My summer "bag" is a flannel sheet or lightweight syn, depending on expected temps. My winter bag is 25F down, good to about 35F. Add 10 dg to whatever the bag is rated at. A liner is highly recommended to protect the bag, esp if down. And/or, use your clothing to compensate for temps below the bag's effectiveness. I've slept with my gloves on and a wool watch cap occasionally. I cut off the hoods to reduce bulk.