Just finished a couple of modifications to my Sub Zero Sleeping System.
The old system, with a Brit Army Bivi bag worked fairly well but the Goretex stopped breathing at low temperatures which resulted in ice forming on the inside which transferred dampness to the synthetic mid layer sleeping bag.
Not too much of a problem but not ideal.
The new Shell bag is made from Ventile which while not being quite as "waterproof" makes up for it in a cold environment by being much more breathable. I made this bag some time ago but haven't used it much because it was awkward to get in and out of. I've improved that now with a water resistant zip on the side.
The rest of the system remains pretty much the same. A down filled inner bag for maximum warmth to weight ratio.
A synthetic mid layer bag which is less damp sensitive as condensation may start to occur in that layer.
Finally the new Ventile Snow Shell.
Underneath all that I have a 9cm inflatable down mattress sitting on a folding foam mat for puncture protection.
The other essential part of this system is a tubular hood made from a microfibre towel.
This is used to absorb moisture from breath which could otherwise condense in the sleeping bags or shell compromising their insulation properties. It can be dried out during the day much easier than the rest of the system.
Simple but effective.