Abstract: A sheet insulator is attached to an item of wearing apparel to provide thermal insulation. The sheet insulator has a fiber reinforced aerogel composite encapsulated within a plastic sheeting. The plastic sheeting is secured to the aerogel composite, surface to surface. The plastic sheeting and the aerogel composite have surface upsets such as elongated depressions or burrows positioned and oriented at a flexing position of the wearing apparel so that the sheet insulator is able to flex with the apparel enabling a more natural movement of the apparel.