Very soft, silk-like polyester fabric. Helps keep the interior of your sleeping bag clean. Can be used on its own on warmer nights. Stuffs down into a very lightweight, compact package (stuff sack included). Available in two(2) Shapes; Mummy: size 83" x 30" x 20", weight 7.4 oz (210 g) , and Rectangular; size is 78" x 32", weight 9.5 oz (270 g).