Flannel is good as a very light weight batting, but be SURE to wash and fully dry it to get the best shrinkage before batting with it.
If you'd rather use a 'real' batting, Quilters Dream Cotton, Request loft, is their thinnest, lightest weight batting. It handquilts beautifully, too. (Sleeping with the window open in the winter in North Dakota -- Oh my goodness, I can't even imagine??!!)