How to Design a Passive Solar Greenhouse : Foundations, Kneewall, Ventilation & Glazing— Part 2 of 4
How do you design a greenhouse to overcome poor growing conditions, provide a winter space that can cure SADS and extend the growing season? In these articles I am going to talk about how to design just a space.
If you are just joining this series you can read part 1 here with steps 1 through 4.
In this article we are going to talk about foundations, knee-walls, ventilation and glazing in the greenhouse.
Step 5 -Foundations
Foundations are a crucial component that dictates the success of any building. It is a component and we often don’t think about, yet a ton of energy goes into the design and construction of a foundation. There are literally enough foundation options out there to write about them for years so I am going to stick to my top 3 options, but first lest start with a few principles specific to greenhouses.
Principle 1) The foundation of a greenhouse should connect the greenhouse to the soil. Plants love getting into the actual earth, not growing above it. A good foundation should allow the plants to dip their roots into subsoil.
Principle 2) The greenhouse foundation should start below the frost line to prevent heaving or, should prevent the frost from entering the greenhouse.
Principle 3) The foundation option you choose should align with your goals.
Principle 4) The foundation should not be toxic to the soil.
My greenhouse ignores the first three of these three principles…here is why. I built a greenhouse on top of a car parking stall and I did not have the heart to rip up the concrete. I was also not has far along in my permaculture career and did not realize the effects of ignoring all these principles. While my greenhouse is perfect for seedlings, and microgreens it will never be perfect for my original intention, 4-season growing of food in the ground. Because I ignored these three principles I have had to adapt the space to its constraints, as opposed to designing the greenhouse around mine.
If you want to see the full case study of our greenhouse, click HERE. We have collected five professionally filmed passive solar greenhouse case studies from geodesic domes to commercial scale greenhouses to backyard scale ones. In them you can learn from the successes and failures of each style of greenhouse.
Option 1) The first foundation that I am a big fan of is frost protected shallow foundations. These are strip footings with an insulated stem wall and a horizontal projection of insulation. This allows plants to express the roots, prevents the frost from entering and does restrict what you do in the soil going forward. This type of greenhouse can grow trees, seedlings or microgreens.
Option 2) The second foundation is a deep foundation that can be built with a strip footing, concrete wall and some form of insulation. Insulated concrete forms (NEXCEM blocks or styrofoam blocks work well here). These are basically soil filled basements. They are a little more expensive than option 1.
Option 3) This is actually my favorite option but a lot of building codes do not recognize it. This is a rubble trench with horizontal thermal protection. It is basically a less expensive version of option 1. This requires little to no concrete, so it is easy and inexpensive to build, and is great for DIY folks. Rubble trench foundations are one of the oldest foundation types and buildings that are built on them have stood the test of time.
Do you research and choosing the footing and foundation for your building, it will pay dividends in the future. If you choose a different option, screwpiles, concrete piles, pressure treated plywood ect. do your research and run it through the principles.
Step 6 — The Kneewall
You are probably thinking, what is a kneewall? It is the front vertical wall in the greenhouse that supports the lower vent system. The kneewall is an important component as it sets the height at the front of the greenhouse (important for the ergonomics of the space for tall guys like me) but it also gives a vertical space for snow to accumulate as it slides off of the glazing. Believe it or not, your greenhouse may still need to open its front vents in the middle of the winter on a sunny day so making sure those vents don’t get blocked by snow is important. Typically I make sure that the kneewall plus the vent system is high enough so the trusses don’t hit my head and this seems to work for me. Generally speaking a kneewall of 3–4 ft seems to work however if you live in a high snow fall environment you may need to think about making it larger or clearing the snow in front of the greenhouse from time to time.
Step 7- Ventilation and Air Movement
Ventilation is arguably the design detail that almost every greenhouse gets wrong.The reason they get it wrong is because it is hard to get enough air through a greenhouse. Under-ventilated greenhouses overheat, cause plant pathogens and heat stress and ultimately crop failure. Don’t under ventilate your space. Here are some general guidelines that you can follow to ventilate your space.
- Have low and high vents. If possible, make your high vents larger than your low vents as air expands as it heats up. This will improve the thermosyphon effect of air moving through the space.
- Install seasonal cross vents that can be opened in the height of summer. Cross drafts can do wonders for cooling a space down. I typically specify double doors or an inexpensive plug (oversized non hinged door) that can be pulled to let in more air.
- If you still don’t have enough air movement add in extraction fans as a last resort.
- If you don’t get enough interior air movement add some fans to “exercise” the plants and keep fungal issues down.
When you are doing design don’t under ventilate!
Step 8- Glazing
The glazing surface to the south is the collection unit. It is the area that lets in the sun, heats up the space and allows plant to grow. It is also the weakest link in the whole assembly. There is an interesting and unfortunate relationship between transmissivity (how much light comes through) and R value (thermal resistance). As the the R value (thermal resistance) goes up (less heat is lost) the transmissivity goes down (less light gets through). If the transmissivity drops below 70% plants don’t get enough light, get “leggy” and never reach their full potential or die. Generally I choose poly-carbonate, but my colleague Curtis Stone has shown that double wall poly (greenhouse grade) is a great alternative at a fraction of the cost.
Here are a couple of guidelines for choosing glazing material.
- Choose the highest R value glazing with a transmissivity of at least 70% or higher.
- Ensure that the glazing will be able to withstand the snow load in your bioregion.
In the next article we are going to talk about artificial lighting, insulation, and thermal mass.
Recently Curtis Stone and I did a webinar on how to design Passive Solar Greenhouses. Take a look at the replay if you would like to see these tips in video format. In addition we have just launched a 10 week passive solar greenhouse course which is now live and it takes you through the process to ensure that you get a productive and profitable passive solar greenhouse at the end.
For more information on greenhouses go to:
https://vergepermaculture.ca/courses-events/#four-season-greenhouse-design
Rob Avis’s Bio:
In less than 10 years, Rob Avis left Calgary’s oil fields and retooled his engineering career to help clients and students design integrated systems for shelter, energy, water, waste and food, all while supporting local economy and regenerating the land. He’s now leading the next wave of permaculture education, teaching career-changing professionals to become eco-entrepreneurs with successful regenerative businesses. Learn more and connect with Rob at https://vergepermaculture.ca/contact/
PS. If you see any typos, please let me know.
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