Go look at them at home cheapo. I've seen them run from $40 to $100 which is a cheap investment. You can get a combo humidistat & thermostat on the better ones. You can add a timer also.
I suggest having a HVAC professional advise on what to do for sure but this isn't a whole lot of money along with a hole in the gable and some wiring issues to deal with. The insulator talked about this subject down lower a ways and you may do a control & f search for insulator as he talked about the very thing you are trying.
I think he had a problem with having 2 of them and creating a vacuum that would pull good air from the house when you don't want it done. Also think about the prevailing winds to help the air flow when you pick an end so it flows with the wind and you are not blowing into the wind.