I've used flannel for the sake of bulk reduction, but I never have thought about it as being a less warm alternative to batting. The quick quilt I made for my boyfriend's nephew has flannel for the batting, because I felt it would be warmer, since it was meant to be used on a tile floor with radiant floor heating. Flannel sheets are lighter than bolt flannel though, aren't they? That might be an option. I could still use it with the sheets to make it a duvet then. I'd have to sleep under it first & see how it goes before I turned it into a duvet. Maybe I can do that while my boyfriend is out of the country for 2 weeks. If I don't like them, I'll just use them for backing for Jamie's quilts for OK!