Here in NW Oregon, Zn 8b, I've been using 8 - 48"x20", 100 watt heat mats on the shelves, with 4 thermostats that can adjust from 68 to 80, to heat the potted plants in my 8'x8'x16' Aadco GH, with a corrugated fiberglass cover and a plastic interior lining . I also use an electric heater with the thermostat set at between 40 and 50 depending on the plants in the house, to supplement on really cold nights. It does a very good job.
