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Step 4: Dome Covering Pattern

Picture of Dome Covering Pattern

Everyone seems to gloss over this part, but I found it the most difficult to figure out.

I used heavy duty 11 mil woven clear poly from for the outer layer of plastic. It has good light transmission characteristics (and strong wind, collision, and UV resistance, which I need), but as a woven material will strongly scatter the light. Even having a second layer of plastic on the inside of the struts will provide some degree of light scattering, though, preventing strong point shadowing from the struts. There will be reflective radiant insulation on the north wall as well, which should brighten things up a bit further by limiting pass-through and providing light from the other side of the structure. The dome shape helps reflect horizontal winter sun down onto the plants from all facets, anyway. The dome is about 13' high, too, and the upper structure captures and bends that light back down.

As far as general constraints on permissivity due to strut coverage, a worst-case back of the envelope calculation would give

Side: 1/4'x4' per strut x ~2/5 dome surface exposure to side radiance

x 60 struts blocking

=60 out of 718 sq ft, or 8% coverage

Top: 1/4'x4' per strut x ~2/3 dome surface exposure to top radiance

x 100 struts blocking=100 out of 1257 sq ft, or 8% coverage

So, 8% is my worst-case blockage due to struts. The actual blockage will be less because this inflates the size of the struts a bit and assumes that none of the radiance on the sides of the struts will make it back into the dome. This will need to be added to shading from the glazing to find final permissivity numbers. Hopefully top dome light gathering and north wall reflection will make up for a bit of this as well.

At this point there is some cutting to do. I've spent a lot of time considering how to cover a sphere with flat plastic without coming to a clear, obvious answer. Covering with a single sheet is not going to happen, because the size of the sheet would necessarily be enormous (better than 80' square) and the waste would be tremendous around the sides. If this were a shelter or something other than a greenhouse I might consider covering with five pieces around the dome (it is 5-way radially symmetric) and refolding wherever needed as I worked my way down the dome. Folding involves greater shading though, and I need to keep the plastic tight for a number of reasons, including standing up to the wind and shedding snow and rain.

I ended up cutting the plastic into the shape of the individual icosahedral patches. If this were a sphere, that would require 20 patches to cover, but it's considerably less for my dome, and should guarantee that everything fits nicely on the structure. The general pattern of these section is in the included picture. The slit is what allows the shape to conform to a sphere and will need to be seamed.

After examining how this would work for the dome, I realized that the top triangle on this patch was not needed. Five of these patches are needed along the bottom (plus five small triangles) and five are needed around the top. However, the top triangle is cut off by the ground line on the bottom patches (these are arranged upside down). At the top, this pattern would result in five corners coming together at the peak, so I opted to remove the top triangles from each piece and lay over a single pentagon piece to minimize problems with leakage.

I'm using the woven plastic material in 10' width, which is just wide enough to accommodate the height of the full triangular patches with sufficient overage to line everything up on the dome and still have overlap. I'm going to use a tarp to trace a template directly on the dome itself, and then lay it under the real plastic to mark each section. I will leave a 6-8 inches of overlap plastic around the pieces to accommodate the seams.The actual attachment to the dome will be done with plastic lathing on the overlapped seam, secured by long staples and wide top nails at strategic points. The trick, of course, will be accessing the top of the dome to put these in.

The plastic is 10' wide X very long, so I unrolled it and restacked it accordion-style to be able to pull it off linearly. After the first couple of pieces I found it easier to use a previous piece as the template for each subsequent piece. The trapezoidal patches are rotated to fit with each other as they come off the roll. Unfortunately, I ran out of plastic, due mostly to my failing to account for the large amounts of overlap that I ended up using on each patch, and I needed to reorder to get a bit more plastic to finish off the external covering. When the second order of plastic came, I had to cut it outside, and that proved more difficult but possible. I would recommend finding a large indoor space if you can, though.

AngelHouse3 months ago
Beautiful job! About how many man hours does it require? And I'd like to know approximate dimensions if you can please provide. I'm thinking of it as an Eagle scout project, or as one for a math class. We're a nonprofit establishing in south Florida but my sister in Texas may establish a chapter as well. We teach students, special needs, elderly & those in recovery to grow food, and this is a very cool place to do it in. Sometimes we need a Wow factor to get the attention of would-be donors and volunteers, to get the programs funded.

In Texas they'd need the double wall construction, easily heated to 20 degrees difference, etc because their weather is Very erratic compared to out stable, warm one. Sure, we get half a dozen cold nights and quite cool days, but nothing like the rest of the country, here in south Florida. I'd think our fan could go out/up in summer to get the heat out and maybe down in winter, opposite yours. Any insight is greatly appreciated.
I'm always amazed at so Few people having heard of Instructables. And how many people tend to jump at the chance to squash the dreams of our beneficiaries. To find themselves still able to garden because someone makes it easy and accessible to them, is a joy to behold!

What we'd like to do is use greenhouses for aquaponics, so the water temp will help too, in TX and there are aquarium heaters of course. Texas winters can be brutal, with 65 in the afternoon, plummeting that same night to below freezing and it occasionally goes down to the low 20's. Also in summer, it can be 105 degrees in south Texas.
At those times, maybe south FL and for sure, Texas, will need a way to also chill the water a bit and cool the greenhouse.
I think in south Florida those "swamp coolers" tend to not work as well with our humidity. But on the other hand, I once had something in our garage, called a "water-heating heat pump" and as the heat from the air in our garage went into the water, the air coming out, was cooled so it was like a mini air cooler for our dogs.
This was on the order of 30 years ago - I'd love to find such a thing today, but may be one of those things that in the US, people have money for a "real" air conditioner, so that it's not used anymore...
If any of the Instructables fans out there can please help me find such a device, I would be forever grateful. We need to keep our greenhouses tolerable for the frailer elderly who will be gardening with us and that's the best way I can think of with our small budget.
Thank you
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