When we think of fiberglass, most of us think of pole vaulting or sailboats or tub/showers, but it is one of the most popular choices for high-performance window frames. Although it tends to be durable since it expands and contracts similarly to the glass it holds (since glass is the main components of fiber glass), fiberglass does need to be protected from UV radiation for long-term durability. Thus, it may be covered on the outside with a thin coating.

It is hard to beat fiberglass for thermal performance. Fiberglass conducts heat slowly like uPVC and wood, and because it is stronger than both of them, it can be shaped under high pressure to thin, but strong, shapes. Window frames all have a cross-section “profile,” and fiberglass profiles have less structural material than most. Thinner material conducts less heat. And it provides even more room within the frame for insulation. Many fiberglass window frames are filled with plastic foams which insulate even better than the multiple air spaces created by the channels in a uPVC frame.