Re: Dehumidifying a dojo
I assume you only run the heat when you're in the dojo (to save energy)? If the structure is well sealed, then running the heat won't help since the water is just absorbed into the air and then condenses when the heat is eventually turned off. Definitely a mold problem.
You might try a decent ventilation fan (e.g. Broan) plus a timer control, ducting (so you could leave it running after you were done training) costs less than a $150 total. You would have to wire it and run the 4" ductwork to the outside.
A dehumidifier is problematic - they are basically AC units and use a lot of power and don't work very well below 40F. If the structure is well-insulated and well-sealed and you are willing to leave the heat on to keep the structure at 60F+ - then a dehumidifier that drains outside (or to the sewer) would be a good choice to keep your dojo nice and dry.