LOOSE FIT FIRM DUCK INSULATED OVERALL BIBERALL - 4 EXTREME WARMTH RATING Engineered For Extreme Warmth in the Coldest Conditions On Earth. See Technology section below for more details. Work doesn't take a snow day. These men's loose-fit biberalls help you get the job done in wintry conditions. They're made of heavy duck canvas with Arctic-weight insulation for added warmth. Triple-stitched seams and durable kick panels mean they're built to last. Ankle-to-thigh leg zips make for easy on and off over boots.