i would be more worried about temp than ventillation by the sound of it!!!

dude if your not carefull they will freeze to death in this weather! you should put the HPS on just to keep the frost off!!!

you say your broke, but a few cheap improvements may help....
invest in some of that insulation spray-foam, use it on the holes under the shed roof, etc, and a heat pad under the pots like whats used for reptiles will help a bit also.its cheap and will keep the cold and draft out a bit...


merry christmas, and good luck!