Garage moisture problems, contractor's bogus solution, or is it?

By code you do need a fresh air vent to supply air for the heater if it is a draft or power vented heater, not nesissary if it is high efficient. I would always recomend one anyways. Like the other two contributors said also, if it is a vent free heater, which i've never seen Modine make, and after scouting there site at a href="http://www4.modine.com/v2portal/page/portal/modine/modineHomeDefault" rel="nofollow"http://www4.modine.com/v2portal/page/por…/a I found no vent free heaters. Unless they installed a greenhouse heater? Read your installation manual or post model and serial number so someone can check for you. What also could be happening is that it is not vented correctly and could be spilling flue ges into the garage which would account for moisture. Put a carbon monoxide tester in there, and don't put the car in there while you test as the exhaust will set it off. Good luck