6 by 10 tent type portable here.
I have a swamp cooler, regular window house type running on a thermostat here. But this is Colorado with low humidity. I can't believe how large a fan is needed. Even then I threw a silver colored tarp over most of the roof for this July sunshine. Bur that cuts down the light and I worry about that.

I am in S.E. Ohio and I use pretty much full cover shade cloths over my houses. This should not be an issue for you with adequate ventilation. You speak to a circulation fan, but that is quite different than a ventilation fan. Circulation fans are just that, to circulate the ambient air around inside your greenhouse to equally distribute the heat and to keep the foliage dry overnight, and not to dump hot air out of it.

You need an adequate intake vent sized to an exhaust fan. Even then, you'll never attain a temp any lower than outside ambient. But that is OK if you have shade cloths. Most of your plants should be out of the g'house by the time summer arrives. They can survive in there through the summer with proper management, but don't thrive. This is assuming you are growing flowers for planting outside, vegetables for planting outside, and winter crops.

I don't know the ridge height of your gothic, but assuming it's only twelve feet, you should have a vent fan capable of moving at least six thousand cubic feet of air a minute from the greenhouse and a vent on the opposite wall large enough to move that amount in and that is in addition to any circulation fans. Greenhouses are subject to the g'house effect. IOW heat will build and it has to be taken out.

Cooling seems to be best handled by a multi-pronged approach: ventilation, shade cloth and misting. Used together they work very well. I love my misting system and find that I enjoy standing under in on days like today when I need a cooling spritz.

I agree with the posts that it sounds like you need more air flow. There are many types of shade cloth available too - and they really do work. Misting takes some fiddling to figure out but it can be set up relatively cheaply and it is definitely effective.

THanks for the ideas. My fan system is ventillation, maybe I need 2 fans instead of one. What mm shade cloth do you use and do you put it in or outside the house? Is there a specific company where you buy your shade cloth?

Saw some misting systems at the short course trade show in columbus, Ohio last week. Seems one could come up with their own,though.

All my plants were outside by 1st week of June. do you turn your fans completely off in the summer since nothing is in there? It gets sooooo hot!

Your ideas are greatly appreciated!

I'm a little further south...but, I can tell you with 100% confidence that you can grow in your greenhouse year round..and, all of my plants not only live, but thrive in the summer time inside my greenhouse.

I use tarps to cover the greenhouse roof completely in the summer, because it easily gets up to around 105 degrees here in the middle of summer....if not warmer for short periods.

I use two circulation fans, and a misting system. I also keep my roof vents open, even though they are covered up.

I would have to say however, it's a full time job tending to plants inside the greenhouse in the middle of summer here....but, I'm a plant geek...so I'd be out there regardless.

What are you growing in there during the summer? Try okra. It will thrive :)

Shade cloth comes in % values that tells you how much sunlight it blocks. We have 50%. It also comes in colors, you can even get striped shade cloth if you want your greenhouse to look circus-like. :)
Aluminet shade cloth is a favorite - it is made of finely woven aluminum threads. Google 'greenhouse shade cloth' and you'll find lots of options.
It's most effective on the outside, but some folks do put it on the inside. You'll want to be careful to not cover your vents, and to install it snugly so it doesn't flap around and scratch your GH panels.

A misting system is easy to make. You need tubing, little misting nozzles, and all the fittings to set it up in whatever pattern you need in your GH. It is also recommended to use a filter so that hard water does not clog the misters or coat your GH with minerals. You also need a timer which is the hardest part to find. After making my initial versions with locally-available parts, last year I upgraded and bought better nozzles and a much better timer. It all came from here http://www.dripirrigation.com/index.php?cPath=39_67
This source was recommended to me here on this forum and I found that to be just what I was looking for!

Setting up a misting system will take some fiddling and a few trips to the hardware store. My advice is to not wait until the hot weather arrives to begin this process, as it takes awhile to get it dialed. It's not hard to do though, and it's satisfying to stand under the mist and revel in your DIY skills. :)

Lastly, I would not run fans in an empty greenhouse. Why use the energy? Just let the vents stay open.

All that said, we are enjoying a perfectly cool summer here in MT so I have barely needed to use the fans and misters this summer. How nice!

Here is a link that might be useful: Shade cloth store

I look at cooling/ventilation pragmatically and so I looked at what you were growing. That has a major impact. What the OP said she grows do not require a greenhouse in summer. If you don't plan to grow inside a greenhouse when it's very hot outside, then you probably don't need to worry about how hot it gets in July. I purposely empty out my greenhouses in summer and let them go fallow because it's a good way to stop the bug cycle. No plants, nasty environment, no bugs when you are ready to fire it up again. Keeping pets in a greenhouse from one crop to another is like handing them a passport and greeting them with Welcome Wagon. Then when plants go back into the g'houses before frost, they are carefully inspected to be sure they don't take passengers in with them.

As for running fans in summer? Well, I don't kick mine entirely off with the stats, but turn the thermostat way up so they don't run continually. And......I leave the manual vents open. Unless you're structure is glass, you are going to reach such high temps you can seriously compromise your plastic, especially if you are using films.

We are rural and when we lose electric, you automatically think of heathers and generators, but we lose it sometimes in summer after storms. I had poinsettia cuttings in, late august or early september and when the power kicked off, the thermometer inside the big house maxed out at 135 degrees in the large house even with the shade cloth on. It's the only g'house I have without only automated shutters and no manual over-riding vents. I was in the process of popping the film clamps to pull the plastic (not something I really wanted to do on an 85 X 100 sq foot piece) when the power came on.

I don't know how much help this will be but we have two VENTILATION fans in each greenhouse. This is not to be confused with CIRCULATION fans. Our larger houses have two 36" fans at one end and manual intake shutters on the other end. The largest house has a big tube that runs the whole length at the top of the house which is connected to a fan for circulation. It has holes in it for the air to escape the whole length.

You CANNOT get too much air movement in your greenhouses!!

Depending on what we're growning and how crowded the houses are, we set or hang 20" portable fans around for more movement. These are not box fans but an inexpensive type floor fan which we get at Sam's Club. The only time we use a mist system is when we're starting unrooted cuttings and our system consists of a mist nozzle turned on in front of one of the portable fans. Cheap, but effective :-)

Ditto shade cloth---depends on where we need it. We have two and sometimes use 'em both on one house.

Our smallest house has two of the 20" fans set in the back pulling air through the house and 2 shutters in the front. We leave the doors open as well.

Now, I'll probably catch some flack for this statement and it doesn't hold true for 100% of what you grow but "generally", the plants don't care how hot it gets as long as you have plenty of circulation and keep them well-watered. There ARE exceptions of course. Its nothing for our houses [w/o shade cloth] to get 115 degrees on some days.

And, btw, we are a small, family-owned/operated commercial greenhouse operation.

Regards,

Tom

I don't know how well this would work in an existing greenhouse, but it's certainly something to consider when designing a greenhouse: the thermosiphon effect. Cool air lays low, hot air rises.

Optimum to describe how it works...

Have shrubs or other low plants fairly close to the north side of the greenhouse. Have low (near ground level) air intake vents also on the north side. Keep the surface of the soil around the shrubs there cool, always shaded; add some moisture if needed.

Have high vents near roof level (or in the roof) on the south side of the greenhouse.

Cool air is automatically drawn into the greenhouse via the low vents on the shady side, and as the sun hits the cooler air it heats up and rises, leaving the greenhouse through the roof vents. As more hot air moves out the roof vents, natural convection draws more cool air in from the low shaded vents. The hotter the sun, the faster the airflow.

When the sun goes down, the thermosiphoning stops, and begins again when the sun shines.

The best example/description of this natural effect that I've found so far is in Anna Edey's book, Solviva. She used this technique in both her home and her large greenhouse, to great effect.

I don't know how much of this can be adapted to your existing greenhouse, but adding some vents low on the north/shady side might help.

Below is a link to a description of natural convection cooling for homes.

Here is a link that might be useful: Passive solar cooling

Thanks to everyone for the information. Hearing from people who actually use these is very helpful. It sounds like our greenhouse is constructed somewhat like Tom's.

A friend just gave us a hose-mister that is attached to fans for cooling livestock. We already have a large livestock fan from back when our children showed at the fair.

Since I'm not growing anything over the summer, it will be a good time to try some methods before growing starts up again next year.

I am conerned with shade cloth on the outside: the name of our farm is "Windy Knoll", so you get the idea of our location and conditions. It did help some on the inside, but i'm sure on the outside is best to stop the heat before it gets there.

I agree with stopping the possible bug cycle. I don't bring anything into the greenhouse from outside, and once a plant is put out, such as my perennials, or potted in a planter, it's out !

Our "inventive" misting system came about due to cattle showing :-)

You really don't need to worry about shade cloth and wind due to all the holes in it. I don't think you could possibly get any more wind than we do and all of our houses are broadside to the prevailing winds.

If you have any questions you can email me at

effna@yahoo.com

...and I'll go into further details about our setup.

Regards,

Tom

We use a combination of 30% Aluminet on the inside of our GH, automatic vents on the ceiling and south wall, automatic fans on the ceiling vents, 2 circulation fans---all thermostatically controlled. We also built out a misting system that is timer based (not temp based). Only through all of these have we found the wort of magic zone the plants seem to like whether it's 60F outside or 100F outside.