stick,
If I remember right, our garden rooms are a similar situation. I am putting back in the propane heater I was using, the girls seemed to like it better. It`s a wall mount, with thermostat, 99.9% efficient and low oxygen shutoff. Have a RV auto switchover valve with two 20 lb. tanks outside the room. Also 2 passive air intakes, 8" exhaust fan with 2" squirrel cage blower as backup, or for real cold weather usage. Do close one intake off also. Need the fresh air exchange year round.