Downside is the propane heaters create moisture. I would go with a simple space heater that you can set on low so it only uses 500-600watts, that should be plenty and the easiest solution.
You are right about the humidity. most already have a dehumidifier in play. honestly my co2 generators (natural gas) help me with humidity because it's so low here in Colorado that I have to run swamp coolers in my room to add humidity otherwise it sits in the 20's. So in my situation the added humidity is a bonus, probably not the case for many unless in an arid state like Colorado.
If trying to keep the power bill low for stealth as is required in some states then the propane or natural gas is perhaps a better option.
Either heat source will work.
Obviously anyway one can use lighting thats growing weed as the heat source trumps other methods of heating. Along with a veg room, I have two flowering rooms in my basement. They are on a flip. Each is 10kW. Because of this I have radiant floor heating in the winter (not so fun in the summer lol) so we only run the furnace during a harvest or if it's below 0F and windy.
A very efficient solution may involve running two tents on a flip and recirculating air between them with a tiny about of fresh air allowed to maintain CO2 levels. May not be right for this grower but perhaps someone will benefit from that setup idea.
Dehumidifiers also add heat FWIW. Can be helpful when lights are off.
Insulation is always good. If possible insulate the whole garage, especially the door. Then a little heat in there would keep things nice and toasty. Thats probably about as much expense and work as setting up another tent but without the extra weed, well maybe not that much money but you get my point.
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