How We Designed Our Solar Greenhouse
by Rob Avis
We live and garden on an urban lot in Calgary, Canada, located on the 51st parallel north and approximately 80 km east of the front ranges of the Canadian Rockies. This northern climate presents many design challenges, including less than one hundred frost-free days, an annual mean temperature of 4.1 degrees Celsius and summer cyclonic weather patterns (i.e. high risk of hail). We are also considered to be a moderate temperate desert as our precipitation is around 500mm including snow. However, one of the advantages of growing food up north is the long summer days. There is no better place to observe this than in Alaska which also has an average of 100 frost free days but is renown for growing the largest vegetables in the world. Also, despite being cold in the winter, it is rarely overcast and we enjoy mostly sunny days. These two factors combined result in Calgary having nearly the same solar potential as Florida.
Traditional European Greenhouse
Passive Solar Greenhouse
In good ol’ permaculture fashion, we set out to enhance sectors and conditions that would improve our growing season (sunlight, heat) while minimizing those that we considered detrimental (cold, hail, frost). We quickly determined that a passive solar greenhouse was just what we needed and we set out to design one for our backyard.
There are two major considerations when designing a greenhouse: heat and light. Interestingly, the traditional European-style greenhouses were developed in the low countries of northern Europe as a response to low level of predominantly diffuse light prevalent in the winter time (think cloudy, overcast winters). This design was brought to this continent with little consideration for the differences in climate and latitude encountered here.
A greenhouse that is better suited to our winter conditions is a passive solar greenhouse. These greenhouses are 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.
Just this past December our greenhouse went up. Although currently empty (we still have to design out and build the interior) we’ve been receiving a ton of interest and questions about how we designed the structure itself. So, here are some photos (including a nifty tim elapse slide show of the construction) and a brief description of the structure.
We chose to site the greenhouse on the concrete pad behind the house (originally designed as a parking pad — who needs all that parking space anyways?). The size of the greenhouse is 10’ x 20’ with a 10’ ceiling (3m x 6m x 3m) which covers nearly half of the cement pad. Generally, this style of greenhouse works best if it is twice as long as wide. We also designed a shed style roof with an overhang to capture rain and reject some of the overhead summer sun.
The entire building (with the
exception of the glazing) is
made from structurally
The building is made from structurally insulated panels (SIPs). These pre-fabricated panels consist of an insulating layer (styrofoam) sandwiched between two layers of structural board. Although the idea of building the greenhouse from natural materials (i.e. straw & cob) was very appealing to us — the truth is that I did some engineering consulting for the SIP panel manufacturer which resulted in getting a sweet deal on the building materials. There are also some great advantages to using SIPs — they are mold and rot proof, which is very important in humid environments. These panels are fire proof and do not off-gas. They are highly insulative with an average R-value of 25 (better than most homes) and because the panels are pre-fabricated, the main structure itself went up in less than one day.
In the end this greenhouse is going to produce far more energy in its life than it consumed in its manufacture. 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.
The angle of the glazing from
horizontal is an important
Glazing on a greenhouse is the surface that lets the light in — usually glass or plastic sheets. Having the glazing at an angle allows us to maximize winter sun (increasing heat in the winter) and minimize the summer sun (reducing overheating in the summer). The angle of the glazing from horizontal is an important design consideration and the optimal angle depends on which part of the season you want to do most of your growing.
As a rule of thumb, to optimize the glazing angle for winter growing, take your latitude and add 15 degrees. 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.
For glazing we chose to use triple glazed polycarbonate with an R-value of 2. This is dramatically less insulative than the walls (R-25) and so to keep the heat in we are going to use an insulated draw-down curtain which will be drawn at night and raised in the morning.
What’s Left To Do (there’s lots!)
With the structure and glazing up, our big project over the next few months is to complete the interior features. There are quite a few other considerations, here’s a brief description of our plans at this time:
Heat Retention & Rejection
Low and high vent holes
have been cut out to
provide passive ventilation
Typically, in passive solar building design, the recommended percentage of glazing to prevent overheating is 7% – 12% of the total southern wall surface. For instance, if your southern wall was 100 m2, you should have only 7 to 12 m2 of windows. If you go above 12% you have to add additional mass in the building to absorb the incoming solar energy. If you go above 20% you are going to overheat your building.
Well, our greenhouse has 90% glazing coverage on the south surface. This amount of glazing is required to capture sufficient energy in the cold months to keep the space warm but is setting us up for potentially major overheating issues in the summer. There are a couple of strategies to deal with overheating which we intend on employing: (i) heat retention and (ii) heat rejection.
For heat retention, we plan to install six inch non-perforated weeping tiles below the raised garden beds. These “earth tubes” will receive hot air directed from the ceiling of the greenhouse using a small solar-powered fan. Effectively, we will be storing surplus heat in the soil of the garden beds.
For thermal mass, we will use black containers of water along the back wall. We are also considering installing some cob features to soak up additional heat, however we are a little concerned about the cob being exposed to high humidity — I’d be interested to know if anyone has experience with this.
For heat rejection we have cut out multiple air vents, both high and low, which will be operated with powerless wax-driven arms (here’s a link to a similar product). A rule of thumb for sizing ventilation is to have the total venting area equal to 25 to 30 percent of the total area of glazing. We may also need to install a shade cloth under the front eave in the summer to further reduce the heating load. Time and experimentation will tell.
Auxiliary Heating System
When we get to the inevitable -30°C day with cloud cover the greenhouse is going to need some extra heat. Our plan is to build a rocket stove back-up heating system. While driving through the Calgary industrial park several weeks back I was amazed at the amount of good wooden pallets that were being disposed of. When I stopped to talk to one small business, the owner pleaded for me to take them away. And so, these pallets will serves as fuel for the rocket heating system and will also make great building material. Sawdust left from the processing will be used for our composting toilets and mulch.
I am very interested in experimenting with combined fish and hydroponics systems — and the greenhouse will provide just the space I need. The added beauty of combining an aquaponics with the solar greenhouse is that the aquaponics system will increase the thermal mass while providing a bounty of fish and veggies. However an important consideration will be the increased humidity. While condensation & rot is not a concern with our mold-proof structure, most plants do grow best at a relative humidity between 45 and 60 percent. Leaf rot and flower, fruit and stem diseases increase in very high humidity environments.
The solar greenhouse is certainly going to make a big re-appearance here in our climate (and similar climates) as people start to re-connect with their food and desire more local and sustainable ways of nourishing themselves.
We are very excited about working on the interior design and construction over the remainder of the winter and have already been perusing specialty seed catalogues looking for appropriate banana and fig tree varieties. There is a steep learning curve ahead as we expand our gardening knowledge with new plant varieties, techniques, time and space stacking and generally about greenhouse operation. But we are keen and eager and look forward to sharing our successes… and failures too!
Stay tuned for updates….
Rob and Michelle Avis are Mechanical Engineers and Permaculture Designers and run their business, Verge Permaculture in Calgary, Alberta, Canada.