Heat reflectors are used inside the bag to act as a vapour barrier. http://warmlite.com/vapor-barrier
I've looked into the vapour barrier idea as well and I thought it was a decent idea because if you look at houses, they have the vapour barrier on the inside so the humidity inside the house won't saturate the insolation and so it can breathe to the outside. Some houses also have the Typar or Tyvek house wrap that is supposed to be waterproof, yet breatheable. You may think the bad thing is the vapour build up inside will eventually turn to liquid, but thats the whole point. Its so you can tell you are sweating and can adjust your exersion and clothing accordingly. Your sweat may make your skin wet, but your outside insulation will stay dry thus, keeping you warm. Im sure you could wear some kind of wicking under garment as well to add comfort against your skin but it will reduce the effectiveness of a vapour barrier.