I have camped on Mt Rainier, Mt Baker and Mt Adams [in Washington State] and also -5 degrees East of Brecksville Ohio. I have an REI Expidition bag rated to 25 below zero. Do not sleep with your head inside the sleeping bag. The moisture from your breathing will present problems and questionable comfort. Instead, I always wear a full fleece hood. In cold temperatures I still wind up unzipping my bag a little to cool it down a bit.

Dick