Building a Biodome Part 2: Design

In part one I went over some of the goals and ideas that informed my biodome design, namely:

• Maximizing passive solar gain and insulation to lengthen the growing season;
• Creation of a 'living' structure that adapts with the climatic conditions;
• Maximization of the amount of plant growing space;
• Ensuring the end result is aesthetically pleasing; and
• Using sustainable, environmentally friendly materials where feasible.

With these design goals in mind, I will go over the architecture I came up with and explain the choice of materials and construction approach.  At this point I will not be providing an opinion on whether the design choices were good ideas or not; in later posts I will go over each part of the structure in the order in which I built it and explain any challenges I faced when theory hit reality.

When considering dome construction, you can get as large or as complex as you want, with many options of geometric shapes.  Dome families include icosahedrons, cubes, octahedrons or the wonderfully named rhombicuboctahedrons.  Looking around at these, I realized that I recognized the shape of the icosohedrons best and it seems that there is more practical construction information available about them online than any of the other polyhedrons.  Beyond their geometric family, geodesic domes are defined by the different lengths of the struts that form the structure.  If all the struts are the same length, that is known as a 1V dome, two different lengths of struts form the 2V dome and so on.  You can get an idea of what each icosahedron dome looks like on the Desert Domes site by mousing over the 1V-5V links here.  The more different strut lengths you have, the harder your design is going to be to build, also the 3V-5V shapes are more suited to larger structures and the 1V-2V more practical for a smaller building.  I opted for a 2V dome, since these are recommended for up to 15 foot diameter and the space I had in my garden was around that size. 

The simpler geodesic domes by themselves aren't the greatest for usable space, as the sides of them slant in a fair bit making that side area harder to use.  Consider this 5 meter (16 foot) diameter 2V dome, with an averaged sized human standing in it:

As you can see while the person can walk around the center of the space it would be harder to work around the sides, with the need to bend over causing the space to be less functional.  So I figured the design should include a wall that acts as a base, raising the dome to around hip height and increasing the amount of usable space and  headroom. 

My choice for the base wall was informed by a passion for straw bale building that I've had for many years. Straw bale construction has very high insulating properties, having an R value of between 30 and 45.

When you apply a natural hydraulic lime plaster finish the wall has the capability to allow air and water vapor to pass through it, ensuring air exchange for the growing environment even when the windows are closed in winter.  The lime plaster also adds  thermal mass to the construction which will help to keep the structure warm at night.

Straw bale structures have very rustic, earthy aesthetics and the idea of the straight lines of a geometric dome contrasting with a very organically shaped circular wall was appealing.

Straw bale buildings come in two main building styles, load bearing and non-load bearing.  In load bearing, the straw bales are what holds up the roof.

In non-load bearing, a post and beam structure is used to hold up the roof, with the bales filling in the walls:

I opted to go with the load bearing approach, since it would use less material (no need for wooden posts and beams) and should also be quicker to construct.

For the foundation of the bale wall I considered pouring a concrete slab, but thought that would add expense that I would rather spend elsewhere in the structure, also I wanted to avoid concrete because its production is very energy intensive and environmentally unfriendly.  So I considered the simplest foundation I could and figured that a trench that was slightly wider than my straw bales filled with 3/4 inch gravel would work OK.  The idea being that any water would drain downwards into the gravel.  I also devised a way to anchor the dome structure to the bale wall, by placing cement blocks in the gravel trench and attaching the base of the dome to the blocks in the foundation with galvanized wire.

When I was first considering the material for the dome struts, I was finding a lot of how-to's by Burning Man festival regulars who were using electrical conduit (metal tubing).  Geo-domes are everywhere on the Playa acting as temporary shelters and hangout spaces for attendees.

I liked the possibility of using metal tubing since it seemed straightforward enough.  Cut your strut lengths to the correct sizes, then flatten, drill through and bend the ends of the struts so that you can bolt them together like so:

The more I thought about this though, this wasn't as easy as it seemed.  You need quite a strong press to crimp and bend the ends of the conduit and I read stories of people busting their tools as they produced the 65 struts needed to make a 2V dome.  Also it wasn't clear in my mind how I would attach a clear membrane to this structure or easily add insulation to the north side of the space.  Metal conduit works fine for a temporary shelter you can erect at a festival and cover with tarps, but I needed something more permanent and practical for making a greenhouse.  I knew it wasn't going to be easy, but I came to the conclusion that wooden struts was the way to go.

When using wooden struts to build a dome you can either go with connectors at the junctions of the struts, or you can cut the wood with a slanted arrow shape at each end and screw it together.  

As I was looking into this, I came across Kacper Postawski's book "The Eden Biodome Revolution".  The book lays out how to cut the angles of the wood for the arrowed struts in a level of detail that I couldn't find anywhere else.  While I thought the book was overpriced at $50, I don't think that I would have been able to figure out how to cut the wood without it.  The book also sang the praises of a material called Polykeder, which I incorporated into my design as the transparent covering for the dome.  Here's some information from Polykeder.com:

"PolyKeder is the name of an air bubble greenhouse film designed for long term, outdoor exposure to the elements and designed specifically for plant production.  PolyKeder is waterproof, airtight, strong, lightweight and flexible enough to be cut with scissors, can be recycled or safely incinerated, can hold the weight of three men, and can be adjusted to virtually any structure.  With careful installation, one layer of PolyKeder can last over 25 years!  PolyKeder guarantees an even light diffusion over the entirety of the structure."

"PolyKeder uniquely allows a substantial amount of sunlight, approximately 83%, 30% of which is healthy infrared light, diffusing through more than 100 air burls per square foot.  The diffused light scatters evenly so as to avoid and eliminate the common difficulties of both shading and burning areas within the structure. Another important point about diffusion is that in the winter months the sun is low in the sky and plants don't respond well to that.  Diffusion tricks the plant because light hits it at all angles, not just from the side. PolyKeder provides an insulating effect that retains up to 95% of heat radiation while providing its users with impressive R and U Values and unprecedented thermal resistivity and conductivity creating the most energy efficient product available in a single layer film without any further enhancements. The R value of Polykeder is 1.7"

I was happy to find this material and it seemed perfectly suited to the job at hand, appearing flexible and straightforward to cut into the triangles to cover the dome.  I figured it was far simpler than trying to fashion custom polycarbonate triangles, which would be the only other way to get the diffusion and insulation properties that are required to meet the design goals.  

By covering the majority of the dome in this watertight plastic material and considering the amount of heat that will build up inside the dome in the sun, it was clear that ventilation was going to be important .  For that I figured windows in the upper part of the structure where it's warmest would be good. Kacper's book turned me on to the idea of self-opening windows that activate with heat and I found these beauties at my local Lee Valley Store:

I was excited about these too, because they are another way in which the biodome acts like an organic system and adapts automatically to its climate.  They are a great example of bio-mimicry,  making me think of flowers and plants that open up during the day and close at night. 

The final piece of my design was to figure out how I was to insulate and cover the north side of the dome, the part that doesn't receive direct sunlight.  I decided to go relatively conventional with this side of the dome with 3/4 inch plywood on the outside, rigid pink foam insulation in the cavity and Reflectix on the inside, creating a kind of reflective mirror to focus the sun's rays into the water tank's thermal mass.

With all these decisions made I was ready to get out of planning mode and into building this thing.  I started sourcing and ordering materials and began preparing the foundation, which I will talk about in Part 3.