Buying guide: Energy Star-certified exterior doors
Doors that are less than one-third glass are confusingly called 1/2-lite doors. Glass allows more heat transfer than the polyurethane core, so only the most expensive triple-pane glass in a door could be expected to meet the U-factor standard of 0.21. Therefore, Energy Star doors with glass sections must reach a lower U-factor bar of 0.27 (the rough equivalent of R-4). They must also have a solar heat gain coefficient (SHGC) rating of 0.30 or less. The SHGC measures the amount of solar heat blocked by the door, and a lower number means less heat transfer, thus lower cooling bills. Keep in mind that in cold climates, higher SHGC numbers may be advantageous for passive solar heating in the winter. Unlike Energy Star's windows and skylights , Energy Star door rating values do not vary by region.