I just built a 10×12 greenhouse and it’s already getting to 100F on a 70F day. I’ve installed two windows and an exhaust fan. The exhaust fan is capable of 860cfm but I might need to install a second one. I bought a 50% black shade cloth but putting it over the top of the greenhouse blocks out too much light and the plants only get about 200-300ppfd with it over the top. I currently have it inside in front of the cucumbers because they seem to need it the most and inside is the easiest to hang it up and take it down. I’d like to get a white shade cloth to hang up on the inside facing south but not sure if I should go with 30% or 40%. Should I also buy a second exhaust fan or will one be enough with the shade cloth? I could also get intake fans for the windows but, can’t find anything besides computer type fans which aren’t strong enough. Any ideas for keeping this thing cool are welcome.
by mobo_dojo
29 Comments
https://preview.redd.it/on549dhmntxe1.jpeg?width=4284&format=pjpg&auto=webp&s=549b537736b15a08d5b37448ec6d29f6c384ba5a
My other images aren’t showing in my post. Here is the exhaust fan
That fan seems very undersized.
I’m in 7b and the door on mine is now propped open. It has a heat operated window up top that opens at about 80⁰f
If you want more heat control with a tighter heat window. You will need controls on the fan but also a way to let air in, louvers, or passive inlets. If the fan isn’t getting it done, you might move to shade cloth on the outside.
Shade cloth
More ventilation, whether another fan on the side or a hatch in the roof. Good airflow.
Can always get a swamp cooler if all else fails but shade and ventilation should fix your problem for that size greenhouse. You can also just get a more powerful fan (like a box fan or I’ve used a dewalt if you don’t have power) and leave the door open
You can have a sealed environment that has negative pressure or positive pressure, figure out which one you want and go from there. I would replace the door with a screen door for max intake, increase the size and add another shutter fans in the back. Courgated single wall polycarbonate doesn’t have any uv protection so you will have hot spots. Shade cloth on the outside should help but active airflow and humidity are needed. Good luck.
As previously mentioned, an exhaust fan pushes the hot air out. You need to let cooler air in at the same time.
You could turn the thermostat down to a lower temperature to get hot air out sooner.
Greenhousemegastore.com has a nice range of shuttered exhaust fans appropriate for your structure. 12” fan ought to handle it easily.
Worth noting that it takes a lot of effort to run a greenhouse at cooler than ambient temperatures.
You need more windows or vents. Even if your exhaust fan were insanely efficient (it’s probably too small), you still need more ways for cooler air to get in and replace the hot air.
Windows are very low. You want something at the roof level (or a roof panel that louvers) to let the hot air escape while drawing cool air in at ground level. Perhaps a second fan at ground level pulling air in?
Put a 50% white shade cloth over the top of the GH. It makes a huge difference.
I’m about to build a greenhouse, and the question I’m asking myself is: why not remove the overhead panels for the summer, once overnight temps are above freezing?
Is there a reason, other than convenience? Are they part of the structural stability?
Keep the door open. The issue I think is the design. I’m not an expert in the slightest. But air has to flow through. Most openings are only on one side. That will not suffice when things can heat up that quick. They probably should be bigger windows on the back and optionally the side or even roof.
Ideally you would have 2 roof vents and 2 floor vents. As the hot air rises out of the roof vent it will suck cool air in the floor vents and keep it much cooler. Shade cloth isn’t going to do much for reducing heat when it’s installed inside, you’ll need to put it outside to prevent the sunlight from entering the greenhouse at all. Doing these 2 things alone will make a huge difference.
The CFM on your fan is probably about half of what it should be too, there’s calculators online you can use as a reference, but proper vents would be just as effective and the fan could be secondary.
I have this in 6B, Western MA, and it’s perfect. 55% Aluminet reflective shade cloth on the outside, spaced about 12″ off the roof (not ON the roof). You might also consider the grey/smokey plastic roof to cool it down a smidge. That won’t kill your plants, promise.
Shade cloth
Two fans on either side. One blowing out, the other blowing in.
Retractable shading.
get an aluminet shade cloth, (mylar, reflective) and put it over your greenhouse on the outside. i’m in zone 6b and use 50% screen from mid April to the end of October, for my orchids and citrus. (recommend greenhouse megastore).
Go 40%. I open the door daily with a box van in the doorway too. Mine is smaller at 6×10 but a window opens up top. I also put an auto opener on that. Only like $20 from Amazon and works like a charm.
You need a larger fan. That tiny fan won’t move enough CFM’s to cool the area.
Second the shade cloth thought. Additionally I added this ‘insulation’ material on the side that gets baking son from the west. It works winter and summer. Last suggestion is don’t plan to grow anything that can’t survive those and higher temps unless you want to invest in a cooling system.
https://preview.redd.it/l4vgm48njuxe1.jpeg?width=3024&format=pjpg&auto=webp&s=c86bf56a134e7f5d37a339eec38671bdc11555c7
Based on CFM calc, the fan they have is recommended. If it’s 100 inside now and it’s 70 outside, another fan isn’t going to solve the problem.
I use 70% black shade cloth over top of my greenhouse, with vents and an exhaust fan. In the dead of summer it’s too hot but otherwise it doesn’t get too bad. You could also install misters, that might help.
It is hard without knowing your climate. I had success with misting fans and a very large evap cooler. This would not work in a high humidity area.
Edit: Also many shade cloths over the entire structure. They were beige.
Are those small windows your only ventilation?
I have a 14ft x 14ft with 2 12inch fans. Still needed a shade cloth. That’s in Michigan.
I use a 40% white shade cloth
I built a hoop house, with 2 ft screened in risers all the way around, and a screen door and screen window on opposite sides. This thing was designed for ventilation. I had to completely remove all the plastic over a month ago, because everything was cooking in the heat.
I don’t have any solution for you. Just sharing your pain. The structure is now just an outdoor shade house, because they get plenty of sun even with multiple shade clothes.
Harbor freight sells shade cloth now. Get some! Put it on the outside. Double it if you have to.