We bought a bat house for the home we're building, that should be ready by
January. Is it best to mount the bat house on the south side of our roof
eaves, so it gets the warm south sun in the winter?

Hey Melissa -- you don't want to mount that on the side of your house.
There are a variety of issues not the least of which is the guanao (bat
poop!). Your best bet is to either put it on a pole 15-30 feet tall or the
side of something like a barn. You can also mount it on a tree but a pole
is usually better (safer for the bats from predators).

James