I can't comment on inflatable GH's, because I've never had one, but as for widths... It would seem to me, that the comfortable reach of whoever will be working in it would dictate the depth of each row, regardless if it's in-ground or on benches. Then, the width of the aisles would also be a consideration. For instance, if you want to be able to operate a small lawn tractor in your GH, you would need your aisles wide enough to accomodate one, perhaps with a small trailer, to haul materials. You might benefit by planning your desired floor plan before you even think about the structure. Lay it out on graph paper and play with different sizes and configurations until you are satisfied that you will have what you need for your desired use.

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.

A few thoughts: 1. What will the framing be between the layers of PC? If wood, then you may have issues with moisture and eventually rot in there. 2. If you can, keep track of the outside face of the PC when you remove it. Typically only 1 side is UV protected and needs to go on the outside. 3. If the PC is getting brittle now, it may be nearing the end of its useful life. Just something to keep in mind. 4. I tend to agree that some sort of supplemental heat will be needed if you intend to keep the GH above freezing in the winter.

Fruitnut, fortunately I have not had the experience you describe using the swamp cooler in another greenhouse. When the swamp cooler is on there has been plenty of venting and I've been able to keep the temp at 95 or below when its been as high as 115 outside. I think the venting has been the key to this. Golden, originally I was thinking the same as you but my research indicates that using outside air is better because adding humidity between the poly layers could decrease the life expectancy of the poly. Something else that made sense to me was that once the inflation is completed, there isn't going to be much exchange of the layer of air between the poly layers. I'm still open to ideas but as of now I'm going with the outside air. I am going to add a filter to the outside air intake though because when it gets windy here it kicks up a lot of dust which dust I also don't want in between the poly layers. What do people generally use for their inflation? I see various inflation kits all over the internet ranging from $113 to around $200. Any suggestions as to a good kit to get?