Not enough light transmission for year round, it would be ok for starting seeds. But OK is probably an overstatement. I'm not a big fan of "hobby greenhouses", they tend to be flimsy and don't last.
Get some 4x4 posts and build your own frame from lumber if you are building one this small.
Buy commercial quality plastic sheeting or Dynaglass, it will last where the cheap crap will deteriorate within a few months. This will cost you quite a bit more money but in the long run will make your life much easier!
Be sure to use galvanized nails or deck screws as anything else WILL rust and deteriorate very fast!
BWI is one greenhouse supply company, not sure but I think they have a warehouse in Jacksonville Florida(you can look for one closer if you aren't a neighbor of the Hawk). I am not sure if they will sell to individuals but most greenhouse businesses probably would sell you a roll of plastic for cost or just over cost. 1 roll of 20' x 100' would last you for a long time as you wouldn't need to change the plastic out every year.
I can definitely help out any one that is between Little Rock Arkansas and Oklahoma City Oklahoma, or willing to drive to OKC to pick it up. Unfortunately it is not feasible to ship plastic and such or I'd be willing to help out there as well. I will check with my sis's BWI sales rep monday and see what the minimum to deliver is and possibly get supplies delivered from the closest warehouse.
Only if it is greenhouse plastic, usually the junk you buy from Home Depot (or any other hardware store) is NOT UV treated. Yes, usually we use 6 mil plastic with UV treatment in a 3 or 4 year plastic, not sure which it is considered now. As long as you don't get a hail storm or wind storm, it will theoretically last 3 years or more.
It is best to use treated wood for the frame and use 1x2 boards for batons to nail through (just using nails will let the plastic rip right through). You build your frame and use 2x4's then stretch your plastic across the 2x4 and hold a 1x2 up and then drive a nail through 1x2 into 2x4 so that the plastic is sandwiched in between and then wrap that plastic back around the 1st 1x2 and hold another 1x2 on the outside of that and nail it again to "double baton" and make sure the plastic does not rip through.
This is a link to a VERY basic diagram of what you should do when nailing plastic(I'm no artist so kiss my butt).
Any one want to build something larger than a 20'x20' building?
As far as heating in the winter time, a small propane/nat gas heater should work in the smaller structures. Doubling your plastic with some kind of air gap will work well for insulation. Be sure not to insulate it too much because in a snow storm you will be raking the snow off the top or you will be rebuilding your greenhouse. Typically they can't stand up under any kind of snow load above 6-8 inches.
The spacing on your posts should be around 4-6 feet apart, I prefer 6 for the sake of saving materials, up north, you folks will probably want 4'. Use the same spacing for your "trusses". Just remember that you are going for MORE light transmission and less shade, so the less lumber you use for the roof, the better you will be at growing things in the winter time. Spacing on the purlins (runners going the length of the house between trusses) should be a minimum of 4', again to save on material costs and maximize light.
I would say that a small house of 20'x20' would probably keep a family of 4 in fresh vegetables through the winter easily. Possibly you would even have a surplus. Imagine the trade value of a fresh tomato in the middle of January in a SHTF scenario.
I have been working on a geothermal heating idea for use in a greenhouse, but I have not had the time to actually try to implement it. You will need to start out by digging 2 ditches probably at least 100' long parallel to each other. These two ditches should be at least 4' deep, deeper the further north you go. One end of each ditch should come up inside the greenhouse the opposite end of course outside. One of the pipes should have an exhaust just above ground level inside the greenhouse the other that the very peak of the ceiling, both should turn 180 degrees to avoid catching water. The opposite ends of the pipe should come out of the ground and then turn 180 degrees to avoid catching water in them. My theory on this is that the natural heating of the air will cause a draft and that draft of air coming from that deep in the ground will at least partially heat the air coming into the greenhouse. I have not myself tried this yet so any results found or ideas to add would be greatly appreciated.
I have read about two interesting techniques for heating greenhouses:
1.) String old Christmas lights (the ones with big incandescent bulbs) inside. They don't generate enough light to mess with your flowering cycle, and the small amount of heat they generate builds up inside the greenhouse.
2.) Use double-walled insulation, and fill the first 4-5 feet of the wall with loose organic material. The organic material will provide some insulation, and when it decomposes it gives off heat.
Some research about the greenhouse kits that use the panels. Looks like Home Depot has the exact same kits as Harbor Freight for twice the price. Lowes is also atleast twice the price but alittle different kits.
We get so much bright sunlight here in the wintertime, I've considered building a geodesic greenhouse big enough for a hot tub in the center, and plants surrounding. My only concern would be heavy snow, but I suppose a solar powered roof-melt system would handle that.