Hey Amanda, I live in Florida and we are going to get a mid 30's night tonight. This is the second winter my rats have been outside. The first, I would bring them inside when it got in the 30's, this year I simply cover the mesh screen on top of their bin in bedding for insulation, add newspaper, and wrap the rack in a plastic sheet. I have found there to be a 20 degree difference inside their newspaper nest and covered rack than the outside temp. The ammonia can get high with this method so clean bins are a must.
I have yet to dose with Tylan for RI, while last year I was dosing continuously. Last year I lost 3 rats (all very new to the colony) and this year with my established colony, I have only lost two that were sudden deaths.

I hope this helps! I bet in an enclosed area outside with this method you wouldn't need to use a space heater.

For the summer, I am sure they stay in the shade and run a box fan pointed down into my rack. Everyone was much happier and I had no losses. It kept the air circulating and the bedding drier. I noticed an increase in litters (too hot to nooky!) and a decrease in how much they drank and urinated. Just be sure to haul old bedding away when you change bins and it will keep the smell to a minimum as well.