How We Designed Our Solar Greenhouse

 In Built Environment, Food, Greenhouse, Permaculture Projects

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 Celcius 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 500 mm 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 results in Calgary having nearly the same solar potential as Florida.

Traditional European Greenhouse (above) and Passive Solar Greenhouse (below)

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 of 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. Here are some photos (including a nifty timelapse slideshow of the construction) and a brief description of the structure.

The General Design

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 (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. We also designed a shed style roof with an overhang to capture rain and reject some of the overhead summer sun.

The Structure

The entire building (with the exception of the glazing) is made from structurally insulated panels.

The building is made from a 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 high 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 supplant 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.

Glazing

The angle of the glazing from horizontal is an important design consideration.

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 linkto 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 degrees Celcius 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.

Aquaponics System

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.

In Summary

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.

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Showing 44 comments
  • Beatrice

    hi I just bought a house with a decent lot in Calgary ,scenic acres. I am thinking of a greenhouse too… Any hints? I am returning in Calgary from Europe after 30 years so I have no idea where to look for help! Thanks

  • Melanie

    I’ve really enjoyed seeing an example of the solar greenhouse done in the same city as I am living in. I’m thinking of converting my large shed into a solar greenhouse so I’ve been pouring over your site and others to determine all the details I’ll need to consider and plan for. What I think would be very helpful would be to see a graph plotting temperatures of your greenhouse over a year. Day and night temperatures, inside the greenhouse and out. It would help to solidify the ideas and the math for me. Is this something you have been tracking? If so, is it something you would be posting about or using as reference in your greenhouse tour?
    Thanks,
    Melanie

    • rob avis
      Rob Avis

      Melanie,

      We have not done this, but it is a good idea. I have been taking a look at Netatmo which is a smart phone enabled weather system (internal and external) which would be great for this. Maybe a future project.

      R

  • Arlene

    Hi…my husband and I built a small greenhouse on the south wall of our house. We’ve only used plastic and wood framing so far, and our tomatoes are still going in the far north of Ontario (near Hudson Bay). We also have lettuce, and we only have planes coming to the community twice a month. It’s really nice to have fresh produce. We are looking at creating more of a permanent structure next summer.

  • brad

    We have a walkout basement with a high deck and I was thinking about turning the underneath area into a green house. Do you think this would be possible? we have a west facing backyard and the other side faces south so we should get good sun from mid-morning on. What is your thought?

    • rob avis
      Rob Avis

      Brad, hard to tell without seeing it.

  • ryan

    Instead of using a wood stove as backup heating in cold temperatures, would it not be possible to have solar panels mounted and then use a thermostat and electric heater? This would save you the trouble of having to monitor the temperature and run a wood, which may not always be convenient. Especially if you don’t happen to be around.
    Although I’m not sure if an electric heated would keep up. Interested to hear what you think. I’m looking at building one just north of you in Red Deer. I was thinking of using the water barrels on north wall method with panels and electric heater for backup. I’m hoping to do it as cheap as possible so I probably won’t use the SIP’s.
    Also, do you have more photos of the finished greenhouse in operation?

    • rob avis
      Rob Avis

      Great idea, except heating is highest when there is the least ammount of sun. Wind is actually a better resource for resistance heating in the winter as the air is denser and blows harder. Thanks for the comment. Check out our most recent blog here. http://vergepermaculture.ca/blog/category/greenhouse/

  • Allissa

    Hey! I just came across this page while researching my own greenhouse… which actually has almost the exact design and the 10 x 20′ dimensions. I live in northern MN, about 47.5 degrees latitude.

    Any lessons you’ve learned since you built?Any tips on shade clothes or insulating covers would also be helpful.

    I’m thinking of using 10mm polycarb glazing, lots of ventilation on E and W walls, and barrels against the N. It’s attached to workshop building with in-floor heating, so I’m trying to figure out the best material for over top… I’m thinking about gravel, but my partner says pavers.

    Many thanks in advance!

  • David Dixon

    I have a solution to summer vs. winter conditions that
    I am using on my greenhouse. Think “venation blinds” like
    you use on your house windows. (sorry spelling?) I am
    building a shading structure outside of my greenhouse
    that allows light in when the sun is low in the sky in
    the winter, but shades it in the summer when the sun is
    much higher. The trick is to get the angle right. A
    wooden framework with fairly heavy metal slats inserted
    is all you need. They never even have to be adjusted, the
    sun does that for you! The shade is “metered” throughout
    the entire year depending on the angle of the sun. Write
    me and tell me what you think. 🙂

    • rob avis
      Rob Avis

      David,
      Can you send us a photo of your design?
      Rob

  • Camille C

    I’m interested in seeing your greenhouse I was wondering if you do tours? I’m passing through Calgary this friday and was wondering if you would be open to letting me see your greenhouse? If you could contact me when you have a chance that would be great.

  • Mark Gillanders

    Great article! Where did you get your polycarbonate?

    • rob avis
      RobAvis

      Ty,

      It has been a while since I was into veg oil. They are probably just over a thousand dollars.

      Thanks for comment.

    • rob avis
      RobAvis

      A place called Acrylco Manufacturing in Calgary.

      r

  • kimberly

    I am interested in building a solar greenhouse for year-round use. I live in northeastern Ontario 48 degrees latitude. I am wondering if your model would suite my purposes, what the cost would be, and how to find someone with the expertise to help build it. Thanks.

    • rob avis
      RobAvis

      Kimberly,

      Yes it would however I have a bunch of modifications that I would recomend. I am about to release a blog with an update detailing out what I would change. The cost is variable depending on how creative you are. To build it with that material it was around 10k. I think you can do it for a lot less. Also I would not build anything less than 400 sqft., 200sqft is too small in my opinion.

      Rob

  • Peter Langes

    We are doing a similar design out here in Nolalu, N.Ontario, 48 degrees north. Putting 1/4 inch thick double wall polycarbonate on the out side that lets 85% light through and then with a moderately thick polyethelene (92% light passage)on the in side for the glazed portion yields an R factor of 3.2 with light being penetration at 75%. We are planning for a concrete floor, did you folks do this as well and could you please share any thoughts you have on this concrete floor thing we are considering. Thanks! Peter from Nolalu.

    • mavis

      Peter, Rob Avis here.
      Thanks for your comment, I would highly recommend using a frost wall system if your site will allow it. Growing on a solid floor is not my first choice unless you are forced to do so. Growing in soil solves a lot of problems both biologically and thermodynamically. I have just written an update to this blog (finally) and it should be out in a couple of weeks. I am going to share all of the learnings to date, both what works and what does not. Keep and eye on the site for more details. Cheers.

  • JINNY

    HI, We live in Calgary too and are thinking of building a greenhouse in our backyard this year. Is it possible we come take a look at yours sometime soon? Thank you very much!! Jinny

    • mavis

      Hey Jinny – Please come by to on of our Open Yard events which we start hosting about once a month in May. Sign up for our Newsletter or watch the website for updates on when those are scheduled. ~michelle

  • Muujig

    So, what happened during the winter? I am from Mongolia, and here in Mongolia during winter climate reaches -25-35 degrees celsius during day time and -35-45 degrees celsius during night time. I have designed a similar winter greenhouse attached to my house wall. The front windows are 65 degrees steep 2m tall and made from double layer glass, but as soon as temperatures began to drop the windows started to crack. non of them had actually broken into pieces but cracked nonetheless, during day time it’s quite warm maybe 10-15 degrees celsius but during night it drops to same degree as outside. I haven’t had the chance to make a insulating sheet/cover to use during the night. So, I’m very interested in your winter experience.

    Looking forward to your update!

    Best of Luck!

    Muujig

  • David Maxwell

    We are in Zone 5b, SouthShore Nova Scotia. I built a solar greenhouse, 12’X16′ to the Brace Institute design, (basically what you have), last year. (Size dictated in part by fact that under 220 sq. ft. did not need building permit.) Glazing TwinWall polycarbonate. Heat storage in multiple 4 and 10 L jugs, (to minimize stratification of heat), filled with dyed water, on North wall. Problems: 1) poor heat transfer into water wall. Interestingly addition of curtain of polyethylene in front of water wall, with fan blowing heated air from towards top of greenhouse _through_ the wall, (conduction from air through walls of jugs rather than relying on solar radiation absorption), worked quite well. 2) heat loss at night. Here I need input from others in advice and experience. Outside temp yesterday (Jan 2, 2013) was -8 C; in greenhouse was 19.2 C – during sunny day. But… night-time low -12 C; in greenhouse -1.2 C. I need an insulating curtain to cover the glazing! You alluded to same in your posting. Did you create one, and what did you use? And did it work?

    • Arun Sivarajan

      I am looking for Mechanical Engineer with Green house experience could you help me please?

    • Pat Cummings

      Hi there, we built a similar greenhouse here in Lunenburg or rather 1 km from town of Lunenburg. we also built a rocket stove with piping underground. the stove has some issues so due to lack of time have been supplementing with a heater to have temps of 15 celcius and be able to start new plants .But it is such a pleasure to pick swisschard,broccoli,celery and the occasional tomato.for smoothies here in the middle of the winter.we would love to get together and see your Gh and perhaps share ideas. Pat and Peter11494 highway 3 ,Centre.

    • Mitchell

      Hi, Thanks for sharing your experiences. I am currently planning construction of a solar greenhouse for Regina. Friends have one and use a large water barrel along with heat tape, to prevent the water freezing at low temperatures. I wonder if the large barrel of water works better than many small containers. The heat loss of water relates to the amount of water, plus the surface area for cooling would be much greater on many small jugs. What do you think?

      • rob avis
        RobAvis

        Mitchell, I have seen people use both. They both have their advantages and drawbacks. Small containers will likely be more space efficient but may gain and release heat faster due to an increase in surface area to volume ratio. Large containers will likely have a slower response (more buffering) but will take up more space as they tend to be large round barrels which are not an effective use of space in a square greenhouse. Ultimately there are a ton of ways to increase thermal mass, I would check out this website for a completely different way to store thermal energy. http://www.sunnyjohn.com/indexpages/shcs.htm . Thanks for your comment. ~ Rob

  • Lenore

    We have been thinking about a greenhouse but I wanted something that could be used year round in Manitoba. Great article with lots of food for thought. Now I just have to check town by-laws and regulations before I go any further. I hope to have a reasonably sized greenhouse going before next winter.

    • Nigel

      I too am looking for a year round solution in Manitoba. Would love to hear about any info you’ve found Lenore. I’m extremely “green” (pun intended) but am just starting to soak this up. Anyway good luck with your research. Hope to hear from you.

  • Anonymous

    ” For heat retention, we plan to install six inch non-perforated weeping tiles below the raised garden beds. These “earth tubes” ”

    I know weeping tiles look appealing, but especially in a humid environment could be a good place for growing unwanted thinks… I suggest smooth and rigid pipe abs or pvc.. (more money) but with a string and a canvas soaked with bleach or other things you can quickly clean them. Because your earth tubes run just in the bed you can maybe just invest less money and once in a while dig them out and replace them if they start to create problems.
    Keep us updated !!
    alvise & paola, bragg creek

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