I have used wool and two layers of cotton with great results. But, the best (warmest) combination I have come up with is Thinsulate batting (same as used for winter coats; not sure if still available --- bought several when they were on clearance at JoAnn's) along with anti-pill fleece for the backing. I know Hobb's makes a batt called Thermore which handles just like Thinsulate. You have to remember not to use any kind of fabric softener (liquid or sheets), though, as it flattens out the loft which is what makes it so warm. I live near Buffalo, NY, and the fleece/Thinsulate combo is warm enough that it is the only blanket I need to use, and I tend to sleep with a window open a crack on all but the coldest nights.