Always look for the Energy Star symbol and label. This label shows that a door or window has been certified to meet the strict and exacting technical requirements to make it one of the most energy-efficient products you can buy.
What’s more, you should only buy windows and doors that are Energy Star certified. The Energy Star program is administered by Natural Resources Canada (NRC), where manufacturers submit their products for testing and certification. Usually an Energy Star certified product is in the top 15 to 30 percent of its class for energy efficiency.
For more information in the Energy Star program and testing methods, visit the NRC here.