should be somewhere in the teens,, generally around the middle but can get lower. weather does vary , the later in jan usually the warmer.. the huts are pretty low in altitude, but not that hot at night. they are so big the fire and bodies dont do that much to push the heat up.. the hut doesnt retain warmth overnight.
just get them to carry as much clothes as you can and you can skimp on the bag warmth..
having said that, read page 11, the temp has gone down to 2 degrees at it's lowest, even though it can reach 30
http://www.franks-alpin.de/contents/Rei ... -08-09.pdf
from the land of the long white clouds...