My grow room is not in a garage but in a underground concrete bunker in northern Alberta that we can't allow to freeze but in the almost 20 years I've owned the house have never had to apply heat tho it has got down to just a couple degrees C above freezing. Even this last summer with the heat domes it never got above 65F down there either so have never needed A/C either.
Couple things you should have is a temp/rh controller for your exhaust fan so it only runs when needed to reduce the heat or rh and a heater with a thermostat to make it come on when it drops below your set temp. I bought a 120/240v baseboard thermostat and wired it into a metal electrical box like the kind you would use for a light switch with cords on each end so I plug the heater into that with it's control on high and let the thermostat turn it on and off as needed. The climate is controlled in a very narrow range whether the lights are on or off. Also wired up a speed controller like the thermostat so the fan runs at 60W instead of the full 120W at full blast. Air is pulled into the room through filters from the rest of the basement and not directly from outside as it gets to -35C and up to +35C out there.
To keep tent walls from getting sucked in some cross bracing with string, rope or strong fishing line to prevent them bulging inwards will do the trick. Heavy blankets, comforters or sleeping bags from a thrift store will do the job pretty cheap too.
what a concept
