I am thinking about getting a big agnes 0 degree and combining it with a bivy for winter camping in the rockies of colorado. I am an avid AT skier but am wanting to get into longer trips. Would this bag/bivy idea be ok for the cold winter high country? How much warmth does a bivy add?
This bivy would be fine for your intended purpose. As far as warmth is concerned, maybe five degrees but it will keep the elements out & this is the most important factor.
I think the ground is really cold in the winter and I cant imagine this thing actually keeping anyone much warm other than a bit of wind chill and water, but I wouldn't expect much more than 3 degree's advantage. They should have it engineered and tested, with specs for those who'll actually be using this type of gear to survive out there.