A passive solar greenhouse is designed to utilize Thermal Mass to either fully heat it or assist in heating during cold winter climates.
The main objective is to retain the heat absorbed during the day and to prevent the cold from entering at night.
While exploring options for building a passive solar greenhouse, you'll discover numerous methods, with some costing over $200,000 and others as little as $3,000 for smaller versions.
A passive solar greenhouse without heating works well only in warmer climate zones such as 6B-9, where it is generally used for three seasons annually.
In Zone 5, where I reside, winter temperatures can plummet to -40 degrees Celsius, so we use a woodstove to heat our passive solar greenhouse.
We cultivate plants in the greenhouse throughout all four seasons. Our greenhouse is considered passive solar because we utilize passive heating from the sun and thermal mass. During extremely cold periods without sunlight, we use a woodstove to warm the greenhouse. The greenhouse remains above freezing in winter.
A properly constructed passive solar greenhouse generally maintains interior temperatures 10-30 degrees Fahrenheit warmer than the outside temperatures during the colder months.
Sweet Million Cherry Tomatoes Jan 10th 2020
Concept
A passive solar greenhouse is designed with its front opening facing south. In winter, when the sun is low, sunlight reaches the back of the greenhouse. In summer, when the sun is high, it receives minimal sunlight, keeping the greenhouse cooler. This design extends the growing season, allowing you to start seeds and plants much earlier than in a traditional greenhouse.
Building
Ours is 20×40 with a 16ft pitch in the back. With a 12ft facing, the back wall is 7ft high. the front is 5 ft high.
Construction Started October 2019
Floor
We lade down rock, gravel and crushed rock as the top layer (the rock warms up and lets off heat.) In the summer it absorbs the heat so the greenhouse stays cool. We chose the crushed gravel so we could ground when we are in there.
Thermal Mass
We use blue barrels painted black filled them with water, doubled up along the back wall. When the sun is out in the winter it is low in the sky, so it hits the barrels and warms them up, the barrels absorbed the heat. During the cloudy days and cold nights it lets off the heat. When we have the woodstove going the barrels are still absorbing the heat and lets it off when the greenhouse starts to cool down.
Thermal mass of various materials
Material | Density (kg/m3) | Specific heat capacity (kJ/kg.K) | Volumetric heat capacity (kJ/m3.K) |
Water | 1000 | 4.186 | 4186 |
Concrete | 2240 | 0.920 | 2060 |
Stone (sandstone) | 2000 | 0.900 | 1800 |
Compressed earth blocks | 2080 | 0.837 | 1740 |
Rammed earth | 2000 | 0.837 | 1673 |
Fibre cement sheet (compressed) | 1700 | 0.900 | 1530 |
Brick | 1700 | 0.920 | 1360 |
Earth wall (adobe) | 1550 | 0.837 | 1300 |
Autoclaved aerated concrete (AAC) | 500 | 1.100 | 550 |
Thermal Mass Table
Insulation
We used regular insulation and vapor barrier for the first year. In 2020 we took everything out and spray foamed 3 inches on the north facing wall. The summer of 2022 we boarded the two sides in and spray foamed them as well. We added each year depending on the winter months of us experimenting
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