Building my first coop. Have read how important ventilation is . . . Am putting heavy OSB on inside and outside walls over 2X4 frame. Floor is concrete. I was thinking fiberglass insulation would breathe better than styrofoam, but husband wonders if styrofoam would work. Will have window that opens and a vent, of course. Our winters in northern Wisconsin get pretty nasty, sometimes 20 or 30 below zero. What R-factor will keep them warm enough without adding heat, if I buy cold-hardy hens? Or is a heat source mandatory? Coop will be 4X8 by 6 feet tall and I am planning on 9 hens.