I probably should have gone with a nice storm door, but I opted for a full, pre-hung, outside door. The frame may not be rigid enough to handle this door over the long run (I suspect the doorway will skew over time), but it has been very nice this last winter.
With this structure I've been trying to stay away from providing a permanent foundation, but this is a problem when it comes to a door which needs to be anchored to the ground in a precise way. Since I planned to add a stone walkway on both sides, I decided to sink some carefully leveled cinder blocks for my doorway support. I know from past experience that this kind of arrangement is very stable in my soil and climate, and unlikely to shift. It would not have worked at all on my previous property where everything was always sinking and drifting, but the blocks I set in my grandmother's property nearby were still level and in position after 25 years.
The uprights were reinforced and braced. This was much harder than I expected because my wood was not straight. If there was one thing I could change about the door, it would be ensuring that both sides were as true as I could get them, and then doubling them up to make sure they stayed that way. As it was, shimming and adjusting was very difficult, and I used special screws so that I could adjust the push & pull between the door frame and the supports, rather than just holding it tight to it. I'm a much better gardener than I am a carpenter or framer.
I used a section of the thick outer plastic as a barrier between the door threshold and the bricks. This was further covered by mats on the inside and out, and sealed around with rocks. The door itself has a low-E (doesn't pass infrared) double pane glass inset and extra weatherstripping for a good seal.
Running electricity was probably the same as for any greenhouse. I dug a trench 24" deep (deepest frost line in the area is 18" here) and dropped a conduit in with four strands of 6 gauge outdoor rated wire. I won't go into wiring details, since you really need to check your local codes and get a certified electrician to help with this part. There is an outdoor spa-type box/GFCI/emergency shutoff on a post in the greenhouse to which everything connects.
Here's part of the trench. The large gap where it turns had a three foot rock in it that had to be removed.
The most important electrical item was the circulation fan. I purchased a wet location ceiling fan, since I had extra vertical space and the shape of the structure seemed ideally suited to it.
The first step to mounting it was placing the fan-rated wet seal junction box at the very apex of the structure. I carefully positioned the tabs over the struts and screwed it down with stainless steel screws. Then I attached the hanging frame that came with the ceiling fan.
Electricity was provided by a half inch conduit attached to the ascending struts. I had to pre-bend the conduit for a while so that it wouldn't pull too hard against the brackets when I attached it to the curving structure. It is glued in to junction boxes at the top and bottom to seal it.
I purchased an 18" extension rod for the fan to drop it down from the curving sides. Wiring it all up was pretty much the same as handling any ceiling fan, according to the directions. It hangs from the bracket while all of the wires are connected up and the covers put in place. Then, each of the blades are added and balanced, again according to the instructions that come with it.
Before the fan was hung, but after the bracket and conduit were in place, I added the internal layer of plastic to the structure. For this, I started with a 20' square piece of 4 mil poly film. Because of the way that it was folded, it was easy to located the center and place it over the fan mounting. A short length of plastic lathing was attached over each of the five central struts to anchor the plastic. This had to be done while carefully supporting the connections from inside the cascading plastic to prevent the weight of the piece from pulling itself down before it was fully anchored.
I worked my way down from center, one tier at a time, tacking a small piece of lathing below each hub and pleating where necessary to take up any slack that appeared as it curved down. As long as I didn't try to move down before every attachment at a given level was complete I didn't have too much trouble molding my plastic to the concave surface. I was impressed by how little I needed to overlap this way.
Once I ran out of plastic at the bottom edges, I started at the door and added 10' tall x 20' wide pieces of plastic around the sides until I had worked my way completely around. These were a bit trickier, and I ended up with a few pockets where I couldn't push the plastic out to all of the hubs. I think when I redo the inner plastic I will attempt to attach these bottom pieces near the bottom set of hubs (at ~3' high) first and then pleat upwards.
The fan has been constantly running essentially since I put it up a couple years ago. In the summer I direct it downward and in the winter I direct it up. Either way, the leaves in the greenhouse show a constant rustling and I'm satisfied that the circulation is adequate. Thermometers placed at various heights and distances from the center have shown an even temperature in both hot and cold weather. Perhaps the greatest advantage to this arrangement has been that I can place strong fans next to the door and keep the internal temperature within 5 degrees F of the outside even in the heat of summer without the top vent that I always assumed would be necessary.
With a single 1500 watt, 120 volt heater I can maintain a 20 degree F differential to outside temperatures throughout the winter. It takes 14000 BTUs to reliably provide the 40 degree differential required by our coldest days most years, including losses due to safety venting. If I were to better insulate the bottom portions of the structure and the north wall, as originally planned, and switch all of my heating to electricity, as still planned (hydroelectric is cheaper than gas here, even for heating) I could certainly realize greater economies of heating. Eventually I plan to run as much as a 65 degree differential, with 30000 BTUs of available heating.
I do maintain ponds, aquariums, and fresh water storage totaling about 250 gallons in the greenhouse, which provides some thermal ballast. I have no idea how much, since they have been present since before I sealed the structure.
I installed low pressure misters on the second tier ring (~7' up) around half the greenhouse. These provided 15-20 degrees of cooling when run 5 minutes at 20 minute intervals during our 115 degree weather, keeping things below 100F. Unfortunately, they clogged after the first year they were installed and I haven't been been able to locate the problem. I will be installing a pump and high pressure misting system this year to take its place.
Most of the floor is currently bare, though I have an ever expanding section of 1-2" river rock starting from the door. I'm conflicted over this, but will probably cover most of the floor with small gravel or rock eventually.
Finally, cost estimates. These are a few years old now, but material costs vary quite about regionally anyway.
Basic Frame - $700
Glazing in and out - $250
Other parts added - $600
This quantity of quality glazing always represents a significant cost, and I don't think the amount listed is unusual. Unfortunately, it is only rated for four years and has to be repurchased and installed. I wouldn't skimp on it, though. I highly recommend the woven material I showed in part 3: Plastic covering for its strength, ease of handling, durability, and scattering light transmission. Still, polycarbonate or something I didn't have to replace so often would certainly be nice. I'll be pricing it out this year.
Other parts' cost will be higher or lower depending on choice of door, vents, heating and cooling system, etc. This number probably has little relation to the type of greenhouse constructed, and is probably least informative for someone trying to construct a similar structure.
For standing by me in the wind and through the single-digit (F) weather while I added all of the above pieces, I need to thank my Dear Wife who helped me plan and keep track of everything, my father-in-law who steadied the ladder and passed things up and down for me, and my mother-in-law who took care of the four curious children watching from the window.
All of the rest of the pieces should be doable from the ground, by a single person, but I'm sure my DW will come up with something I've forgotten which requires her assistance.
She still thinks I'm crazy for putting up a dome, but I'm not sure that has changed her opinion of me any. My father-in-law is trying to convince my mother-in-law that he needs to put up one of these for storage at his house. My kids just think it's cool.