For backpack high country hunts and fishing trips I use a 25 degree synthetic (primaloft) bag that compresses very well and also take a Sea To Summit "Reactor" liner. The liner is amazing. It weighs 9 oz., rolls into a 3x5 inch stuff sack and adds about 15 degrees to your bag (manufacturer's claim and I agree that's about right). It is made of dupont thermolite which I would describe as a very thin polypropylene type material. At 9 oz., if you take it and dont need it, don't use it!