If there’s one failing of most warmers it’s that they absorb water like a sponge. On rainy days they end up waterlogged and providing compromised warmth. Nanoflex is a proprietary Castelli fabric. A super-warm and ultra-stretchy Thermoflex fabric, used in many of Castelli's winter garments, and then coated outside with ultra-tiny nanofilaments that cause raindrops to run off the fabric without soaking through. Despite its water-resistant finish, Nanoflex fabric remains highly breathable, keeping you comfortable in almost any conditions.