7 Tips for Passive Solar Greenhouse Design
After many years of designing and building passive solar greenhouses (and running two of our own) in the Canadian North, I want to share a few lessons I’ve learned on designing them.
Tip #1: Orient to the Morning Sun
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.
Tip #2: Choose Glazing for Transmissivity and R-Value
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.
Tip #3: Insulate Based on Climate and Heating
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 (two, three, or four), how cold your environment gets, and how you plan on heating the space. R20 is a good place to start for the Canadian prairies.
Tip #4: Ventilate, Ventilate, Ventilate
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.
Tip #5: Provide Rich, Deep Soil for Deep Roots
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 first greenhouse had 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 we had 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!
Tip #6: Set up Timed Drip Irrigation
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!
Tip #7: Add Thermal Mass for Longer Growing Seasons
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.
To hear more about each of these tips, catch my interview on the Permaculture Voices podcast, where I did an hour-long piece on passive solar greenhouses!