I have the 0 degree (F) mummy bag with the insulated bivy shelter and it works very very well, especially when coupled with a quality ground pad(I use the thermarest accordion style pads). I was out last week in 12 degrees (F) ambient and -5 with windchill. I wore long johns and a fleece top, wool socks and a fleece beanie--I was very comfortable....Until it was time to get up.