I may be wrong but I was under the impression that synthetic bags dont really have the same loft per degree rating that down does. So in other words a synthetic bag can be as warm as down but with less loft since depending on synthetic type. I think I read this on a discussion of clo values for climashield. So I guess a synthetic bag of 2 inch of loft might not be needed to get 20deg, but like I said I may be wrong.