No, the floor is not insulated, which is why I'm even asking. I would think that insulating the underside would add considerable help during cold weather and in the summer heat. No, there area no vents in the garage, but thats to be expected. Garages were mainly designed for vehicles not people but its been quickly found that when the toys stay home or are outside, this extra space is a great "extra room". I would think hard foam insulation would be better than soft batts but I'm not sure I'm going to hold anything up against the underside without letting moisture or debris from getting in between and causing issues.