A base layer like no other - these seemingly futuristic pieces by Seirus® warm you to your core while perfectly managing ventilation and perspiration. The heat-reflective properties of heatwave provide the most warmth in the lightest weight – which means no more piling on multiple layers just to go outside. The Mapped series utilizes heat-map studies of the human body to determine which areas need heat, and which need to breath. Making these the pinnacle of hitting that perfect warmth level. But if all you want is raw warming power, the Reversible series features Heatwave throughout, making it powerfully warm while staying remarkably thin. Brand new to the season, our Base Layer Lite™ offers the best warmth-to-weight ratio out there. See here for more sizing information.