Comfy at -28 F

Wow, it got really cold last night.  My thermometer said -28 F at sunrise.  Inside it was a comfy 68.  In the greenhouse, low and outside was right at freezing and high and inside was 40.  The 5 kW infloor heat has been running continuously and has also been circulating in the greenhouse floor.  A nonfreezing greenhouse is a good thing.