Gas, however, has a tendency to seep out. Many window makers guarantee against leaks, with 20 years being fairly standard. “In some aspects you get what you pay for,” said the NFRC’s Bush. “Some will last 50 years easily. Some aren’t designed to last that long.” In the NRFC’s testing, windows are checked for leaks in an “accelerated weathering test,” which simulates how the window will perform over long periods of time. To pass the test, a window must retain 90 percent or more of the gas. That result is not on the sticker, but it can sometimes be found in the NFRC database or may be available on request from the manufacturer.