The profile design and methods of assembly determine the strength of a window. Just as beams or roof trusses are designed for specific mechanical loads, so are window shapes.
The strength of the sash and frame dictate how the window will maintain its shape under high winds. A weak window can flex and reduce the effectiveness of the weatherstripping, resulting in air and water infiltration, and subsequently, higher energy costs. A weak window design also requires near-perfect window installation to ensure an effective seal. You’ll have windows with long-term strength and squareness if you select windows with all fusion-welded joints and glass sealed to the sash frame.
If you are considering vinyl windows, you may also want to consider certain other qualities of window frames. For example, window frames with corners that are welded together with heat (often referred to as fusion- or heat-welded) are usually more durable than frames with plastic corner gussets or corner keys, or those that are glued together (solvent-welded).