Underground Greenhouse Top 10 List
There are many reasons why underground greenhouses and other non-traditional structures can be used to cultivate food in the United States. One goal with the design of this particular greenhouse was to blend the functionality of a utilitarian structure with the aesthetics of a retreat center so that it didn’t stand out as a blemish on the healing space the team envisions.
The main reason is energy savings. Many of these subterranean projects around the world are used for seasonal extension. The greenhouse will be used year-round and due to size will be easier to cool smaller ones of construction. By using the insulative factors of dirt as well as concrete filled cinderblock walls with insulation added on the exterior of the wall, the amount of insulation that you can achieve is pretty high. Contrast that with traditional greenhouse builds, where heat is lost as a matter of fact throughout the winter due to the light transmissive walls And roof leak.
A second reason for the choice of underground greenhouses is more of a fun one. Utilizing the greenhouse space as a catalyst to inspire positive change and sustainable choices in construction. The visionary of Bask Retreat Center and I discussed the prospect of such a structure early on in the design process, and she saw immediately that the structure would not only be productive for food and medicinal plants, but also an inspiration Point. After reviewing about 300 hours worth of YouTube footage, I can say with certainty that only a handful of people have documented the process well but only few have creative and cost saving solutions.
The third reason we chose to build an Underground Greenhouse is long-term cost savings. A commercial scale Underground Greenhouse is not cheap to construct. However, the long-term cost savings by using solar power, and battery banks to heat and cool the greenhouse along with the long-term cost savings of using a ground air heat transfer system, made the prospect of such a construction project much more appealing.
Reason number four involves site placement and the more subtle appearance of just the roof of a greenhouse being exposed to the environment versus a traditional greenhouse at retreat center. Bask's greenhouse is sited very close to the transition line between a dry meadow and the rare sunny wetland Meadow to the north of the structure. The perimeter of the greenhouse will eventually be planted with native wildflower seeds, and shrubs to help it blend back into the hillside.
Coming in at number five is utility. Growing fruits, vegetables, herbs, and additional plants on a scale to be a greater benefit to the community requires seasonal extension. The greenhouse will act not only as a production site where food is harvested but also as a nursery or better yet a jumping off point for restoration. Ceilings can be germinated in the greenhouse for restoration projects during the winter and planted in the spring. Spring and summer seeds can be planted in late summer and fall.
A sixth reason was to build a ground to air heat transfer system. Essentially a dehumidifier, supplemental heater and air cooler all rolled into one. While powered by fans to move the air underground and back out, the majority of the work is done by the earth, sand, and gravel mixture under the greenhouse floor. Our greenhouse is about 8’ underground with the lower air pipes a further two feet below that, so the air system is even more efficient. A traditional greenhouse can use the same underground air flow system, but the speed it would heat up would likely require other energy needy exhaust fans.
A seventh reason we chose this design is we have suitable build locations. The slope we chose was one of two options to receive full sun from the south. We are working with the existing trees by having light transmission pass through deciduous forest when the oaks, hickory, and dogwoods they shed their leaves in autumn. This is exactly when the earth’s tilt makes the sun appear lower in the sky if you’re in the northern hemisphere. Most of the completed projects I’ve seen that are more than 50% underground have awkward ramps and stairs for ingress - egress. At Bask, we want transitions through portals to flow seamlessly. It’s also practical to not push a wheelbarrow of food up a slope, so we are considerate of those working in and appreciating this space.
The eighth reason to underground with our build is protection from the wind. Moving air outside any greenhouse will strip heat from the roof and walls of a traditional greenhouse. By only having the roof above ground, the cold wind will only be able to pull so much of the heat out, which makes the structure as mentioned earlier more efficient. Efficiency is the key as greenhouse cultivation does require a high degree of energy and cost input to replace plastic broken glass and uses a number of petroleum products that degrade and require replacement. Our greenhouse does have petroleum based products, but their lifespan is greatly increase by being buried.
Reason number nine is that the presence of the greenhouse on the property will not be felt as heavily because it is mostly hidden by earth and plants. For a retreat center to have a utilitarian structure such as a greenhouse it needs to perform multiple functions as classroom and visual in the positive sense. The greenhouse represents abundance both in the form of edible and useful plants. We want the area surrounding the structure to recover from the construction phase, and we saved the topsoil which practically re-estsblished a plant community where we mounded the dirt.
The final reason that an Underground Greenhouse design was chosen is for the community. The neighbor closest to the greenhouse property line has a cattle and horse farm. With the greenhouse on a slope above his ranch, we did not want him to look at a visual eyesore nor did we want the next steward of that land to have to look at something looming over the valley below. With a design built into a natural slope, the amount of man-made material is as close to a minimum as we can get while still having a functional structure. We also don’t want the guests to see a production facility unless they engage with the space. The goal of creating parasympathetic states in the minds of Bask's guests is high on our list of priorities.
Greenhouse projects like this should be adopted when possible to not only save money long-term, but also not create another above ground scar on a landscape that we should really be removing buildings from. One could argue that building an underground greenhouse is just as environmentally destructive as an above ground greenhouse. However, the amount of non-permeable surface is much lower as the perimeter of this greenhouse does not need to be graveled or have a gravel floor that is completely exposed to full sunlight. There are many more variables that led to our decision.
One unknown is how to allow pollinators to commute. Another is how many tenants will be encouraged to build their summer homes in the greenhouse. Are we building a buttery conservatory? What wire mesh size keeps the possums out and lets the bees in? We will share our process as well as refinements in the coming months.