I looked around at greenhouse kits online and most of the conventional ones didn't appeal. I was surprised to find that even a very basic 6'x8' aluminum framed conventional greenhouse kit will set you back around $3000, such as the one pictured below from Backyard Greenhouses.
On looking a bit deeper at this model, I figured that to extend the growing season where I live beyond a couple of weeks one of these probably wouldn't cut it. For one thing, the insulating effect of the 6mm double-pane polycarbonate covering has an R value of 1.54, which while not bad, is still at the lower end of the possible options. Selecting a good cover is one the most important considerations when buying or designing a greenhouse as articulated very effectively in this article in GreenhouseCatalog. In a colder climate every bit of insulation counts as protection for your plants. Here's a table from that page, which is a handy reference.
| Greenhouse Covering | R-Value | U-Value |
|---|---|---|
| 5mm Solexx Panels | 2.30 | 0.43 |
| 3.5mm Solexx Panels | 2.10 | 0.48 |
| 8mm Triple Wall Polycarbonate | 2.00 | 0.50 |
| Double Pane Storm Windows | 2.00 | 0.50 |
| 10mm Double Wall Polycarbonate | 1.89 | 0.53 |
| 8mm Double Wall Polycarbonate | 1.60 | 0.63 |
| 6mm Double Wall Polycarbonate | 1.54 | 0.65 |
| 4mm Double Wall Polycarbonate | 1.43 | 0.70 |
| Single Pane Glass, 3mm | 0.95 | 1.05 |
| Poly Film | 0.83 | 1.20 |
The second problem with the conventional kit was that it was too small. I'd need a good amount of space inside, since I'd have to sacrifice some of the growing space to thermal mass. If you want to keep the plants warm at night, it's not enough just to have warm air in the space, since that will cool as soon as the sun goes down. Thermal mass is a repository for the sun's energy that is an important element of passive solar heating. Typically in houses it is a mass of stone or brick, but water is also a very effective medium to collect heat during the day and slowly give off that warmth at night and that is normally the choice for greenhouses. There's a great article about building a solar greenhouse here that gets into some of the facts and figures such as how much water you need versus the dimensions of the space.
Another thing about the rectanglar design of the conventional greenhouse is that it may not be the best shape to withstand the high winds I get in my region. I've seen the straight lines of my backyard fence heave in heavy storms and I had to replace it entirely when Hurrican Juan whipped through our area. You can get rectangular greenhouses with curved roofs which seems better, but still I'm not convinced it is the best shape for aerodynamics and longevity in an area that gets regularly whipped with Nor'Easter's.
Finally I don't find conventional boxy architecture aesthetically pleasing or mentally, emotionally or spiritually satisfying to be in. I'm a great believer in the idea that architecture shapes our consciousness and that the built environment can restrict or liberate our lived experience. Many traditional dwellings are circular, from the tipis of the Native Americans to the yurts of Mongolian herders to the thatched mud huts of the Masai in Kenya. I think there is a good reason for this. Circles are everywhere in nature, from water droplets to stems and branches, the shape of the Earth and even in our very cells; try finding a rectangle anywhere in the natural world! In the words of Oglala Sioux medicine man Black Elk "There is no power in a square house".
After going off the concept of a conventional greenhouse, I recalled the concept of the biodome. I had seen them used before at the excellent Eden Project in southwestern England. Based on the classic Buckminster-Fuller dome geometry, these unique spaces take greenhouses to the next level, becoming more like mini-climates, where every element of the structural design is a conscious part of a living system.
What I didn't realize is there is recent movement of people making them for their own gardens, which I started to uncover in my travels around the internet. Following dome links I found the excellent designs produced by Growing Spaces and it all seemed to fall into place - yes, this is what I was looking for.
The shear and deflection inherent in a dome fares better than a square shape does in high winds, pushing the structure down, rather than over. The dome shape is an approximation of a more natural spherical form, providing an element of bio-mimicry and a more natural shape to capture the sun's rays as it travels across the sky. The north interior side of the Growing Spaces domes are insulated and a reflective material added to focus the sun's rays onto a large water tank at the back of the dome.
The thermal mass of the water retains heat and solves the problem of cold nights, allowing for the possibility of radically extending the growing season, not just for a few weeks but for months. Stories of people harvesting lettuce in cold climates in March warmed my heart, as did those of dome owners being able to keep tropical plants alive through the winter. Then I looked at the prices of the kits and even for the smallest 15' dome it was in the region of $7,000 just for kit and additional materials would still be required. Being someone that likes a challenge I thought I could do just as well, if not better with my own design, admittedly borrowing most of the ideas from the Growing Spaces concept. I set my budget to half of buying a kit, thinking that $3500 would be more than generous for what I had in mind, then I set to work on my own design, which I will detail in part 2 of this series.
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