I had to wrap my GH (8x16) in a solar pool cover to help keep the heat in. That helped a lot, too. Where do you get solar blanket material, Ken?
I use one of those oil-filled heaters; they seem to be less expensive to operate and have worked quite well for me. If the outside temps don't get too low, it will maintain an indoor temp a little over 15 degrees warmer than outdoors. Of course, I also use seedling heat mats when I'm starting seeds; much easier to keep the flats warm than the whole GH.