In that part of the country you will need to think about shade cloth too. Did you put up a kit or build your own? One of the problems I see with kit greenhouses is not enough emphasis on cooling.

I had to learn the hard way from building my first greenhouse that cooling is just as important as cooling unless you want to use the GH for only a very short period each year.

Marv-Montezuma,IA

Here is a link that might be useful: Photo Albums

Do not put the exhaust fan a few inches away. They really need to be spaced equal distance. But since you have one in the middle, put another on one side. Who knows you may need to add 3rd one.
The attic exhuast fans even though they saw so many CFM's are not really that strong. They also are not made to run all day long every day.And are not made for moist enviroments.
You would be better getting a large greenhouse fan unless you could start over and add 2. One on each side of the same end. You can get one to fit the same space but more CFM's.
In my opinion the calculators are a little short. More CFM's is better. I almost doubled what mine should have.
The greenhouse people told me it was too much. Can you have to much ventilation in an oven?
You should be able to keep it at least the outside temp. and maybe a little lower.
Yes you need a misting system(Low cost) or a fogger(big bucks).
And take Marv's advice about the shade cloth. Not only will it cool it will keep your plants from burning.
I'll be glad to advise you.

Here is what I would do:

1) install another 1500cfm exhaust fan, on the same wall as the first fan but not right next the the first fan

2) get shade cloth

3) make sure your intakes are large enough for your exhaust fans

4) mist system

Recently retired from the greenhouse business. We always put houses up with enough cfm to change the volume of air in the greenhouse 2 times per minute. However, we only used them spring to June in the midwest. Shade cloth will difinitely help, but I would never expect to get the temperature in the house lower than the outside temp in the summer time without shade. And then I doubt if you'd get 10 or 15 degrees lower. To do that you would have to have evaporative cooling, but that's less effective in high humidity locations. Try shade and take your sides off. Good luck,Burlap

I designed the greenhouse myself and my husband built it.

The poly cover I have lets in about, roof 70% sun and sides 75 to 80% sun. I did this with the heat in mind. The only wall that has clear is the front. I had hoped that would keep the temp. down. Boy was I way off!

Found a fan at the Greenhouse Mega store and says it's a 20" fan. 3200cfm for $195.00 or 4500cfm for $220.00.
Greenhouse is 18 x 24 x 8 1/2.

Which of these would do best for me? And is this a good price? If anyone knows of a better place to get one wholesale please let me know.

Thanks everyone
Patris

You know me I would go with the larger one. Esp. in zone 9.
You will also need two inlet shutters approx 24in. If you get the motorized ones go with the commerical 21x21. They will do just as good and are cheaper. If you use the non motorized one be sure to turn the shutter fins into the greenhouse.

I'm building a greenhouse soon as well. I'm a little bit confused. I know there needs to be an intake and an exhaust. Do both sides need fans or only the exhaust side? If the intake side doesn't need a fan, how do you calculate how large you need it to be?

http://www.igcusa.com/greenhouse_cooling_information.htm

Try the above link. It will give you a little more info than I can.

You need the exhaust fan at one end and intake at the other end. Hope this will help.

Good luck and if you need more help let us know.
Patris

An air inlet down low on the opposite side would be a good addition, or simply removing the side panels as already mentioned.

I went with the cheap double wall poly panels from Farm Tek this year, they have about 75% light transmission, but also really seemed to help regulate the temps.

I have a full length roof vent with a simple box fan on a thermostat blowing out. We have yet to get anywhere near our outdoor highs, but so far it's regulating well.

I have screened windows on all sides, the roof vent, plus one removeable screened panel for a total of 35 sq. ft. of openings. I'll screen the door for even more ventilation later on this summer.

Natural cross-ventilation helps a lot, the roof fan kicks in when it warms past the thermostat setting.

-Ed

If you use fans only you CAN make it work but need a LOT of fan power. Using a fan that can exchange the air once per minute (go to the site below to calculate that) usually keeps the temp about 9 degrees (F) higher than outside on sunny days. Doubling that (one air change per 30 seconds) would keep it within 5 degrees, but would take some serious fan power! The site below tells you what you need for one air exchange per minute-if that's what you already did then try more.

If you're worried about getting enough sunlight, you CAN shade partially over only your more shade-tolerant plants which will reduce the TOTAL solar radiation inside the structure and the air will of course not heat as much.

Here is a link that might be useful: Fan Calculator