ONE GOAL: KEEP YOUR FEET WARMWe set out to create our warmest bootie ever, which meant rethinking how booties are made. For the windproofing we went with GORE-TEX INFINIUMWINDSTOPPER® fleece-backed fabric, which also lets more moisture escape than any other windproof fabric. Then we added a second fleece layer with Polartec® Power Stretch® Pro, which is unrivaled in overall loft. The two layers are thick enough that we had to do a wheel well single-layer construction by the ball of the foot to prevent the fabric from rubbing on the crank. We made the bootie high for extra warmth and gave reasonable splash protection. We recommend our Venti Soft Sock for maximum warmth.Key Features:Castelli's warmest shoecover for extreme conditionsGORE-TEX INFINIUMWINDSTOPPER® fabric for windproof water protection with maximum breathabilityPolartec® Power Stretch® Prolining to keep you warmReflective details provide high visibilityYKK® AquaGuard® zipperHigh-durability material under foot-10°-5°C