Pella Impervia Fiberglass Products Pell'e Made from Duracast® the strongest, most durable material available in windows and patio doors 1825 Hwy 93 South Pella Window & Doors Showroom Kalispell, MT 406.257.1516 In testing performed in accordance with ASTM testing standards, Pella'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 made from a non-Duracast fiberglass composite. Pella Impervia Fiberglass Products Pell'e Made from Duracast® the strongest, most durable material available in windows and patio doors 1825 Hwy 93 South Pella Window & Doors Showroom Kalispell, MT 406.257.1516 In testing performed in accordance with ASTM testing standards, Pella'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 made from a non-Duracast fiberglass composite.