The proof is in the window. Attractive, durable, virtually maintenance free and professionally installed, Infinity by Marvin fiberglass windows are made inside and out with ULTREX – a pultruded fiberglass material that outperforms and outlasts all other window materials. For windows, the strength of Ultrex fiberglass means less material movement for better seals and long-lasting windows. Tough, dependable, non-corrosive, non-brittle and low maintenance, Ultrex is the last word in replacement windows.