Pella Impervia® Fiberglass Products Made from Duracast the strongest, most durable material available in windows and patio doors Pellie 1825 Hwy 93 South Kalispell, MT 406.257.1516 Pella Window & Doors Showroom In testing performed in accordance with ASTM testing standards, Pela's Duracast has displayed superior performance in strength, ability, to withstand extreme heat and cold, and resistance to dents and scratches. Special shape windows are mmade from a non Duracast fiberglass composite. Pella Impervia® Fiberglass Products Made from Duracast the strongest, most durable material available in windows and patio doors Pellie 1825 Hwy 93 South Kalispell, MT 406.257.1516 Pella Window & Doors Showroom In testing performed in accordance with ASTM testing standards, Pela's Duracast has displayed superior performance in strength, ability, to withstand extreme heat and cold, and resistance to dents and scratches. Special shape windows are mmade from a non Duracast fiberglass composite.