A duvet is to protect your comforter. You can remove the duvet cover and launder that much more often than the comforter. We have a down comforter and no way am I going to launder that more than once a year. I made a two-sided pieced duvet a few years ago. I have 90" wide muslin inside and stitched thru the muslin and both the top and the other side thru the muslin. I wanted the muslin to protect all those seam allowances from the pieced top and second top side, when I put the down comforter in and out. Just by itself it's four layers of fabric. That's all I have on the bed for warmth in the summer besides a top and bottom sheet. When the down comforter goes inside, it doesn't feel heavy, but when I need to fluff it and rearrange it on the bed, I notice it is much heavier than the old duvet cover I made that was just two layers of fabric. I'm wondering why you think you need to put a thin batting between your two layers of outer and inner fabric layers? The duvet is a cover not a quilt.