Heavens, my backside has always had enough insulation. Sweats will work 'til the 40's. THen tights under the sweats. Then tights and thermal undies and sweats. Then the really, really warm tights, the silk undies, and the thick sweats. That works for 4 degrees (F) and 8 miles, and I haven't had to do more than that.

I'm unchamoised in the winter.