The exact temp, I don't know, but I do know someone pretty well who kept rabbits outside and didn't do a good job winterproofing. in the winter, one froze they think. (then again it was also a very old rabbit, but they asume it froze which I was so upset about :''( )
Around me it can get down to 0 in the winter (and every once in a blue moon slightly negative temps) with negative wind chills like alomst 15 below. We have heat lights (just low watt light bulbs actually) in they hutches that we turn on. I also got Ripley a fluffy dog bed and Pile it up with hay. I'm thinking the lights don't actually do so much and I might be better piling hay even higher so they can borough and tunnel through more and not turn on the lights ( i worry about a fire anyway, though we try to keep it VERY safe.)
Mine have always been fine. MOre i've found the hazard to be bringing them in out of the freezing cold to the warm. I think that's alot worse then leaving them out if they're used to it already.
Also I up the pellets some in the winter for warmth, and give them each they own blankies to cuddle in.
Also we brought plastic that we stapled on as flaps to the hutches at the hardware store. In the summer you flip them up on the top of the hutch. For winter I staple or bungee them down, and pile several THICK blankets on and a tart or two.
I always try to keep the hutch FILLED with hay.
We go through lots in the winter. lol
Also, they hutches we try to keep out of the wind to help keep the windchill out.
