Designing Your Own Passive Solar Greenhouse, Part 2

Welcome to Part 2 of this series on how you can design your own greenhouse that can extend the planting season, enhance growing conditions, and provide a cheery space for those dark winter days. You can read part 1 here to see steps 1 through 5. Onwards to Steps 6 to 9!

Step 6: Design Your Foundation for Crops and Season Length

An attached passive solar greenhouse with an insulated concrete form (ICF) foundation.

The foundation is a crucial component that dictates the success of any building. It’s something people often don’t think about, but in reality the time and energy you spend on its design now will save loads of trouble later. There are a lot of choices out there so I’ll limit discussion to my top three, but before we start, here are a few principles to keep in mind:

• Your foundation should always connect the greenhouse with the soil. Plants thrive when they can reach into the earth, and a good foundation should allow roots to dip into the subsoil.
• Your foundation should start below the frost line to prevent frost entry or any heaving issues.
• Your foundation should align with your own specific design goals. More on that later.
• Your foundation should not leach toxins into the soil.

Full disclosure: My first greenhouse actually ignores the first three principles. I wasn’t as far along in my permaculture career then and didn’t recognize the effects of ignoring these wise rules. I had built the greenhouse on top of a parking stall and didn’t have the heart to rip up the concrete. So while my greenhouse is now perfectly suitable for seedlings and microgreens, it will never fulfill my original intention of four-season growing. Because I ignored the principles, I had to adjust my expectations instead of having a space that could fulfill my original vision.

The long and short of it is this: Don’t do what I did. Learn from my mistakes!

Back to foundations. Here are my top three recommendations:

• Frost protected shallow foundations. These are strip footings with an insulated stem wall and a horizontal projection of insulation. This allows plants to stretch out their roots while preventing the frost from coming in. It does restrict what you can do in the soil going forward. Greenhouses with this type of foundation can grow trees, seedlings, or microgreens.
• Deep foundations. These can be built with a strip footing, a concrete wall, and some form of insulation. Insulated concreted forms (NXCEM blocks or styrofoam blocks) work well here. These are essentially soil-filled basements and are a little more expensive than option 1.
• Rubble trench foundations with horizontal thermal protection. This is my favourite option, but a lot of building codes don’t recognize it. It’s basically a less expensive version of option 1, requires little to no concrete, and is great for DIY folks. Rubble trench foundations are also one of the oldest types of foundations, and buildings built on them have stood the test of time.

My advice is to do your research and choose the footing/foundation that’s right for your needs. Trust me, it will pay dividends in the future. If you choose another option (screwpiles, concrete piles, pressure treated plywood, etc.), always run it through the above principles first.

Step 7:  Size Your Kneewall for Head Space and Vent Space

You’re probably thinking, what’s a kneewall? It’s actually the front vertical wall of the greenhouse that supports the lower vent system. The kneewall is an important component to consider because it determines the height at the front of the greenhouse (which is important for the ergonomics of a space for tall guys like me) and it also represents the amount of vertical space for snow to accumulate as it slides off 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 something to consider. Typically, I make sure that the kneewall and the vent system are tall enough so that I don’t hit my head on the trusses. A kneewall of 3-4 feet should be sufficient unless you live in a high snowfall area. If you live in that type of environment, you may need to think about making the kneewall taller or clearing the accumulated snow on a regular basis.

Step 8: Ventilate for Airflow and Plant Health

The Groundswell Network Society Community Greenhouse in Invermere, BC.

In my experience, ventilation is the one design detail almost every greenhouse gets wrong. The reason designers get it wrong is because it’s really hard to get enough air through a greenhouse. Under-ventilated and overheated greenhouses can lead to increased plant pathogens, heat stress, and crop failure. Here are some general guidelines you can follow to ensure your space is properly ventilated:

• Have high and low vents. If possible, make your high vents larger than your low vents, as air expands as it heats up. This will improve the “thermosiphon” effect of air moving through the space.
• Install seasonal cross vents that can be opened during the height of summer. Cross drafts can do wonders for cooling a space. I typically specify double doors or an inexpensive plug (an oversized non-hinged door) that can be pulled open to let in more air.
• If you still don’t have enough air flow, add in extraction fans. Or place some floor fans to “exercise” the plants to keep any fungal issues at bay.

I can’t stress the need for ventilation enough. When you’re doing design, don’t under-ventilate your greenhouse space!

Step 9: Choose Glazing for Transmissivity and Thermal Resistance

5 wall polycarbonate glazing from Co-Ex Corp.

The south-facing glazing surface is your primary solar collection unit. It’s the area that lets in the sun, heats up your space, and allows your plants to flourish.

It’s also the weakest link in the whole assembly.

There’s an interesting but unfortunate relationship between transmissivity (how much light comes through) and R value (thermal resistance). As the R value goes up (less heat is lost), transmissivity goes down (less light gets through).  If transmissivity drops below 70%, plants don’t get enough light, become “leggy”, don’t reach their full potential and may even die.

For materials, I like polycarbonate, 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 the two main guidelines to follow for choosing a suitable glazing material:

• Chose the highest R value glazing with a transmissivity of at least 70% or higher.
• Ensure that the glazing is structurally able to withstand the snow load in your bioregion.

Ready for part 3?

In the next article in this series, I’ll go over artificial lighting, insulation, and thermal mass.

Here it is – Part 3 on Designing Your Own Passive Solar Greenhouse

Interested in learning more? Check out our Advanced Passive Solar Greenhouse Design On-Demand Course!