My rabbits in the cold.?

okay i have 6 rabbits and they live in there cages on my back porch. I had them for a little more than 5 years. Well in the winter i would cover there cages with blankets and old sweat-shirts or jackets at night. But this winter it is really cold and there water bottles freeze. I have to de-freeze and fill up there water bottles with hot-warm water twice a day. I can't bring there cages inside because i have four big cages and i have no where to put them and i can not leave there cages inside with my dogs. On my porch it is completely enclosed but there is no insulation, and there are windows completely surrounding the porch, right now the windows are all shut. When i walk out on my porch wearing just a t-shirt it is chilly but not freezing cold, i can sometimes at night see my breath if i breath. In there cages they have a little house they can hide in and they have shredded newspaper all in there (i don't use bedding anymore because two of my guinea pigs died from inhaling the cedar dust). Is there anything else i can do for my bunnies, maybe should i get a small space heater and keep it on low.???? any other suggestions.???