Fourth Element Xerotherm Hat Constructed from Polartec Power Stretch fabric which has been used on several Everest expeditions, the Xerotherm is the most effective next to the skin garment for ensuring warmth & comfort in demanding environmental conditions. The Xerotherm is made from the fastest wicking fabric in existence. Moisture is moved away from the skin. Air, which is a far superior insulator, is held at the skins surface. This dramatically reduces conductive heat loss from the body.