I have used both a 0 degree and a -9 degree bags at huts depending on the hut... If it's a busy hut where I know there will be a lot of people stoking the fire during the night,then I go with the 0 degree bag. If it's a more remote hut that doesn't have a fire place, then I go with the -9. That being said, if there is a chance you are potentially going to be be stuck out in an emergency bivy then plan accordingly... Not sure if that answers your question specifically... I've done Bow to Peyto several times with the -9 and it worked out fine. I have not done the other huts on the Wapta so cannot comment, but know several people that have had to bivy going to the other huts on the Wapta due to limited viz, so be prepared...

The above is a good response.  

I've only ever used a 0C bag in a hut and I've been too warm most of the time.  But if you have to bivy...