You defeat the purpose of the sleeping bag by preventing heat from reaching the sleeping bag and warming it. Use extra clothing as insulation under or over or to fill extra space in the bag or as a pillow. Pack it in a protective waterproof stuff sack that is lined with a garbage bag. Wrap the stuff sack in your tarp or carry it inside your internal frame pack to give it added protection. Use adequate full-length ground tarp and insulation.
More heat is lost through heat conduction to the ground that to the air.
Use whatever is available in an emergency to get off the ground. Backpack, extra clothing, ropes, etc. Remove constricting clothing that restricts circulation Don't breath into your sleeping bag. A large amount of moisture is exhaled with each breath of air. Wear a hat and keep your face out.