Since I'm two full zones different, I don't feel quite competent to address the questions, but I'll take a crack at them. At least until someone with more relevant experience wades in!

We have a 20 x 24 foot hoophouse, intended for use year-round (deep winter slowdowns are expected of course). We have doors at each end and roll-up sides. The layers are plastic with a blower fan to inflate some space and keep the plastic taut.

Ventilation
We do not have any form of mechanical ventilation and we do fine. Our hoophouse does not get any shade in summer. Air temperatures peaked at a little shy of 120 F last summer--hot, but all the plants--including cool-weather varieties--lived.

It is often safer to keep the ventilation means simple, to avoid complications if something goes wrong (power goes out for example). But if there is not someone who will be around to correct things in case you forget to open doors or roll up sides, then a backup system (like an automatic vent) would bring some peace of mind and perhaps save some of your gardening efforts from setback!

Securing plastic
Ours does have the baseboard and roll-up sides. I wouldn't do without them, especially since it's so easy to make adjustments. In fall and spring, yes, one typically rolls the sides up every morning, and back down by late afternoon--all depending on the temperature in the hoophouse, ambient temp, and cloud cover.

It's not a difficult task--only takes about 20 seconds a side (not including walking and checking out what's going on in the garden). But if someone isn't available, then once again some automatic system might help.

The sides typically stay up in late spring, summer, and fall. But if a severe storm threatens, or even just a strong winds, then one has to run out there and roll down the sides and shut the doors to keep the structure safer.

Insulation
Particularly with the winds around here, I wouldn't consider doing anything but two layers with inflation. I don't know what wind conditions you have there--that might be something others will like to know in discussing your project. :)

I hope that helped somewhat and didn't muddy the waters. I love hoophouse growing though and am always very happy to talk about them with others. May your project bring you much pleasure and many wonderful veggies!

Great advice IMO Ralleia. The only thing I might suggest is that you can save on an intake louver expense if you just crack the door enough on the end opposite the exhaust fan every morning during cooler night weather. Used exhaust fans are common but not so much the intake louvers. And you can get by without an expensive rollup kit if you just want to use a few pipe sections and insert a "T" on the end that you can insert a smaller pipe into to roll up and then brace into place. I would still strongly recommend the "Wirelock" base and wiggle-wire above, at ends and below the rollup side and whereever else the plastic needs anchored.

I have a very cheap used barn fan as my smallest (15x40) G.H. since 1994 and I value it on sunny days. I have a screened window on the opposite end but now just open the door and at the same time flip the power switch to the exhaust fan in the mornings. A thermostat to actually activate the fan is important, but you can usually get those for $25-$40. I also have a house box fan hanging in the center to circulate air day and night- it helps to avoid temperature variations. This is the structure that I have a layer of solar pool cover between two film layers, inflated on either side and it is fairly efficient. In fact I cover the exhaust fan inner opening with a sheet of plastic and bungie cord every night (early spring) to prevent minor heat loss while I'm heating the structure. I just make sure I'm always available to open up and throw the proper switches. just a thought for you. You might not want to go to that bother.

For my larger units I have intake louvers along with the exhaust fans and hanging internal circulation fans. These have rollup sides. When the sides are rolled up the only fans that do any good are the circulation fans since exhaust fans would just pull air from the ends just below them. I guess my point here is that you install enough switches to allow you to override any fans you install.

Thanks for your great advice, Ralleia and bmoser. It sounds like roll-up sides are a must (we don't have strong winds here, btw), and not too big a deal. And it sounds like having an exhaust fan doesn't have to be a huge expense -- and that things will be okay without it, too.

Again, thanks so much for your help and encouragement! It sounds like the main thing is just to get going with the project.

Nancy

We haven't built ours yet but had an ag expert look at our site. He told us to keep well away from trees so falling branches don't puncture the plastic. We have a very old maple that has lost some large limbs and we are also in NH where ice storms can break branches. Maybe it wouldn't be a problem in your area.

Mine GH is double walled with no rollup sides and manual ventilation at the gable ends. Only one end has a door. I woul d like to increase my cross ventillation with a door on the other end.

I've run the GH for a couple of years now with no real issues. I grow all year with it. Unheated. The pictures you see here were taken today. Much of the produce has been eaten through the winter so there's not much left.
New seedlings are growing now.

Here are some links to the photos:

http://pvjsl.com/images/poppa/greenhouse/GH%20in%20February.JPG

http://pvjsl.com/images/poppa/greenhouse/GH%20and%20garden%20area%20february.JPG

http://pvjsl.com/images/poppa/greenhouse/Greenhouse%20inside%20a%20greenhouse.JPG

There was a good tutorial on youtube I think it was on Farmtek's channel I can't remember. They attached pvc pipe to the bottom of the plastic. Fabric clips are definitely a must and rolling things up during the summer. You can roll it down at night. One thing I might do is get that lightweight row cover stuff and skirt my new hoophouse during the summer. Even during the spring plants near the walls of the plastic can be fried. If I had say a 1 foot tall plant near the side I'd roll things up a bit to give him some relief. Even it's just a crack it'll help. Plants kind of like air too. Just monitor temps.