12Vdc Motor/Generator

It will be relatively easy to produce 12v DC. (off load)
The problem will be when you try to draw power (current) from it.
A roof ventilaltor is likely to be of small diameter and thus produce little torque.
Have you ever seen a windmill with short sails? A watermill can have a smaller diameter because water is much much denser than air.

You will maybe get a few watts, enough to drive a micro controller and some sensors and maybe feed back into a heating/ventilation control system, but you won't generate economic amounts of power.
You need to consider this fundamental problem before proceeding.
Del