Right there is why I am glad I live in a southern state. Heating a Gh will kill the wallet. A GH down close to me with 18 large GHs, I think its 18, spent around 18,000 on propane in Jan or Feb alone. I am so glad my two 1500 watt space heaters keep my GH at 60 pretty easily, and I have two layers on 6 mill plastic for covering with an airspace in between. I am going inflated for next winter after some renovating. Goodluck Krik -20 is pretty cold to try to be heating back up. I hope you get something that works for you!
Just thought of this. You could get some large containters like those plastic 55 gallon drums and fill them with water under your benches. Put as many of those as you can fit. They will collect yeat during the day and release it at night. It might help a little bit. I know a guy down here that does that to save a little bit on his heating bill, but I duno how effective it wil be at -20. I would also go with 220 heaters if you could. There is some I was looking at for garages and things. They need to be hard wired in, but they will heat faster and run less then the 110's because they get hotter. Might be an idea. I was thinking about upgrading to those after the renovation, but they are more expencive than 150 though. A GH is not an inexpencive venture I have come to find out.