There are many different shapes and building styles of passive solar greenhouses. The most successful building styles (commonly referred to as “geometries”) integrate four basic principles: orientation, glazing, insulation, and ventilation into an energy-efficient grow solution that can perform year-round. Two common solar geometries, besides Ceres’ standard greenhouse design, are the walipini and geodesic dome greenhouses. These styles are intriguing because growers think they are easier to build and require less energy consumption than your standard all-glazed greenhouses. While many growers have successfully grown in walipinis and geodesic domes, it’s important to understand how the design principles of each style aids their function if you’re considering either one for your specific climate and growing application. Walipini or Pit Greenhouse A walipini is essentially a pit greenhouse, or a hole dug in the ground with glazing laid over it. The name “walipini” originated from a 2002 agricultural project where volunteers from the Benson Institute went to Bolivia with the goal of building low-cost pit greenhouses for local farmers to use year-round. The pit design was called a walipini which means “place of warmth” in the indigenous tongue (Schiller 162). Essentially, a walipini is a 6’-8’ hole dug in the ground that is covered with a single or double layer of polyethylene glazing. Recently walipinis have gained popularity thanks to articles circulating on the internet proclaiming that it only takes a few hundred dollars to build an underground greenhouse. Unfortunately, there isn’t a lot of consideration of the serious modifications growers need to make a walipini function in North America. Orientation The pit style greenhouse works quite well near the equator at high altitudes in mostly dry climates. In these areas, neither one of the walls will cast a deep shadow into the growing space and the roof water runoff will not damage the walls of the excavated pit. Considering the much lower light angles for most of North America, especially those in the winter when the greenhouses are more valuable, the walipini floor will be shaded to such an extent that growing is hardly possible. For this reason, we do not recommend walipinis as a year-round growing option for growers living in North America. If you do decide to build a walipini in the northern latitudes, you will need to think carefully about your roof slope and the angle of the sun during the winter solstice to ensure there is no shading in your greenhouse. To overcome this issue, you can build an earth berm on the northern side of the greenhouse to create a steeper roof slope. Glazing Because a walipini is built underground, glazing is one of the only building materials you’ll need to consider in your upfront costs. Like all greenhouse designs, it’s important to use a glazing material with high light transmittance and insulation value because the glazing surface is where most of the heat loss is going to occur. A popular glazing option for walipini greenhouses are either poly sheets mounted loosely on 2×4 wood members or, as a longer lasting and more stable alternative, polycarbonate panels. Either method will work, as long as snow can slide off the roof without accumulating due to a lack of surface run-off. Insulation The perceived benefit of a pit greenhouse is that the earthen walls surrounding the grow space provide ample insulation to the growing space. Even though this form of insulation is free, it is not as effective at capturing and retaining heat as people perceive it to be. The reality is that the earth only has an R-value of 0.125 to 0.25 / inch, as compared to Ceres’s standard R 8/inch walls. In other words, one needs 11 ft of soil to achieve the same insulation of a standard insulated wall. In addition, unfortunately, the lack of reinforced walls poses an issue for long term durability. In areas with at least one wet season per year, the water runoff from the roof can easily find its way through the soil into the greenhouse itself. As a result, the greenhouse walls can crumble. This can be an even bigger problem during freezing winters with high snow loads because your walls are more susceptible to erosion and collapse. And, if you consistently get water in your greenhouse environment, you’ll need to design a drain system to prevent your greenhouse from becoming a mud bath. You might find that all of a sudden your simple pit design is becoming more work than reward. Last but not least, the design of the entrance area is also tricky. Entrance areas that are well below the ground can turn into swimming pools in strong rain events. Ventilation Walipini ventilation can get a bit tricky because the greenhouse itself is underground. Some ventilation options include digging out doors on the east and west ends of the greenhouse, installing a vent towards the top of the back wall of the greenhouse, or installing a trap door vent in the glazing material on the upper north end of the roof. Each option has their advantages and disadvantages in regards to labor, cost, thermal mass, and airflow so your ventilation system will depend on your climate and resources (Benson Agriculture and Food Institute). If you are curious to learn more about walipini greenhouse construction, download this paper published by the Benson Agriculture and Food Institute at Brigham Young University. Geodesic Dome Greenhouse Geodesic dome greenhouses are another popular geometry in the realm of passive solar greenhouses. The geodesic dome originated in the 1960’s and 1970’s from a designer and futurist named Buckminster Fuller. In addition to its greenhouse application, the design has been used for houses, churches, auditoriums, stadiums and other event spaces (Chandler, 2011). A geodesic dome greenhouse consists of small sections of glazing (mostly triangle shapes) – cut out and fit over a wood or steel frame. An appealing characteristic of the geodesic dome is it’s increased structural strength. In fact, the dome structure uses a thinner frame to achieve the same strength as a rectangular greenhouse. Besides its durability, the dome shape is attractive because of its unique aesthetic. Orientation The geometry of the dome greenhouse is great at letting in a lot of light for its small surface area to footprint ratio. The many sections of glazing that make up the dome shape allow light into the greenhouse at any angle. Therefore, building orientation for the geodesic dome isn’t as crucial. Although, if you live in a more northern climate, you might consider insulating the north side of the dome to allow for year-round growing. Glazing One major thing to consider is that the increased number of framing members creates more points for air to get in, if the dome isn’t sealed well. Most geodesic domes use double wall glazing while Ceres greenhouses, in comparison, use high performance triple wall glazing to find a balance between solar gain and insulation value. Insulation As spoken about in The Year-Round Solar Greenhouse, “Most domes, particularly kits, don’t include an insulated north wall, which greatly reduces performance in cold climates. To create an energy-efficient structure, the north half should be insulated, but the options with a dome are more limited.” The easiest and most common way to insulate a dome is to install silver foil covered bubble wrap to increase insulation slightly, which also happens to come at a reasonable cost. More efficient insulation includes spray foam insulation or cutting out individual sections of rigid foam board insulation to fit the glazing panels. Ventilation It’s easy to find prefabricated greenhouse dome framing kits online that don’t require construction experience to put together. These kits usually come equipped with vents and fans that integrate cohesively into the final product. But if you’re doing the dome framing yourself, cutting rectangular openings for vents and fans can be extremely calculative and time consuming. If a vent protrudes too much from the frame or doesn’t seal completely when closed it can create a lot of problems for the growing environment when it’s windy outside. Other issues to consider with geodesic dome greenhouses: Space usage: The ratio of grow space to walkways is usually smaller in round structures. In other words, you will need relatively more walkway space per greenhouse area. Thermal mass water tank: It is great to have a thermal storage space in a greenhouse. Many grow domes accomplish this by housing a big water tank on the north side of the structure. While this tank is indeed a great energy storage sink, we advise you not to keep an open water surface in the greenhouse in the winter months. Excessive humidity is a big concern in every greenhouse, having a warm and open water tank will create powdery mildew issues quickly. The Ceres Solution As we’ve learned, the geometries of the walipini and geodesic dome greenhouse do not fit all growing climates for their own reasons. On the other hand, Ceres design considerations like roof angle, insulated metal panels for the east, west and north walls, east/west building orientation and reflective interior north wall ensure that our growers get plenty of sunlight and solar gain year-round – especially those living in more northern latitudes. At Ceres we design and engineer a whole system solution that will work for every one of our growers, no matter their climate or weather conditions. Not only does this ensure a long-lasting product, but it also ensures lower operating costs for the lifetime of the greenhouse. If you’d like more information about greenhouse options specific to your location, budget and growing goals, contact us today. A greenhouse expert is waiting to hear from you! Citations: Schiller, Lindsey. The Year-Round Solar Greenhouse: How to Design and Build a Net-Zero Energy Greenhouse. Gabriola Island, New Society Publishers, 2016. Nathan Chandler “How Geodesic Domes Work” 13 September 2011. HowStuffWorks.com. 22 October 2020 Benson Agriculture and Food Institute, “Walipini Construction (The Underground Greenhouse)” Revised Version 2002. Brigham Young University. http://www.solaripedia.com/files/1257.pdf