If you’re serious about creating conditioned space you’ll be best off sealing the room, insulating the floor and walls and mechanically controlling temperature and humidity. That’s a lot of expense and maintenance. We hate to turn away from free space but attics and basements were never meant to be broadly used. The cool floor is collecting condensation. More ventilation would create still more water. Underground space requires a lot of modifications. That’s why garages and sheds do better.