The THERMOLITE® Reactor™ Fleece sleeping bag liner is a luxuriously soft and super warm liner that can bump up the warmth of your sleeping bag considerably. It can also be used as a stand-alone sleeping bag in warmer climates and has a quarter length zipper to control temperature and allows the user to get in and out of easily. The hollow core fibers that make up this fabric trap heat effectively, keeping you toasty warm. If you're looking for a high warmth to weight ratio or a lightweight summer bag, this is an ideal choice.The THERMOLITE® Reactor™ liner is our top seller in the sleeping bag liner range. It's an excellent cold weather option for those who need additional warmth. The hollow core fibers trap heat and offer extraordinary insulating properties. This is a super stretchy, soft, and comfortable choice with a mummy shape, foot box and a drawcord hood. Compact and lighter weight than fleece, this is a best seller for good reason.