Good info for humidity. Thanks Ken.

Dave since you are digging a cistern anyway you could dig a rock bed as well. Dig a pit and run a pvc pipe (type with holes in it) under the rocks use a fan to pick up the heat at the roof line and blow it down a solid pvc pipe and into the rock bed heating it with the extra heat at the roof. Hope I made some sense of this. We did pour an insulated concrete floor with a dark color to collect more heat with a brick center for drainage and Ric found some old metal radiators they were removing from a church that he is filling with antifreeze mix and putting under the benches on the south side that should also add heat

I have a greenhouse just for my Orchids and C&S and some odds and ends, and it is part of the house. I love it!
You are in zone 8b, and as you said you will heat it.
So, personally, I would always prefer to have a greenhouse attached to the house, if somehow possible/there is room for it. It will be more energy-efficient, since you can circulate the heat from the house through the greenhouse and back. The greenhouse will heat your house on sunny Winter days. ( many times the oil burner here doesn't need to kick it until the sun goes down) And it is much easier to step into it during the Winter.