louisgreenhouse,
With any clear greenhouse covering alone- glass, polyethylene, vinyl, etc., your structure will hold in heat during the day, resulting in higher than outside temps. You have mentioned some of the options to reduce the heat gain in the greenhouse, including shade cloth. This will help reduce the heat build up. Also, exhaust fans should always be placed to vent to the outside, helping to remove the hot and/or excessively humid inside air. The fan should be placed high on a wall, and an intake vent low on an opposite wall will give the best results. Your gravel/stepping stones for flooring should work just fine.

Hi! Thanks so much. I will turn my fan around tomorrow. It seems like it will be another hot day. I built some panels high up so most of the plants are not in direct sunlight. I guess the purpose of the greenhouse is that the plants should get as much sun as possible. It seems rather pointless to have most plants in the shade now. Hopefully the fan blowing air outwards will do the trick and I can take down some panels and netting.

Louis,

Another way of thinking about the greenhouse environment is not that it should be as sunny as possible, necessarily, but it should be an environment most suited for your plants to grow. So if it's very sunny, yet way too hot, then that won't work. You are on the right track in setting up some options for cooling. If you have a hot summer, you may want to consider moving some things outside, or adding shade cloth, or taking other measures to keep it cool and comfy for your plants.

It always takes some experimentation to figure out what works. This is part of the fun, so don't worry if you have a few setbacks. Good luck!

You have a beautiful greenhouse that you built...and you want to use it. HOWEVER...think that if you had a new pair of snow skis you would not be trying to use them in the summer in Iowa. Your greenhouse will be good to extend the season in the Fall, and to get a jump on Springtime in the (late) winter. No need to try to use your greenhouse in the summer. Take all the plants out of it, and begin to think about what you will do in the Fall to utilize it. How about some tomatoes plants started in July that you will move inside in late October.

Thanks Karin, also lonmower for some great advice. Hopefully I can start using my "contraption greenhouse" with some actual benefit soon! - must say at the moment it is keeping out the rabbits, groundhogs etc. Actually I also thought of sleeping in it this evening ...iowa city is quite a chilled out place to be. Anyway, so if I plant seeds in july I can plant them in the greenhouse when it get cold right? Can I plant my mature tomatoes in it as well around August so I don't end up with 100 green tomatoes again this year? Also what else will grow quite "fruitfull" in till say Desember when everything freezes I guess!?

I'd love to see pics of your green house if you have any. Also wondering how the temp regulation has gone for you during July. I'm also in Iowa and looking forward to putting up our first gh soon or using coldframes/row covers of some sort.

Sure I will take some pictures. You might laugh for it is a rather weird looking contraption. The last week or so it has been around 100 degrees here in Iowa City. I went away for three days and got back to see that everything inside survived. A little dry but I guess if the plants could survive the hottest weather I have seen in Iowa for three days without water my cooling system works. So what I ended up doing was: Bought a very light cloth at Wal-Mart...about three yards and hung it at the very top of the greenhouse. It is the lightest cloth I could find. A green see-through type cloth. I also cut a hole near the bottom of the greenhouse about the size of my hand. (Where I attached a mister) Then I have a bigger hole higher up on the other side with a big box fan blowing out the air. I keep it on a medium setting but put it on high when it�s really warm. It sucks the air out of the greenhouse very well. The vinyl of the entire greenhouse is actually sucked in tightly when switched on. I planted a type of grape that has gone up to the top of the roof so I plan to take the cloth away seeing that it makes sufficient shade. I also planted some moonflower seeds which are creeping like crazy. They seem to like the thin metal chains I got at Menards. Snapdragons are growing very well inside. I found about 30 dead Japanese beetles inside. I figured they died like flies at a window in about a day, not being able to get out. Since then I have not seen any, probably came out of the soil. I planted out all the vegetables and other flowers I had inside but plan on planting seeds as soon as I do some more research.
Louis