I always follow the posts in the greenhouse thread, but I usually don't really have anything to add, since my greenhouse is not free-standing.
My greenhouse was built on top of the garage ( in 2001) and I walk in through a room upstairs, dimensions are 20 x 12 feet. If you look at the specifications of the Geneva model at the Florian greenhouse site, walls and ceiling are double glass panes. The heating system is connected through three ducts, (pulled from the oil burner in the cellar through the garage and back up into the greenhouse) with the warm air circulating through the whole house.
Now to the good part
- I have the thermostat set to night time minimum of 60 degr F and noon time maximum to 70 degr. I was holding my breath if that would hold at night time temps outside of - 2 degr F and quite a bit of wind. Yes, this greenhouse held up just fine, temps were 60 degr when I checked early in the morning. It has been toasty and comfortable in there this whole miserable Winter here in NJ. On sunny days the temperature climbs during the day above the set temperature in the greenhouse and so circulates through and warms up the whole house until sundown. On the sunny 18 degr F daytime outside, it was 78 F in the greenhouse, very nice!
So, if you ask how is your Gh holding up in this cold, I would say, very well.