Designing Your Own Passive Solar Greenhouse, Part 3

Welcome to Part 3 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 and Part 2 here to see Steps 1 through 9. In this newest entry, we’ll be talking about lighting, insulation, thermal mass, and subterranean heating/cooling systems.

Step 10: Do You Need Artificial Lighting?

If you live in a cold and cloudy environment or in the extreme north where daylight is very short, you might consider adding artificial light to supplement your greenhouse needs. Generally, this is a situation I try to avoid because lighting costs can add up quickly. My advice is first to explore whether your greenhouse can operate without artificial lighting. If not, the next step would be to quantify how much it’s going to cost per month and per year to make sure the solution makes financial sense.

Here are several scenarios where artificial lighting may not be required:

• The greenhouse only operates three seasons out of the year and is used as a hangout spot during the dark winter season. I don’t consider this a bad thing, as in my climate this means I’m still able to get 200-250 growing days a year instead of the measly 100 days I would get outside.
• The greenhouse is growing tropical plants that live in the understory of rainforests and can tolerate low light conditions.
• You’re simply growing microgreens, which don’t require additional light in most scenarios.

If none of these situations apply to you, then you might need to design your lighting apparatus. Do so carefully. Here’s a light calculator I built into the passive greenhouse design tool so you can easily estimate power consumption and costs:

The calculator takes into account various technologies with their respective power draw per square foot, their different pros and cons, and the ability to estimate the number of hours needed to operate per day and per month. Once you enter in your lit area and the cost of power, the calculator can estimate the operating cost of lighting in your greenhouse.

[By the way, the passive solar greenhouse design tool described in this article series is included in our updated and enhanced Passive Solar Greenhouse Design Course, starting soon in the spring of 2022!

If you want to calculate lighting costs manually, simply estimate the area you need lit, the wattage of your lighting technology, and how many hours you need to light your greenhouse per month.

• Cost = cost/kWhr* Power consumption (kW)* hours of operation/month

Use this formula or the design tool to make an informed decision on whether artificial lighting is right for you.

Step 11: Calculate Heat Loss and Insulation Value

Correctly placed insulation is what makes a passive solar greenhouse different than a conventional gable-style greenhouse. Everyone knows that insulation is important, but interestingly it has a diminishing return in passive solar greenhouses.

Since the southern glazing surface (south-facing glass) has such a low R-value, there are massive diminishing returns in adding additional insulation to the north, east, and west walls.  Adding more beyond a certain point has no effect on mitigating heat loss – you’re basically spending more money and getting no additional benefit.

That’s why I’ve built another easy calculator to help you determine the optimal insulation value for each wall and glazing surface:

Just enter your greenhouse’s surface areas and proposed R-values, and the tool will generate a pie chart that shows you a breakdown of heat loss for the space. It’s super simple – you don’t need to understand what a BTU is, just how much of the pie makes up each surface. You can then go back and try swapping in different R-values to find an optimal insulation design that works for you (for me and my area, I started with R-values of 20 for walls, 15 for the foundation, and 1.8 for glazing)!

You may note that in the image above that 58% of the heat loss occurs through the glazing while only 10%, 6%, 15%, and 11% occur through the footing, infiltration, roof, and walls, respectively. That means doubling the R value of the walls, which only contribute to 6% of total heat loss, is financially difficult to justify. Instead, we should take another approach. 

If most of the heat loss occurs through the glazing at night, one solution would be to add a thermal curtain. A thermal curtain with an R value of as little as 2 can reduce the total building heat loss by 25%, and for a fraction of the cost of doubling the R-value of the walls!

Step 12: Add Thermal Mass for Heating and Cooling

Thermal mass is critical for any passive solar building. It helps cool the structure in the summer and keeps it warm at night and in the winter. There are several options you can choose from including but not limited to rock, concrete, cob, water, water and glycol, metal; you can use any cheap material capable of storing a lot of heat in a small space. We chose to use cob in our first greenhouse, with a rocket mass heater, but there are an endless number of elements that could work for you.

To help design for thermal mass, we once again can turn to our handy passive solar greenhouse design tool. The calculation is based on the glazing surface area and the type of material you choose to use:

Different materials have different pros and cons – I usually categorize them into solid and non-solid materials:

• Non-solid materials: Water can freeze, which can make it a challenge to use, but it has a high thermal capacitance (it holds a lot of heat energy) and it’s cheap, making it an attractive option.
• Solid materials: Solid materials hold roughly four times less energy than water, but they don’t freeze and can be easily set up inside the greenhouse without any hassle.

Generally speaking, more thermal mass is better than less, but there’s a point of diminishing returns if the mass is eating into the greenhouse’s production area.

Step 13: Include a Climate Battery for Heat Transfer and Storage

Climate batteries, or subterranean heating and cooling systems (SHCS), are heat storage technologies that hold heat underground for later use. These systems capture hot air at the top of greenhouses and pump it below grade through a pipe network. Hot air in a greenhouse usually has a high humidity, which means it can hold a lot of thermal energy.  As the warm humid air is pumped into the cold ground, condensation occurs and releases heat into the ground. This water then infiltrates through the SHCS pipe network, irrigating the soil while warming it in the process. SHCS stores surplus energy during the day when temperatures rise above 21 C (72F) by turning on a fan. The fan will also turn to extract thermal energy from the ground if the temperature in the greenhouse goes below 11C (~52F).

In order to properly design a SHCS, you need to determine the right duct and fan sizes to ensure that you’re not using more energy to push the air than you are storing in the ground. Otherwise, it would be cheaper simply to add heat inside the greenhouse with direct resistance electric elements. I size my SHCS manually with a “ductulator”, a calculator for mechanical engineers that do a lot of duct design work. Since not everyone is a nerd like me, I’ve taken the liberty of building in a SHCS design tool so even non-engineers can do this for themselves:

The calculator allows you to choose duct diameters and through a little iteration, a fan size as well. I highly recommend the use of variable-speed fans because they increase the number of options you have with the system. After everything is installed, you can play around with the fan speed to find the optimal air flow for your greenhouse. In general, you want your SHCS to change the air volume over between two to six times an hour, with the ability to increase or decrease this rate if required.

For Large Greenhouses: Thermal Dynamic Modelling

The tools and processes I’ve provided in this article series will work for any greenhouse size. That said, construction costs and design challenges for greenhouses bigger than 1,500 square feet go up substantially. Buildings of this size are generally when I get involved as a consultant to perform what’s called thermal dynamic modelling. This is when additional optimization and design may be necessary.

Did you enjoy this series?

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