Your (for a single window or whole house). The climate you live in (extreme cold, coastal, or desert).
Look for the label and pay attention to these ratings: U-Factor: Measures heat loss (lower is better). how to buy windows for house
Ask about the "labor warranty" in addition to the "product warranty." Your (for a single window or whole house)
While DIY saves money, professional installation ensures the warranty remains valid. Get at least from local contractors. how to buy windows for house