The most efficient system for an exhaust fan is to have the fan up high (hot air rises) and the intake vents low. This will bring in cooler (hopefully) outside air in the bottom part of the greenhouse and vent out the warmer air through the high-mounted exhaust fan. If you have an exhaust fan and only a ridge vent for intake, you will probably just create a venturi effect in the upper area of the greenhouse, which would not be very efficient in relation to heat exchange. The shade cloth is basically a trade off, in very general numbers, 50% shade reduces the heat load 50%, but also the light 50%, and plant growth 50%. You want the minimum amount of shade that you need to keep heat loads down to reasonable levels. Bedding plants and rooting cuttings have a lower light requirement than tomatoes or other fruiting plants, so you can have higher shade if needed.

Thanks for the reply! Can I put the exhaust fan and intakes on the same wall?
Sarah

Sarah,
It is best to put the exhaust and intakes as far apart as possible, including exhaust high, intake low. If they are close together, the exhaust will tend to just pull air directly up from the intake, and not completely ventilate the rest of the greenhouse. CR

Sarah,
I have found some cheap auto vents at Lowe's. They are called FOUNDATION VENTS and they are designed to replace a concrete block so they are 8" high by 12" long (I think that is the size of a block). They have a bi-metal coil that opens and closes them. I am looking for my notes now and will post when I find them. They only cost $13.00 each and are UV protected and have screens. That is what I plan to use. I plan to mount them as low as I can along the long sides of my as yet incomplete GH.

1eyedJack and the Dawg

What a nice size greenhouse you have!

As Cuestaroble says, I think the idea is to pull as much air as possible across the whole length of the greenhouse, so that's why the exhaust and intakes are usually on opposite walls, moving the air from low to high.

Trying to get a better picture of your situation...I understand that one wall of your GH is attached to a structure. Is the attached wall the long side (26') or the short wall (18')? Which wall(s) have the side vents? If you designated north, south, west, east then folks might be able to toss a few more ideas your way.

If you can draw cool air in at floor level and pull it up and out the roof vents, you can drop the temp in a GH to a considerable degree.

So, if you have ridge vents and can install some aluminum braces to serve as rafters, for a GH that size, about 4 - 6 of the largest, 5 blade outdoor rated ceiling fans will outcool any side wall mounted exhaust fan by a mile, and run silently to boot.

As already noted here, you need to have your air intake as close to the ground as possible, preferably on the north (cooler) side. If the GH is not on a direct N-S axis and you must mount the intakes on a west or east wall, it's best to keep the area directly above them shaded from the sun.

With a glass GH, you could also go back to the old, but still effective method of applying GH whitewash to the exterior in late spring, which will cut the heat but still admit quite a bit of bright, diffused light. Then in fall, just power wash it off.

MudHouse,
Thank you for the reply. The attached wall is a short wall(18'), on the east side. That back wall, which is the north wall has the side vent, but behind that wall is another structure, about three feet away. The south facing wall has no side vent and the west side has the door going outside. I hope that this helps you get a better picture.

It seems like such a wierd setup to me, my employeer bought it this way, and some how the old owners managed to keep it cool enough to grow cut flowers in, i just can't figure out how!!

Ole Dawg,
The only place i could replace a block would be on the north side, which is only 2 or 3 feet away from the other part of the office building, which I feel is to close to draw in air. The south wall has 4.5'x5.5' panes of glass that go to the floor(or just about to the floor)so i can't put one there.
Thanks everyone!

If that area is shaded as you noted it would be a perfect place for low mounted vents. Remember, these vents don't have to replace blocks. You can put them anywhere you have a mounted surface.

1eyedJack and the Dawg

ole_dawg 7

At what temperature do they open?

I juist went on line to check. It says full open at 70 and closed at 40. BUT I thought that when I bought one to look at it several months ago it stated closed at 35. I could be wrong. CORRECT size is 8 high by 16 long

1eyedJack and the Dawg

Thank you birdwidow. That is a good idea, i will check into that. I will also check into the foundation vents. Thanks everybody, i am feeling so much better about this now! HAPPY THANKSGIVING!

ole_dawg 7

Thanks. 40 is too cold for the plants I grow in my GH. The Rion auto-vents start to open at 70 and close at that temp also.