I think more what you made want is a hoophouse.

Here is a link that might be useful: Hoophouse

I can't give you any input on a GH, because I'm still to much a neophyte on the subject myself, but can't resist asking, because I've long wondered:

What do giant pumpkin growers do with their monsters AFTER the contest?

Lots of pumpkin soup.

I thought it was pumpkin pie.

Too sweet.

Jack-O-Lanterns
I'll ask a focused question instead of the generalized one above. How about passive natural ventilation verses exhaust fans. Those with automated side and ridge vents look interesting. Probably would require much less energy then exhaust fans but how well do they work. The one's I looked at have 2-3' ridge vent and 4-5' side vents on both sides. I would think that does a wonderful job keeping the temp down. Any thoughts

I believe that practical greenhouses can be heated and cooled entirely passively (well, including those autovents). My waterfall keeps my greenhouse below ambient temperature without any fans. In mid winter I have no trouble keeping things warm (once I turn the waterfall off). Shade cloth in summer is also helpful, and making sure you properly insulate the north side, and take care to seal up gaps.

Good day.

I'm a greenhouse manager and I have a few thoughts that you might consider in building a greenhouse.

1. Plastic vs. Glass
Double-poly is much better for heat retention (higher R value) but transmits less light than glass. Poly does not last as long, but is much less expensive to set up. Generally, unless you are building a GH (greenhouse) for commercial practices, the savings from building plastic are probably worth it, especially in a climate where you want to heat.

2. Carbon Dioxide (CO2)
Generally the bottle-neck of photosynthesis is CO2. In the general atmosphere there are about 340 ppm. In a greenhouse, the plants tend to use CO2 quickly as they grow and so the CO2 ppm (parts per million) drop.

One way to solve this is to vent a lot of the time to keep the ppm closer to the ambient levels you would find in your garden. A commercial greenhouse would increase the ppm to 800 or 1000 ppm (maybe higher depending on the crop) and get about a 50% increase in photosynthesis. If they are venting a lot, however, they may only try to maintain 500 ppm, because it's a cost to add CO2. You might look into a CO2 generator that runs on natural gas. They are not terribly expensive (although I have no idea what budget you are working with) and could give you a 50% increase in photosynthesis (and, thus growth).

3. Moisture
Regulating humidity levels is tricky, and if you're not around a lot can be a problem. This is especially true in a soil floor GH. In the GH, the relative humidity will climb, especially at night, leading to problems of botrytis and other diseases much more easily than would affect a garden plant. (This is one of many reasons that almost no professional greenhouses leave exposed soil in their GH, though there are some old school places and a few types of cut flower production still done in the soil where exposed soil is necessary.)

Commercial greenhouses use computers to keep the RH (relative humidity) regulated, generally below 86% depending on crop, since many serious diseases require environmental conditions slightly above this to germinate.

If you really have to grow in soil, you might consider using plastic mulch and planting the pumpkins in the holes in the plastic sheets. This would minimize the exposed soil, and help keep your RH more in control.

You also really should get Horizontal Airflow Fans (HAF). Install them so that there is a continuous circulation of air above the crop. This keeps pockets of higher moisture from accumulating around the plants and significantly decreases diseases, aside from a number of other benefits.

4. Pests
There are a number of pests that are more likely to infest a GH crop than a garden crop. The conditions are particularly perfect for certain insects. It means that it is well worth your while to learn to identify them, and watch your crop more carefully than you would a garden crop. You might want to put in yellow sticky cards and keep track of how many insects of various types are trapped on it. There are a lot of good biological insect controls available that are more useful inside an enclosed space than outdoors. If you know what insects are affecting your plants and in what quantity, you can use these biologicals and avoid spraying (in Canada, almost no commercial vegetable greenhouse sprays biologicals are almost exclusively used. This is a change in the industry in the last five years.)

Having an open floor also means that some insects, such as thrips, are more able to breed. (Another reason to cover it, in my opinion).

5. Watering
The greenhouse gets hotter easier and plants dry out more quickly. But timing of irrigation is important because the moisture from irrigating is more likely to affect your RH negatively. Be more careful of when you irrigate (try to avoid it at night even more so than in your garden).

Anyway, I've rambled long enough. Good luck with it all.

I've always found CO2 producers a classic example of wasteful operation. Normally plants can be considered carbon sinks, reducing atmospheric carbon and producing fuel. But with current commercial greenhouse practice we have people who just burn fossil fuels to produce greenhouse gases!

If you want to increase CO2 levels there are a number of more justifiable solutions: combined heat, power CO2 - generate electricity and use the waste heat and CO2 directly in the greenhouse; beer production - brew beer in your greenhouse; even collecting the waste CO2 from your DHW or heater (ventless) is more justifiable than burning fossil fuel just to produce CO2.

Regarding watering, even mid summer in Melbourne (a very dry climate with a pan evaporation of over 3m) I have never need to water more than twice a day - once in the morning and once mid afternoon. I expect having 4kL of water sitting in there probably helps.

And don't people normally aim for a constant VPD rather than RH? 86%RH at 25C is a VPD of 3.5, a long way from the 'optimal' of 1. I suspect RH86% is a rough rule of thumb that has been encoded into monitoring programs by people who don't really understand what they are doing.