Won't they go into the barn when it's that cold on their own?
Btw, once they have found a spot for the night, do they ever, that you know of, get up and move either into the barn if they aren't already or just to a different spot? Like if they were hunkered down for the night outside and unkown to you it got pretty cold, would they get up and move into the barn on their own, or stay put for the night?
~ Some go in on there own and some do not, truly no consistency with any of them. They do not understand cold and take preventive measures and self enter the barn, LOL. They do shift, mostly in the early morning hours they begin to all move to the doors in anticipation to be let out.
I have two 250w heat lamps hanging in my barn and I mounted a small fan on the light that blows air over the bulb. It spreads the heat all over the barn. Its a lot smaller barn than yours but it works pretty good.