BMW thermal long johns
I recently shelled out AUD175 for some BMW "phase-change" thermal long johns. They have a neoprene – like material which reacts to the wearer’s temperature by opening and closing it’s pores to either trap heat when you're cold or release heat when you’re hot. The sections of this new material run from below the front of the knees up along the top of the thighs, and on the seat and up to the (high) waist.
I was surprised at how warm they were in 5ºC conditions under my thinnish Tiger Angel Element pants ; they are considerably warmer than polypropylene or polartec thermals and in spite of the cost, I would recommend 'em.
A top made from the same materials is also available.