LaCrosse makes some very warm pac boots that you might want to look into. The IceMans are advertised with a cold temp rating from-45 to -80. They also have the IceKings which are suppose to be rated for -80 to -150 degrees. I've had a pair of the IceKings for ten years now and my feet have never got cold with them. Already this year I've hunted in single digit temps with twenty mph winds and my feet stayed nice and warm.