The Altitude Liner is designed for the best warmth under the severest conditions. It is constructed from a double layer of Polarguard 3D VaprThrm®. This bulky mitt is the warmest one we make! The Altitude VaprThrm® liner features full box construction with anatomic curve for increased warmth and comfort. A sweat skirt Velcros into the shell to prevent vapor from contacting the Polarguard insulation.