Design Basics for Growing Year-Round in a Solar Greenhouse
It’s a little-known fact: most year-round greenhouses are energy guzzlers. To grow a variety of crops year-round in cold climates of North America, a standard greenhouse requires large amounts of heating. Commercial greenhouses in North America can be incredibly energy-intensive, costing about 10 times as much to heat and cool as a standard home in areas like Colorado.
Their inefficiency is largely due to their materials: glass or plastics, generally referred to as “glazing”. If you’ve ever sat by an old window on a cold night, then you understand that glazing materials are very poor insulators. They let lots of heat and light in during the day, but also let all that heat out at night. The result is a structure that swings wildly in temperature, both overheating and freezing, making year-round growing expensive or impossible.
Fortunately, there are easy and affordable solutions to this dilemma. Passive solar greenhouses – often called “solar greenhouses” for short – are those designed to minimize external heating and cooling, and maximize use of free solar energy, for both light and heat. Using time-tested principles, a grower can enjoy an abundant, lush garden all year without relying on fossil-fuels. Solar greenhouses typically incorporate thermal mass or other means of storing excess heat during the day for use at night. We’ll have more on thermal storage strategies in our next blog but for now we explain the principles of passive solar greenhouse design, the topic of the first section of our new book The Year-Round Solar Greenhouse.
The first tenet is to site and position the greenhouse to maximize use of solar energy for both light and heat. In the northern hemisphere, this means positioning the greenhouse so that most glazing faces south.
A second tenet is to insulate wherever possible. In most North American climates, the north side of a greenhouse receives very little light in comparison to the south side. Thus, this area is best insulated like a wall in a home. While a larger investment, insulation goes a long way to minimizing heat loss, allowing for abundant year-round gardening without needing excessive heating in the winter. Examples of different construction techniques can be seen via Ceres Greenhouses’ commercial greenhouse kits or residential solar greenhouses.
The downside to insulated walls is of course they do not transmit light – an essential factor for plant growth. Designing a solar greenhouse comes down to striking a balance between glazing and insulated walls. The mindset that greenhouses must be constructed entirely out of glazing is a vestige of their origin in cloudy climates of northern Europe. In those areas and similar overcast climates, a fully glazed greenhouse is probably the best choice. In most of North America, however, greenhouses can grow abundantly year-round with much less glazing.
The optimal area of glazing depends on your climate, what you are growing and when. While there are no hard and fast rules, broad rules of thumb and examples of other greenhouses in your climate help narrow down the choice. We elucidate this process more in Chapter 5, and via many case studies in the book.
Another strategy is to use glazing strategically by paying attention to the placement and angle so it transmits as much light as possible. In this way, a solar greenhouse makes the most of a smaller glazing area. Glazing angles are a complicated, and often oversimplified. We explain the issues in the book and Ceres blog Find the Right Angle for your Greenhouse Roof. When it comes to glazing placement, it’s important to think about both shading and achieving sufficient light throughout the day. It’s also crucial to select the right glazing material for your climate, factoring in sun intensity, snow and wind loads, outdoor temperatures, and your growing goals. We delve into the pros and cons of glazing options, both glass and plastic, in Chapter 5.
These design principles create a vastly more energy-efficient greenhouse. However, solar greenhouses typically require another means of climate control, such as thermal mass. A final option is to add solar panels to power the greenhouse, if it has electricity. While often seen as “sexy”, solar panels are not always economical in a greenhouse application. We explain the reasons for investing in solar PV, and strategies for doing so, in Chapter 16, “Powering Your Greenhouse.” You can also get help from the Ceres blog, 5 Tips for a Solar Powered Greenhouse.
Once you have designed the structure, you can go even further by adding sustainable growing systems such as aquaponics, permaculture gardens, vermicomposting beds and more. Solar greenhouses have grown almost everything – from cold weather crops to tropical fruits like bananas and avocados – in most every climate in North America. With a sustainable and efficient year-round greenhouse, the gardening options abound.