Thanks Dave. I think I read somewhere that in a well insulated relatively confined and closed space that one body there for a couple of hours is enough to raise the temp by a couple of degrees Certainly once I shut the darkroom door and am in there for a couple of hours the temp will regularly rise by 2 degrees.
They report no increase in electricity consumption overall in garden shed darkrooms in Scotland but that's because they turn off the heating in the house as they leave it
Oh and wipers blades when travelling under bridges!
Mike