My darkroom is a wooden shed in my back garden and during the winter months I permamently run a trickle heater designed for use in garden greenhouses. It is fixed to the wall in brackets, is about 24" long and is 180 watts so the electricity usage is minimal. So long as the door closed it maintains a low heat and keeps the darkroom quite warm at the times that I am not working there. When I go in to print I have a 2KW convector heater that heats up the room to the 24c that is set on the thermostat. I never have any condensation problems no matter how cold it is outside. I know that we don;t get the low temperatures that some of you guys get but I do live in Northern England where it does regularly fall below freezing and is always damp and wet.