Castelli set out to create their warmest bootie ever, the Estremo Shoe Cover, which meant rethinking how booties are made. For the windproofing the designers tapped into the wealth of knowledge and multi-layer textiles of GORE-TEX. The INFINIUM™ WINDSTOPPER® fleece-backed fabric, is amazingly protective and flexible, and also lets more moisture escape than any other windproof fabric. For more insulation and some airspace they added a second fleece layer with Polartec® Power Stretch® Pro™, which is unrivaled in overall loft. The two layers are thick enough that Castelli had to create a "wheel well" single-layer construction by the ball of the foot to prevent the fabric from rubbing on the crank. The bootie was built high beyond the ankle for extra warmth and to ensure reasonable splash protection.

You have insulation, breathability, comfort, wind and water protection. Add in a full length rear facing YKK AquaGuard zipper, a smartly placed pull tab, reflective details, and a super durable base and you have a full featured, amazing extreme weather shoe cover that is rated for temps from 8-41 degrees Fahrenheit. Estremo = Extreme.