My Thermarest has worked fine from an insulation prospective down to 27, the lowest I've experienced. I always sleep in what ever layers seem justified by the predicted low and my 25 degree bag, actually good to about 35 degress. I wear a wool watch cap over head and ears, having sacrificed the bag hood to reduce bulk. Good gloves and wool socks complete the package.

These heat packs might have a place in your sleeping bag. Mine too.