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Build Your Own Passive Solar Greenhouse on Your Driveway

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This DIY passive solar greenhouse was designed and built right on his concrete driveway parking pad by Rob Avis, who writes up his experience at EnergyBulletin.

Building in Calgary, Canada, he took into account local conditions. The region, located on the 51st parallel north, requires a different style of greenhouse than the traditional European style that is designed to receive diffused sunlight from all angles.

He chose to site the greenhouse on the concrete driveway parking pad (“who needs all that parking space anyways?”). The size of the greenhouse is 10’ x 20’ with a 10’ ceiling (3 m x 6 m x 3 m) which covers nearly half of the cement pad.

Generally, this style of greenhouse works best if it is twice as long as wide. He designed a shed style roof with an overhang to capture rain and reject some of the overhead summer sun, while being open to the lower winter sun.

He used pre insulated SIPs to build, because of their very high insulating values, (in this case with an average R -value of 25) far better than most homes. They are also fire-proof, don’t out-gas, and are mold- and rot-proof, which is very important in the high humidity environment he is building in.

Because the panels are pre-fabricated, the main structure itself went up in less than one day.

Despite being cold in the winter, it is rarely overcast and almost every day is sunny.

Here the best design is only open to the South – a passive solar greenhouse, designed to accept and enhance the direct sunlight and heat from the south while preventing heat loss by insulating the north, east and west sides.

Normally in passive solar building design (not greenhouses) the recommended percentage of glazing to prevent overheating is 7% – 12% of the total southern wall surface.

The greenhouse has 90% of the Southern exposure glazed; perfect for the chilly winter. To ventilate it in summer he has openings under the window area and will experiment to see what works best to cool things a bit, because that is too much for summer.

The glazing angle is chosen to maximise exposure to the winter sun, while minimising exposure to the summer sun (preventing overheating).

“As a rule of thumb, to optimize the glazing angle for winter growing take your latitude and add 15 degrees” says Avis. “In our case the optimal angle would have been 51 + 15 or 66 degrees. However, as long as the glazing angle is within 45 and 75 degrees you will be within 5% of optimum – therefore it often makes more sense to design the building to height restriction and material constraints vs optimal glazing angle. In our case, the actual glazing angle is 55 degrees.

He used triple glazed polycarbonate, with an R-value of just 2, so at night they will draw down a blanket to keep the heat from escaping.

So, the next step will be finding plants to begin growing. Perhaps overly ambitious for Calgary, Avis is thinking Banana trees, and maybe Figs.

But whatever he chooses, it is a big return for a small outlay. “In the end this greenhouse is going to supplant far more energy in its life than it consumed in its manufacture” says Avis. “Every calorie of food that is supplies to my family is 10 – 25 that do not have to be expended in the industrial system when you consider tractors, fertilization, pesticides, shipping, refrigeration and transport. With a life expectancy of at least several decades we are quite satisfied with the energy payback”.