first of all, wtf is a fart fan?? lol..
secondly, yah, 88 is a bit on the warmer side of things unless you're running co2 like you said... if it's cooler outside of the shed, pulling the cooler outside air into the shed would be a good idea..
if it gets too cool, just make sure that the airs not coming in and going directly on your plants as if it's too cold it can also harm your plants as i'm sure you're well aware of..
i would just vent it in as far from the grow as you can so it has a lil time to warm up before it gets to the plants if it's going to be getting rather cold.. if it's not going to be getting really cold, you'd be fine to vent closer to the plants, lol..