You have done almost exactly as I did, except for the flooring. We get even colder than you and have already had nights/mornings of 22 (twice), 21 (twice), 20, and 19. Brrrrrrrrrrr
I have now placed another layer of solar blanket over those two sides and roof (one piece) and hope it will give me even more insulation value. I'm using 16 mil material.
