: Frames and sashes are fusion-welded to prevent gaps or splits that could lead to air leakage. Choosing the Right Rating for Your Climate

Your target U-rating should depend on your local weather patterns: Guide to Energy-Efficient Windows

A U-rating measures the rate of non-solar heat transfer through a window assembly, including the glass, frame, and spacers. : Ratings typically range from 0.20 to 1.20 .