Hey Moonwoolf, we must be neighbors- we are in zone 3 in Minnesota. We started raising rabbits last January. We keep them in wire cages inside an insulated railroad car which sits directly on the ground- no wheels. We leave one half-door open year round. We had no problems with the adult rabbits, but we were breeding and we lost our first two litters (the bunnies froze). After that we started pulling the nestboxes and keeping them in the basement, returning them to the does for feeding twice a day.
We just set up a new rabbitry inside an old dairy barn, I think it might get even colder in there than it did in the rail car, but we are going to build different nest boxes for the does for winter use. We got one from a neighbor which is a plywood box with a hinged lid and a round hole cut into the front. It hangs on the cage in front of the cage door, which is then left open. The entrance hole is several inches above the floor of the box so the bunnies won't be able to fall out of it. The box itself is approx 12"x12", so it can hold a lot of nesting material. We are hoping that the bunnies will be kept warm enough in these- we might also try an electric light bulb or a heat mat under it for the first few days for extra heat, we'll decide when we see how cold it is in the rabbitry when the time comes. But, I definitely do not want to haul nest boxes in and out of the basement next winter.
I just put some redworms under our rabbits last week, but I am not sure if they are still alive or not- we used sawdust as bedding and it may have been toxic to the worms. Our chickens also got in there a few days ago and may have eaten the survivors- I have to go out and dig around in the bedding tomorrow and check :grump: Worms are definitely the way to go, though, but take my advice and don't use sawdust as bedding unless it is well aged. I knew it could be toxic but thought ours had enough manure in with it that the worms would be ok- maybe not.
We are discussing the possibility of making some kind of indoor chicken tractor to use for housing the chickens inside the rabbitry during the winter. We might also house a couple of feeder pigs in a pen adjacent to the rabbitry. Using the deep litter system these might add some warmth to the rabbitry- although, the area above the rabbits is open and the old barn has a high arched roof, so not sure we could keep any heat in where it would do the rabbits any good anyway. We considered using visqueen to try to keep some of the heat in, but with rabbits you have to be careful of condensation- too much can cause respiratory problems. Just kicking some ideas around- we'll have to see what works when the time comes. But, I think you should be ok with adult rabbits as long as they are dry and you can keep the wind off of them. If you are going to breed, you will have to be careful with the bunnies. We asked for and were given advice before we bred our does last winter- were told that the does would make nests and the bunnies would be ok- that was not the case here and we were distressed over losing the two litters, so, take all advice with a grain of salt.
Good luck- we really are enjoying our rabbits- great eating, great manure/compost for the garden, and as soon as I can find some old sawdust we are going to re-stock the worms, lol!