Hi in PA! How many chickens what age and breed? When you say the bottom of the barn I assume you mean the old barns that you can drive underneath whick typically means at least one side is earth? Maybe not, but more info is always better.
Anyway, I have mine in a coop with an attached run, six per unit. At night I lock them into the coop portion our of the wind and elements and I stays approx 8-10 degrees warmer than the outside temp and no wind. I am in New Hampshire and perhaps a bit colder than you.
It has been as low as 8 degreees with winds up to 50 mph and my girls are fine without any heat. Heat is dangerous and can make a bird stationary which could also be bad. Let them be birds if you dare, after all they have nice down coats on!!