Millet, insulating the southern wall is a very good idea. I expect I will do this if practical. I may have considerable choice in the direction of the greenhouse... would it make sense for the longer length to face north-south, or east-west? I would think north-south would receive the most light in summer, but maybe not so much in winter (because of lower sun angle) when it is needed most. I also may be able to build the greenhouse into a slope somewhat, so the rear wall could be partially underground and insulated in that way, or the whole thing could be a lean-to into a slope depending on the lay of the land.
Cory, yes I was thinking I may have a section for higher humidty and heat and grow some true tropicals. I'm excited about this prospect.
Tom, rather than have multiple greenhouses I'm thinking of adding a polycarb wall or similar and having different sections as I mentioned above.
I've done somewhat more research, I'm using GreenhouseMegaStore.com as a reference for now. Most greenhouse kits are expandable, so I can start smaller and add space later if it isn't too difficult to expand. However, it could be complicated if I need to get a new permit each time and add concrete footers or dig. It is an option though. There does not seem to be meaningful cost savings when buying larger sections at once versus adding sections over time, though perhaps freight cost if that is not included.
I ran through the heating calculators on
www.littlegreenhouse.com and if I'm doing the calculations right the insulation advantage of 10mm versus 8mm polycarbonate may not be enough to justify the added material cost, especially if I may have to replace it every decade. It looks like I definitely want polycarbonate, though, as PET film doesn't look sturdy enough and glass & acrylic are more expensive than polycarb for no clear advantage to me.
My current question that I have no good answer to is - How much ventilation is required to maintain reasonable midsummer temps? It seems there are a few styles of ventilation:
natural: thermostatically controlled vents throughout the entire length of both walls and a ridge vent through the entire length of the ridge. This is the design of the Ventmaster link above.
most expensive)
fan-powered: vents on short ends of greenhouse, powered by fans. Either a pair per end or the entire length of the short ends (minus doors)
fan-powered plus Active cooling: (condensors, water-wall swamp coolers, heat pumps)
I am wondering how much is required in my climate. I'm willing to do some labor twice yearly to simple remove wall segments or something to allow extra ventilation in summer if that somehow introduces huge savings versus a more complex ventilation option. I was initially hoping that the cheapest option would suffice, but if I remember correctly Millet has both shade cloth *and* active cooling to maintain temps and he is in a similar climate to mine. So perhaps he can offer advice on this portion.
Ventmaster design comes with PET sheeting roof but it looks like you could use PC instead
Ranger Series powered fan vents on ends instead of side/ridge vents
EDIT - found some great information on the PolyVent system the Ventmaster design uses:
"A second automated passive ventilation system is the Poly Vent from Poly-Tex Greenhouse Co. This system is a sidewall formed from a series of polyethylene tubes that are connected. When the tubes are inflated, the Poly Vent makes a tight-fitting, double-layer greenhouse sidewall. The inflation fan is controlled by a thermostat so when the greenhouse is warm, the fan switches to off and the wall opens; when the greenhouse is cool, the fan switches on and inflates the tubes thus closing the wall. This system has worked fine on research greenhouses at the South Farm for six years. Each wall is controlled by a separate thermostat, so staged control is achieved. This system is best for small greenhouses, up to 60-80 feet long; with longer greenhouses, wind and rain tend to move the tubes from their enclosure. This system is less expensive than fan ventilation, initially; but the tubes have to be replaced every 2-3 years so the system still has a recurring cost.
Contact us if you need an address for Poly-Tex. This has been a simple, reliable system in our use at South Farm. It is designed for straight sidewalls, but might work on a curved wall of a quonset. Remember, when the power goes off, the wall deflates and the greenhouse is wide open; this is quite a disadvantage during winter power outages! They do sell a battery back-up system."
On evaporative cooling: Pennsylvania average summer afternoon humidity is 56% per
http://www.currentresults.com/Weather/US/humidity-by-state-in-summer.php.