Not much, Alan. I've got a little electric radiator thingy in there. Doesn't do much, but so far I don't need much.
I know what I want to do: a nice gas heater up near the ceiling, vented through the existing chimney. Hopefully I'll get round to it, too. The hold-up is running the gas line out there. Needs to go through some concrete and then into a crawl space and then up through a wall. It's a bit of a project, and I have not yet got up for it. Plus: money.
Today it was 53 in there and I worked for 4 hours or so cutting and bending aluminum for the tub. Should be on schedule to lay it up over adhesive by tomorrow afternoon.