For winter camping in the snow, I've been using an additional groundsheet which goes inside the tent on top of the existing groundsheet. It's an off-cut of Cromar Vent 3 building membrane which my sister gave me. It's brilliant and seems to insulate very well against the cold - it's actually comfortable to the touch, when the groundsheet underneath is freezing.
I've used it for the past two years and wouldn't camp in snow without it now.