I'm moving from San Jose, California 9b to Atlanta, Georgia area (7b) this year. Having grown my own subtropical plants in ground and enjoying the fruits of my labor I'd like to continue to enjoy homegrown subtropical fruits in Georgia. My goals are to grow most everything in the ground as it's much lower maintenance than potted plants. I'd also like to keep it above 32 degrees at all times, which is different than here as we get a few dips below freezing. If I can do that I can add some of the things zone 10 grows, which would be exciting.
Atlanta has a median 45 days where it gets to 32 degrees or less. The first/last frost dates are about 5 months apart. The winter is generally mild (avg 3" snow total). The summers are hot and humid. I know most growers in the south say a greenhouse becomes unusable in the middle of summer without a lot of ventilation and shade. My idea is to have the greenhouse be up only for those 5 months are for the area to be open for the other 7 months.
Let's look at warming for the winter.
Of the 45 days of winter lows many of them are only a few degrees below freezing. Most nights a passive greenhouse should easily keep the temps above 32 degrees. One idea I have is to dig down 2 feet. (Provided the water table isn't shallow).
Lower the grade:
The frost line in Atlanta is about 12 inches, the ground never freezes below that. If the greenhouse is set two feet below grade warmer air will be radiating up into it. This will give the bonus of allowing for trees to get taller as well as the roof will be two feet farther away. A concrete block wall would be created around the perimeter. It is cheaper than poured concrete and has a higher R value because of the air gaps in the block. I might as well insulate the wall too from the outside ground there is a lot of used poly insulation boards on craigslist to pick up cheap.
To see the success of going down a couple of feet, check out this guys blog, in a colder zone he stayed above freezing with no heat: http://www.greenfingardens.com/p/semi-pit-tunnel-greenhouse.html
I want to catch rain water off the house. The plants much prefer this to treated tap. 4 used IBC tanks would hold 1100 gallons of water and would take up around 8-9 sq. feet of space if stacked two on two which you can do with the metal frames. The water should help mitigate the temperature along with raised areas lined with whatever I can get on the cheap, field stone, concrete block, etc.
Building against a south wall:
This is the wild card. I need to buy a place where the south wall of the property has access to winter light. Lots of tall trees in Georgia and I don't mind cutting down trees but sometimes if the trees are in a neighbors yard you have no choice. The south wall will become the greenhouse north wall, helping to retain heat. And what I feel is the most important part; for the rare night a few times a year it goes down to 10-15 degrees, air can be blown in from the house to warm the greenhouse. Having the greenhouse attached to the house will be a huge energy saver, as the times when it gets to warm in the winter the hot air can be vented into the house, saving on heating bills. Surely the house can return the favor a few nights of the year? This avoids having to install a heating system in the greenhouse. Also, being attached to the main house electrical is close by if needed, and plumbing, of course the north wall of greenhouse is already built, saving time money. I'd like a greenhouse as long as my post stamp backyard, which is 64 ft here in San Jose. I plan to grow a lot of stuff!
Let's look at cooling:
I think summer will be too hot to cool. Having roll up side walls on all sides (except for the north wall which is the house) will allow air flow. I'm not sure if the roof would need to be vented as well if three sides are fully open.
I think the tricky part will be managing the in between months where spring has started/fall is still warm but freeze is still possible. Being 2 ft below grade will mitigate the heat somewhat, but probably not enough. Rolling up the sides a bit to allow low cool air in and then having some kind of ventilation in the roof, a solar attic fan, or vents that open automatically with temperatures, or a roll up roof would be needed I think.
Height: I'd like the greenhouse to be 8-10 ft high starting at the south wall and gentle sloping up another couple feet to the north wall. Being dug a couple feet down this will allow plants to get to a decent height but still be manageable. Length & width will depend on the property, but if a guy can dream 60 ft long by 30 ft, which is my back yard now!
Glazing: For the sides double poly inflated. It's cost effective, lets in a lot of light, and can be rolled up! I think for the length of the greenhouse a door would be added dividing the south wall into two roll up parts. This will allow a way out from the house to outside and cut down such a long run of roll up (although I see go that long in youtube videos)
Roof: The roof is what baffles me the most. Should it be able to be opened aside from some venting spots? One thing I like about a permanent roof is I can control the watering, fruits won't get "washed" out and it should help cut back on fungal diseases I think. I know it wouldn't be like out here where it's so dry but humid alone seems better than humid and rain water clung onto flows & leaves for hours at a time.
Options for roof:
Solexx - Pros: long lasting, durable, easy to install, good r-value. Cons: cost (maybe not a con if long lasting) 72% light transmission. Seems ideal for a desert/full sun climate but if it's cloudy out (and Atlanta has 215 "sunny/partly sunny" days out of the year which means 150 cloudy days) then getting 72% of what may be only 50% on a cloudy day doesn't sound great.
Solawrap - Pros: long lasting, durable, easy to install, decent r-value, 83% light transmission. Cons: cost (although if claim of 15-25 years is true then no)
Other - Double Poly? Seems tedious to replace a roof every 4-5 years but maybe not, I've never worked with it. Damaged easily though I don't want to be messing around with it at 3am when some hail or something ripped through it.
How much of an issue would leaving the roof on all summer be? Would wind getting under it cause a problem is all three sides are open? Would heat still build up? What would be best glazing for roof?
I've been thinking about this myself for a while and need some feedback from others. I'm sure I'm missing some things I've never had a greenhouse before.
UGA has great weather data for Georgia: http://weather.uga.edu/?content=calculator&variable=CC&site=ALPHARET
If it helps to know what I would be growing; citrus, passionfruit, guava, fig, pomegranate, avocado (which I grow here), mango, carmabola, sapodilla, atemoya, papaya (I'd like to grow and think I can if I stay above 32 degres)