If you can’t save your historic windows, then it’s time to find energy-efficient replacements. And selecting new windows involves industry knowledge. It’s important to have a grasp of the alphabet soup: U-factor, SHGC, air leakage, VT and LSG. Then, after you understand all the different rating systems, you can choose windows based on your climate and the requirements for your house.

But who rates these things anyway? Can you depend on the manufacturer to be truthful? Just like food labels are monitored by the U.S. Department of Agriculture, window labels are certified by the National Fenestration Rating Council. And all Energy Star windows should have an NFRC label. (Note, though, that Energy Star bases its ratings only on U-factor and SHGC, which are explained below.) Windows that don’t have the Energy Star label may or may not be rated; then it’s time to check with the manufacturer.

Read on to learn what all the acronyms mean!

(Note: This article is in reference to American standards. While the fundamentals remain the same, Canadian readers are advised to utilize the information as provided on the Fenestration Canada (formerly CWDMA) website.