7 Tips for Passive Solar Greenhouse Design

 In Built Environment, Design, Featured, Gardening, Greenhouse

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After five years of running a passive solar greenhouse in the Canadian North, I want to share a few learned lessons with you on designing one. And you won’t want to miss it, I was just interviewed  on the Permaculture Voices podcast, where I did an hour-long piece on passive solar greenhouses. That link is at the bottom of this blog!

Greenhouse Tip #1: Get the Orientation Right

Solar Greenhouse Orientation

You might think that a greenhouse should capture the maximum amount of solar energy and should be oriented to straight south. Not so. Believe it or not, the optimal orientation is actually slightly to the east. This gives the greenhouse early morning sun and rejects sun towards the end of the day when it’s most prone to overheat.

Having said that, if your greenhouse is oriented within 45 degrees of south, it should still work fine.

Greenhouse Tip #2: Use 70%+ Transmissivity Glazing

Solar Greenhouse Glazing

 

Light drives the entire system, so you want to make sure you choose a material that’s going to let at least 70% of it through. If too much light is blocked, your plants will get leggy and things won’t thrive. But as you increase transmissivity, the R value (the thermal resistance) of the glazing will decline. In northern climates you want to strive for a balance between these two variables. We use a polycarbonate product that gets 72% transmissivity and has a R value of 2. There are materials with even better performance out there but as performance increases, so does the cost – keep that in mind.

Greenhouse Tip #3: Insulate, Insulate, Insulate

Solar Greenhouse Insulation

In cold climates, insulation is the name of the game. It’s the element that makes the biggest difference in the thermal performance of your space. We chose to go with R20 walls all around, which ironically is better than most homes in our neighbourhood. The R value you choose is going to be a function of how many seasons you want to grow (2, 3, or 4), how cold your environment gets, and how you plan on heating the space. R20 is a good place to start for the Canadian prairies.

Greenhouse Tip #4: Ventilate, Ventilate, Ventilate

Solar Greenhouse Ventilation

On the flip side, I’ve come to believe that there is no upper limit to ventilation in a greenhouse. I recommend that people install a ventilation area equivalent to at least 30% of their glazing. However, this will only keep things cool and manageable in the shoulder months. To get enough air through the greenhouse, it’s important to have other ways of seasonally increasing ventilation to keep temperatures below 26˚C. We have a blowout door that can be removed seasonally . The other reason for excess ventilation is to allow the plants to get “exercise” – it’s important to let them blow around in the wind.

Greenhouse Tip #5: Get the Soil Right

Getting the soil right is as important as any of the other design criteria. Plants growing in a greenhouse grow faster, harder and longer than outside plants. Why? Because you’re creating a perfect environment for them. This means that you need to make sure your plants have a soil medium that can keep up with them. To get that medium, partner good biology with good subsoils. Our current greenhouse has wicking beds and mediocre soils. We made the mistake of building on a concrete pad, which precluded us from getting access to the subsoils. So now we have to continually add minerals, compost and compost tea to keep things running. So learn from us. Plan your greenhouse on soil and plant your plants into the ground it encloses. Use heaps of compost and mulch, cycle beds with cover crops, and you’ll be laughing!

Greenhouse Tip #6: Get the Irrigation Right

Irrigation is one of the first things to go wrong in a greenhouse. When that isn’t right your plants will get stressed, and diseases (plant pathogens and pests) will follow. You want to hit the Goldilocks zone of moisture: Not too much, not too little, just right. I’m a fan of drip irrigation on a timer – Set it up once and forget about it.

I also recommend using rainwater in your drip system as rainwater’s loaded with nitrogen (from lightning) while being free of chlorine and fluoride. Plus if you have hard water, drip irrigation’s not going to last very long before it needs replacement. Rainwater is soft and works great!

Greenhouse Tip #7: Get the Right Thermal Mass

Solar Greenhouse Thermal Mass

Thermal mass is crucial to your success if you want to extend seasons. Water’s often talked about as the ultimate thermal mass, but it can freeze and make a mess. Even though they have a quarter of the thermal capacity of water, I’m a fan of stone, used concrete or cob – you don’t have to worry about them during the winter.

If you are using water as thermal mass, use 8-12 L of water per square foot of glazing. If you’re using stone, concrete or cob, go with 40-60 kg per square foot.

Interested in more tips & tricks for designing a passive solar greenhouse?

One Hour Interview with Diego Footer of Permaculture Voices Podcast

 

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Showing 9 comments
  • Kathy

    I am very interested in designing my own greenhouse.

  • Kathy

    I just stumbled upon your website and can’t believe this is going on in Calgary Alberta!! I live 2 hours drive SE in Brooks and I am very interested in the permaculture philosophy. I would like to take the PDC foundation course but will have to wait and see … $$ are tight – but maybe next year. Thanks for all the great information. Wonderful website!!

  • luke

    Very nice. Where did u purchase the polycarbonate material from?

  • Cam

    Having a house with a crawlspace, the connection to soil worries me for rodent and insect entry. I’m inclined to favour the full enclosure of a concrete floor, footing and up to walls and roof. For sure there are improvements that could be made to the early 1900’s crawlspace I reference, but how would you deal with this aspect on your greenhouse? Extend the concrete footings several feet down below grade? Rodents are less likely to tunnel than they are to just stumble upon an opening. This is of particular interest as any greenhouse or sunspace I would desire would be attached to the house essentially as an extension of livable space. It would exchange C02, humidity, oxygen, and heat with the house. A win-win.

    • rob avis
      Rob Avis

      Thanks for the comment Cam. I still would not build on a concrete pad again. Build it away from your house to eliminate the rodent vector.

      R

    • LindaP

      Yes, I think deeper footing would be helpful, but rodents are not limited to below wall entry. I might suggest plants and oils that discourage rodents. They seem to avoid many of the aromatics we find pleasant.
      Oils or herb sacks are recommended for attics and engine compartments so well placed plantings in your space might help keep them from nibbling your greens.

  • Anna @ NorthernHomestead

    Thank you so much for this informative post. I so agree on the building on a concrete. Our GeoDome – on soil preformed great! Our Garage- Greenhouse on concrete (of course) is challenging to say the least. We are planning to do aquaphonic in the future. Hoping it will be better.

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