Will,
You're getting answers, but no explanations. Grey Knob is an odd facility. There's a wood stove that is almost never used, and the word is that if one leaves water on the ground floor, it may be freeze because the ground floor is not all that warm when it's very cold outside, and it is also very very humid.
However, one sleeps on the second floor. If one is there on a night when there are many others, and in winter that is often the case, all the body heat rises and stays up there. The result is a rather remarkable heat differential between the two floors. Even on the coldest of times when I have been up there, the second floor is still above freezing, and maybe into the 40s.
So I agree with TJ that a twenty degree bag should do it.
Brian