A proprietary waterproof, breathable membrane that lets vapor out without letting water in.
THERMAL HEAT SHIELD FOOTBED
On the outside, wooly softness; in the middle, support cushioning; and inside, a thermal foil barrier to harness radiant heat. Keeps the cold out and the heat in.
Three layers of innovative heat trapping materials get sandwiched under foot to hold warm air in. Works a lot like the insulation in your house.