I have a question about the temp ratings of a sleeping bag. If I buy a 20 deg bag can I decrease the rating to say 0 deg by adding a poly fleece liner or will I not get that much benifit? I have 3 bags now, one is an old military extreme cold weather bag, I've never gotten cold in it even down into the teens but it's very heavy, I have a 30 deg summer bag really light weight but still bulky and cheaply made, and I have a mummy type sold by REI but I don't know its rating as it was giving to me because someone didn't want to backpack anymore, they cut all the tags out of it but don't remember the temp rating. I've used the REI bag down into to teens and wasn't cold but then again I wasn't warm either. I thought about a poly fleece liner to the REI bag but would just as soon get a new bag instead. Any ideas?