How cold is too cold?

Well winter is coming. My rat room is outside and I have been thinking of getting some kind of supplemental heat for my rat rack as it can get really cold outside. I have had a year where the water actually froze . I saw the heat cables and wanted to know how hot they got, if they where good for multipule cages (or levels on a rat rack) I would like to rap it around the water bottles or beside the water bottles so they don't freeze and along one side of the tub so that they have one side that is warmer. When it is cold I have shaving and put hay in so they can make a nest so the can keep warm but they don't produce much. Last year I quit during the winter and I have had a heck of a time getting started again. I can't just put a heater in the room as it doesn't have a door so all the heat would just drift outside. I already have a heat lamp that I can aim toward the rack but it can't cover all 3 levels that I use. Thanks for the help. I would also like to know if the zoo med repti heat cable is any good or if there is a better product out there. Thanks

Can you hang a tarp or something over the door opening?That would help some.I bring my rats in for the winter form October till April here in Pennsylvania.I keep 1.3 to a tub and have to clean them every 5 to 7 days when kept inside.I put a few drops of vanilla in the water bottle,it keeps the ammonia smell down a little.They are in my furnace room so they have heat all winter and actually produce better in the winter that outside in the summer.