Sorry for my delay on replying here- but Tony is much more knowledgeable than me on this anyway
but here is some information from me anywho.
As far as expandable my first greenhouse comes in an expandable manner. 8x12 can be taken to 8x16 for $1000 this greenhouse is on the expensive side though- just under $3k for the 8x16x10 greenhouse. The construction is redwood and was very easy to build. I love how it looks and it's very sturdy! the main downside (for me) is the high peaked roof we don't have snow to worry about so all that does is makes the edges of the g/h too short to have full height shelves (which brings me to shelving... I like freestanding shelving. I've already had to move it all around many times to make more room) The greenhouse I'm talking of is made by Sunshine and I really do like it (even though they won't let me sell them
).
My foundation is not sunken (raised filled with rock).
As of now it is ran by extension cords but this year at some time my main g/h will finally be ran with power but that doesn't help the sake of conversation now
.
I also don't really use any vents or fans since it is so hot and dry here blowing in hot dry air to expel hot humid air really doesn't do any good. So all my ventilation is done via my swamp cooler.
I do have a generator but it is not set for automated back up I think it's a Troy-Bilt 5500w but that really needs to "match" your draw, so use some calculators to see what kind of back up power you would need.
My swamp cooler is a "portable" master cool swamp cooler
http://www.lowes.com/lowes/lkn?action=productDetail&productId=215791-420-MMB12A&lpage=none (I assume your normal conditions are pretty humid so a swamp cooler probably won't work for you) but this is the thermostat system that I've installed on it to make it automated:
http://www.lowes.com/lowes/lkn?action=productDetail&productId=155861-420-CC2000
We barely get cool (just started my heater last week) so my heater is small. I have one oil filled heater used when it isn't needed to start the propane heater and also to supplement the propane heater (on the other side of the g/h) when it does start getting cooler. Right now I have a solar fusion heater but I don't think it was really the right choice.
http://products.desatech.com/product_info.php?products_id=514 (has a built in thermostat) I've decided something with a blower would be better to distribute the heat. Right now i have a fan at the roof blowing down since all the heat just goes up to the peak.... It has worked fine- but before I was only heating a part of the g/h. more on that later. I have already ran natural gas to the g/h so perhaps next year I'll pick up a new natural gas heater.
Up until this winter I have always partitioned off my greenhouse to have one side for temperate stuff and the other for tropical. The side the cooler blew into was the cool side then a partition and the heater on the other side. The problem with this for me, is even in the winter our days here are pretty warm. So without any cooling on the warm stuff it would get hot (over 100 easily) so I was always playing a balancing act of opening / closing the partition to allow some cool air to come over to keep that side from over heating but at the same time heat would go over to the cool side... it just never really worked well. so this year I built a second greehouse, a cheaper one (just like vraev showed actually) it was DIFFICULT to build (took longer than my larger redwood one) and the rubber they give for insulation is a joke. If you want to seal it buy 6 tubes of silicone.
I also know nothing about a heat exchanger composter... sorry.
Both of my greenouses are 4mm twin wall polly and I really like the stuff. I like that you can't really see into the greenhouse, I like the efficiency of it but I love the way it diffuses the light. Refer to this
page for information on that. as far as longevity... my sunshine greenhouse has a 5 year full warranty and a 5 year prorated warranty but is said to be used for 30 years in Europe with no discoloration (from their site) on my new
(cheaper) greenhouse (it's nice having my wife work as lowes and getting special order stuff at cost +10%
) there is said to have 12 year warranty on the poly. I did have one panel go yellow on my sunshine and they sent me a replacement right away. On the other hand, I have heard some discussion about the g/h from Harbor freight yellowing and cracking in just a few years under stronger sun of Arizona/Texas etc. So make sure you buy quality poly if you're going that way.
I think that's all I had to say at this point!
Andrew