Hi, Artstone. I have been greenhouse growing for five years in the desert climate of metro Phoenix, AZ. Wherever you are, greenhouses will easily heat up + 20° - 30° F over ambient temperatures. The fan will help, but not fully resolve the heat problem.
I purchased an expensive ($500 +)
evap cooler, which only kept the temps between 100° - 105° F and required constant maintenance. Buds were good quality smoke but larfy due to the heat. I'd simply keep your fan on the ground to draw air in through the cooler end of your polytunnel and keep all, if any, side vents fully open. Place shade cloth on the exterior to reduce interior temp build up. Honestly, I fought through two summer heat photoperiod greenhouse grows, before coming up with this solution: My winters are mild, so I switched to a late fall/winter/early spring growing season. A couple of $40, temp controlled space heaters keep the evening interior 55° - 60° F. You will need to grow autoflowers since the photos will flower immediately due to the short daylight hours. And, expect at least 2 - 3 weeks longer seed to harvest, also due to less daylight.
I have a 10' x 20' x 6.5' Quictent heavy duty polytunnel - grows 12 plants easily. I installed a power panel and water spigot in one corner, which makes chores 200% easier. I also laid old paving bricks on the entire interior floor which, along with some Stepmaster greenhouse shelving, makes for a super clean, healthy plant environment. My total cost, minus the evap cooler, about $700. I use 50/50 coco perlite, 7 gallon fabric bags and GH nutes.
Good luck with your setup.
