, assigns a standard new home an Index of 100 and a home that uses no energy a 0. So, the lower the score, the more efficient the home.
Conversely, the U.S. Department of Energy’s E-Scale, based on the HERS Index, associates a higher score with better energy efficiency.
Then there’s the 1–100 ENERGY STAR score, which gives a score of 50 a median score, while a score of 75 or better means a building is a top performer.
Me and U and energy-efficient windows
The U-factor, also called the U-value, refers to the rate of heat transfer in a window or skylight. The calculations involved are pretty technical, but what you need to know is that the lower the score, the better the window insulates. ENERGY STAR considers a U-factor of 0.30 or lower to be good.
The R-value refers to how well an insulating material resists conductive heat flow. The higher the R-value, the more effective the insulating material. Your recommended R-value will depend on your region’s climate.
No air leaks here
Some window labels will include an air leakage, or AL, rating. AL is measured in cubic feet and refers to the amount of air passing through a square foot of window area. Again, not exactly scintillating conversation to bring up over cocktails, but a good rule of thumb is to choose windows with an AL of 0.30 or lower.
Big gains in energy savings
Another number is the solar heat gain coefficient or SHGC. It is a comparison of the amount of solar heat that reaches the window and the amount that gets inside the home. The score ranges between 0 and 1. The lower the SHGC, the less heat that makes it into the house. The best number for your purposes will depend on your climate. If you live in a colder area, you might want windows that help keep the house warm, but if you live in a warmer area, you don't want to overheat your home.