Fiberglass window frames are essentially composed of glass fibers and resin, materials that expand and contract very little with temperature changes in the weather. Stronger than vinyl, more affordable than clad wood, they stand out for their strength, low upkeep, and good looks. It’s stiffer and lighter than wood, as low-maintenance as vinyl, and unaffected by water or temperature fluctuations.
Most parts for a fiberglass window are fabricated by pultrusion. In this automated process, lengths of fiberglass roving and strand mat are bathed in a resin, covered with a fiberglass veil, and pulled into a heated die that hardens the resin. (Separate dies are used for each window part.) The smooth, rigid lineal that emerges from the die is then typically cut to length, coated, and fitted with hidden nylon-reinforced corner blocks. When screwed and glued together, lineals and blocks form tight, clean, nearly indestructible joints.