How large is your greenhouse? You can see pictures of how I mounted a 12" exhaust fan in my greenhouse here:

Mounting the Exhaust Fan

The plastic does not support the fan. The fan is supported on two steel angle irons that are held in place with "T" bolts in the upright aluminum channels. I was able to mount the fan and then mark the position on the polycarb panel. Putting in a fan in an already existing polycarb wall is going to be a bit harder since the hole for the fan clearance will determine the position of the mounting holes in the angles. I sealed the joint between the exhaust fan box and the polycarb with silicone after taping the upper and lower edges of the twinwall opening with tape:

{{gwi:287678}}

If you make a bigger hole you won't need a fan, saving you money and installation cost. A 2 foot square would be adequate for your ventilation needs. Put some flyscreen or shade cloth over the hole to keep out buglies

I'm weighing here only because Nathan's suggestion MIGHT work but only if you carefully consider a number of things.

He is often the one to suggest low tech solutions (of which there is nothing wrong) BUT....any solution high OR low tech is a balance of a number of factors. "Cutting a bigger hole" only has a chance of working IF you also incorporate something to allow "make up air" to enter the gh through low placed intakes (shutters or vents) AND the "bigger hole" has some kind of heat sensitive device to get it to open when you need it to open.

Of course, you'd also need to incorporate intakes for a "higher tech" exhaust fan as well.

You should also IMHO consider what you plan to put in there. I have orchids in my gh (moderate tech at this point..moving toward higher tech)...I'm only an orchid newbie but while many can tolerate mid 90's, they sure don't thrive above the mid 80's.

Finally, I'd also suggest you seriously consider(as in honest with yourself - no one else will know) how much attention you REALLY will pay to the gh day after day after day.

Obviously the lowest tech solution is to go out each day (as needed) and open the door and manually open the roof vents and manually check the inside temp and then, when the sun lowers (or sets) and go back out and manually close the door and vents etc. Forget to open it up just one day when it's bright out and your orchids are steam wilted greens.

Next step up would be non-electric powered vent opening devices, next above that and you are into thermostat controlled shutters and vents fans, misting systems etc.

BTW, NONE of these approaches are bullet proof. I go off to work each day confident that the shutters will respond to the thermostat and the vent fan will kick in 4 degrees later. But if someone takes out a power pole and the gh sits for as little as an hour with no electricity in the mid day sun, well there is very good chane that I've got wilted greens for dinner too.

yep, I agree with everything clavero has said. I'll just add that a lower tech solution might be easier to fix and is less affected by external problems. An autovent is about as robust as you can make, and having a couple reduces the chance of catestrophic failure dramatically. For example, if one fails on average once a year and you take a day to notice, then the chance of both failing on the same day (assuming independance) might be once in 365 years. The same logic doesn't apply to fans because fans usually fail because the power failed, removing the independence assumption.

How valuable are your plants is the other question you should ask yourself.

Low tech won't work here.
the green house is home attached and small (6x8) and will be facing due south. I know that if I don't have a fan my plants will fry.
Also I work full time so I need something that will work by itself. I am getting a solar powered vent fan and will hook it up to a thermastat. I have that planned out already.
I have been growing indoors for years and want to make the leap to a greenhouse. I don't have room for a large greenhouse though so this small one will be fine. That is enough room to fit my plants. They are currently in a similar room.
I will have a screened door so I don't think I will need an intake. The screened door should act as one. The greenhouse also has top vents with an opener that I can set to open at a certain temp. I will have to close this during the summer when I am using the exhaust fan though.
I won't fill the greenhouse right away untill i have logged temperatures. I don't want to ruin the orchids that I have collected. I feel it will take me about a full year to fine tune things and then I can start moving more plants in. I have hoyas and they are hardy so I think they will be the first to move along with some citrus I have. Then the orchids and finally the violets.

CottageFarmer: was it difficult to cut the hole in the polycarbonate? My greenhouse isn't up yet. I am just researching before I buy everything. We plan to put it up in June with help with friends so I want everything to be ready and on-site as they are only up for a couple days.

-renee

Sorry, there was a leap of logic there that I missed. Why wouldn't a large vent work? It works here in Melbourne on 45C days with clear sky. I had a vent that opens at the top of the wall with an autovent set to about 28C. Never lost a plant to cooking as a result. Perhaps you are right, but I'd like to know how you know.

Solar powered vent sounds very expensive! Does the device in question have some kind of (mini-)maximiser?

I cut the hole in 4mm twinwall with a utility knife with a new blade and a metal straight edge. It took at least three passes, the first through the top layer, the second through the ribs and the third through the bottom layer.

Cottagefarmer's suggestion on the on the utility knife is right on. You'll find others have used skill saws and jig saws and all manner of power tools but the resultant chips and dust are HARD to get out of the cells without compressed air. Just be careful as the knife catches and skips if you don't go slow.

I am about to buy a Cross County twin wall polycarbonate and had the same question regarding the installation of fans.

The representative from the mfg. told me to use angle iron, so the photos on cottagefarmer's link would seem to a great guide for that part.

The Rep. also said not to use a saw. He didn't say why, but avoiding dust in the channels makes sense.

For the cutout, we plan to make jig from luan, clamp it to the polycarb panel and hopefully, have both a perfect fit and cutting guide edge.

I do know we would rather mess up some scrap ply than a poly panel.

I just marked the cutout and used a jigsaw. You will need a compressor to blow out the chips. I tried cutting with a razor knife and and found the jigsaw easier to conrtol. If the knife gets away from you it can leave a nasty slice in your PC or your fingers.
Dave

Well we have a hand held jigsaw and dremel. So the PC doesnt melt when you cut it with the jigsaw? I have cut plastic before and the friction has caused the plastic to melt a little.
-renee

Wet the plastic as you go with a spray bottle (or do it in the rain like I did :).