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My greenhouse (3mx3m) came with my allotment, fairly knackered, without any ventilation & I've just had it repaired. However, the panes are not standard size (24"x24"); they measure 24 1/4" by 28 1/2". We couldn't put in any automatic opening vents or louvres because they only come in standard sizes only (18"x18" or 24"x24"). There is no manufacturer's name on the greenhouse. I found a Sun Vent 100

Smart Solar Vent 100 , with FREE Delivery, Accessories

and installed that. It puports to extract 25m cubed of air per hour, so I thought that would do it, but I went down there today to find it really hot inside, and some of my lettuce wilted :-( presumably from being too hot, as it recovered after I left the door open.

How can I get some automatic ventilation installed?? Getting down there first & last thing to open and close isn't an option.

I thought about getting a perspex sheet cut to size and then getting the standard size louvre inserted in that.

Any ideas or tips most gratefully received!!