Welcome to a series of blogs about Cultivate Cochrane president Jackie Skrypnek’s backyard greenhouse building process. The intent is to share a realistic account of one couple’s experience to help inspire and inform other community members in their own season-extending, food-producing ventures.
With the shell now formed, it was time to add all the elements that would turn our little structure into a functioning passive solar greenhouse. Installing polycarbonate panels on the south face felt like a defining moment - you could no longer mistake this thing for a garden shed or chicken coop! There are several options for greenhouse glazing, each with pros and cons. Poly film has a short lifespan, is a very poor insulator, and is prone to damage from wind and hail. But it’s cheap. Glass is heavy, prone to breaking, and would be very expensive if you went with high-efficiency windows. But it looks nice and gives you a crystal clear view. We chose polycarbonate because it’s extremely lightweight, durable, and offers a good balance between R-value and transmitting adequate light. It’s not cheap, though, so it makes sense to take a look at the dimensions it’s sold in and adapt your design as necessary to avoid waste. We purchased from EcoFort in Calgary.
Ventilation in a greenhouse is vital both to prevent disease through air movement and to avoid overheating. To passively ventilate, you essentially need intake vents located near the base of the greenhouse and exhaust vents near the top so that as hot air moves up and out, cooler air is drawn in. We settled on two custom-built intake vents that will passively open and close using heat-activated openers, and two solar powered exhaust fans connected to a thermostat. The exhaust fans are designed to seal well in a roof application - something we couldn’t confidently say of installing custom-built vents in the same spot. The cost of the fans was also likely no more than what we would have spent on two more heat-activated openers and miscellaneous material to build our own watertight exhaust vents. We can add to this ventilation system, of course, by simply opening the door and window.
A key ingredient in passive solar building is thermal mass - those dense materials that absorb and slowly release solar heat. Barrels of water, cob structures, concrete, brick, and stone are examples of such materials. As I’ve noted, our indoor space was at a premium given the small square footage, so we installed 2” thick sidewalk blocks up the north wall to act as thermal mass while keeping a slim profile. I’m happy with the fact that the blocks are simply strapped in place with strips of wood and, if ever the greenhouse was deconstructed, they would come out intact and reusable. We’ll add a little more thermal mass, likely concrete blocks or stone, in the floor between growing beds.
Concrete sidewalk blocks strapped to north wall for thermal mass
Because passive solar greenhouse design restricts light entry to mainly just the south side, it helps to reflect that light around as much as possible by painting interior surfaces white. So, apart from the concrete blocks and a bit of wood detailing my husband couldn’t bear to see coated, we applied white paint to everything: the walls, door, and ceiling. The increased brightness was immediately noticeable!
With some finishing touches to the exterior, we very nearly have ourselves a finished and functioning passive solar greenhouse. All that remains is to set up the growing beds inside, a rain capture system outside, and any other fun and functional details as they occur to us.
Finishing the exterior with cedar shingles treated with non-toxic Lifetime product
The price tag for this whole undertaking, you ask? Well, brace yourself - it may strike you as quite high, especially considering our labour was free. Our current rough calculation comes out to about $7,000. That’s a lot of food this little building will have to produce to pay for itself!
I will, in fact, loosely keep track of the pounds of tomatoes, cups of basil, etc. that come out of it each year. But, of course, you can’t really compare zero-mile, nutrient-rich, soil-grown produce to anything commercially available. And there are yields from this greenhouse that go beyond filling our fridge - an all-season feel-good retreat space, a venue to trial and get up close and personal with some crops that go beyond our Zone 3 limits, an experiment in design for climate extension, increased resiliency for ourselves and anyone who inherits this property, and a privacy screen between our yard and the next. Heck, maybe even a special place to share a glass of wine with friends!
In the next instalment we should have plants in there trying out their new home…