We built a greenhouse two years ago and it has a roof vent and a side vent. Even with the roof and side vents and shade cloth it gets really hot. We installed a fan with a thermostat for those hot, hot days. I'm on the Mississippi Gulf Coast and in the summer there is no planting in it because temps are in 100's. Our winters usually aren't super cold and the temps get warm in the greengouse, so the fan helps.
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