Passive solar greenhouses | Columnists
To build a passive solar greenhouse, whether small or large, is to create an ecosystem, a biosphere and basically, a very special place.
In 1977, my husband and I moved to a house built in the 1890s. Quite by accident, the new foundation was built 3% from perfect south. This meant that the longest wall of the house was ready for a passive solar greenhouse.
In 1980, Sheridan College offered a course in passive solar design. Taught by Wyoming Energy Extension Service, the course provided training in passive solar principles. For the student’s final project, the class would build two passive solar greenhouses to illustrate and solidify the concepts learned in class.
My husband took the course and offered our house for one of the greenhouse builds. WEES felt that a 24-by-12-foot structure was too big and looked for other sites for two smaller projects. Luckily, a large group signed up for the course. Due to the high number of students, WEES chose our house for one of the greenhouse projects. At that time, we had built the foundation for a two-story greenhouse measuring 12 feet by 36 feet.
Passive solar greenhouses are the most efficient structures for collecting and storing solar energy. This style greenhouse faces south and for best use can be attached to the south wall of a building. Ours is attached to our house.
Originally our mass storage, to retain the heat generated by the sun, consisted of 55 gallon black water filled drums placed along the north wall. Eventually they leaked and took up so much space that we replaced them with a 5ft by 24ft raised bed to grow herbs and vegetables on the south side of the greenhouse. The bed is 4 feet deep and is bordered on two sides by interlocking concrete blocks used for retaining walls. The remaining bed walls are against the east and south exterior walls. Our mass storage is now the planted bed where we grow our cold winter crops. The amount of mass, for each passive solar storage, is measured by a mathematical calculation.
The frame of the greenhouse constructed with two-by-six-inch boards, plywood, and the existing wall of our house are painted white, to reflect sunlight on all sides of the plants growing in the raised bed. The exterior of the structure features LexanTM Thermoclear Multiwall Polycarbonate panels on the front, half of each side wall and half of the roof. The rest of the roof has shingles. The remaining half, of the east and west walls, is clapboard. Inside, the solar plastic is stapled to all exterior LexanTM walls. This air space helps insulate the greenhouse. The shingle portion of the roof helps the greenhouse cool during the summer months when the sun is directly overhead. In winter, when the sun is low in the sky, the rays shine more directly on the bed and store heat. For the most part, this keeps the temperature at a sustainable 50 degrees Fahrenheit. When outside temperatures drop below freezing for several days, we use a small heater and direct it towards the concrete retaining wall. The greenhouse stays above 50 degrees Fahrenheit. However, if the temperatures are -20 degrees Fahrenheit and the sun is shining, the greenhouse temperature can reach 110 degrees Fahrenheit.
The size of the covered roof is calculated with a magnetic variation map, which indicates the magnetic variation away from solar south. This simply means that the map determines the location of the sun, in summer to minimize heating and in winter to maximize solar gain. Thus, the need for this calculation.
Exterior ventilation is important for a passive solar greenhouse. For hot summer temperatures the greenhouse can be above 120 degrees Fahrenheit, a vent is located at the top and center of the greenhouse roof. A window is located on the west wall and a door on the east. Together, all three are open to ventilate and regulate summer temperatures. Their placement creates a natural convection current and moves warm air outside, keeping greenhouse temperatures low and our home at least 15 degrees cooler than outside.
In winter, the air circulation is reversed. All exterior openings are hermetically sealed. The ground floor and basement windows are all open. Floor vents inside the house, near the north walls, create a convection current inside. This moves hot/hot air from the main floor windows, down through the vents in the basement and out the basement windows into the greenhouse. This cycle moves warm air and moisture throughout the house. Static electricity is non-existent with us.
High humidity can cause problems for plants and sometimes for the structure. The floor of the greenhouse is made of recycled bricks, laid a bit like cobblestones, with a base of sand and sand between each brick. This allows excess moisture to be absorbed by the soil. A friend of ours built a passive solar greenhouse, with a concrete walkway. Moisture was continually a problem, with only one solution, removing the concrete.
We are now looking for a solar powered fan for the east side of the greenhouse to move air and as an extra gauge to reduce summer temperatures. This temperature controlled fan will turn on when temperatures reach 75 degrees Fahrenheit. This will further control humidity and circulate existing air in all seasons.
It is important to water the plants in the morning. When possible, ventilate outdoors, but not at extremely low temperatures. For a few weeks, the greenhouse has been open to recycle the air. A timer is set to ensure that it does not stay open for long and damage the plants.
One year we grew a banana tree. It was near the door. When we opened it, the biting cold killed the tree instantly.
Our greenhouse is now 42 years old. The planted bed is lush with celery, Swiss chard, collard greens, carrots, radishes, onions, leeks and herbs such as chives, mint, oregano, rosemary, sage and thyme. The calamondin orange tree, covered with flowers, will soon offer kilos of oranges. Although bitter, they are perfect for certain recipes and you don’t need to peel them.
There’s something amazing about cooking with fresh-picked produce. One of my favorites is baking fish in brown butter with crunchy sage leaves, lemon and butter. Yum yum.
In our greenhouse, we find peace and contentment working in the land in all seasons. People with seasonal affective disorder would do well to have a passive solar greenhouse for atmosphere and light. There’s nothing like warming up in a 100 degree Fahrenheit space when it’s freezing outside.
The clean air and abundant sunshine of the passive solar greenhouse warms and relaxes our bodies, while the produce nourishes us throughout the winter months.
The Sheridan County Fulmer Public Library has several resources related to passive solar design and greenhouses.
Val Burgess is a Sheridan County Master Gardener.