I would think if the flannel is 100 per cent cotton it will be cool as it will breathe, so will wool. I would be more concerned with the drape and ease to quilt, but that is a personal preference. (My DD's wrap up in my quilts even when it is 80 degrees, and we do not have air conditioning.) I like a bit more body to the quilt. (When I tie my quilts I use the high loft batting.)