Yes there are probably better alternatives. First is to remove the hot air by one or a combination of many methods: Natural ventilation. exhaust fans, evaporative wet wall combined with exhaust fans, etc.

If that isn't feasible or adequate then reduce the heat load with shade cloth. There are many other options but that's a starting point.

Thanks for your reply. That's something I forgot to add. The entire southern side is covered in shade cloth, and there are 6 windows on the roof, as well as 3 slatted windows on the sides. This is a 16m² greenhouse, if that helps.

Sounds like your greenhouse is grossly underventillated.

Light is the LAST thing I sacrifice. You have to get the heat out by circulation. Not just vent out, you need to get as much air in as you take out.

My greenhouse has a similar floor area. I have a 1.2m2 top vent and 2.6m2 of bottom vents which provide passive ventilation to replenish co2 levels. For heat control i use a 170w fan and 200ft of perforated drainage tubing buried in 30T of cool clay soil beneath the greenhouse. My neighbours thought i was a little crazy when i installed it 6-7 years ago, They prefer the traditional method, block out the light with shadecloth and leave the doors/vents wide open all day and night :)

Right, so it sounds like the greenhouse needs some active ventilation. The greenhouse I'm in has about the same areas of windows as yours, Hex, but more on the roof than below.

Thanks for all your advice.

For effective passive cooling your top vents need to be 20% of floor area. That still leaves you 5-10C warmer inside than out. Active ventillation will give a little more cooling than that. But to maximize cooling you need a wet wall and exhaust fans on opposite ends of the greenhouse.

The vent area will depend on the height difference and temperature gradienr between the upper and lower vents. In my case its around 3m, the greenhouse is a dome so the shape acts like a chimney. The top vent is dead center with 14 lower vents arranged around the entire base perimeter.. The air flows up the curved walls and exits via the central top vent, it can bring the greenhouse down close to outdoor ambient fairly rapidly. When outdoor ambient tops 90f, you need more than passive venting