Summer Greenhouse Update, Plus How We’d Design the Next One!
Our How we designed our Solar Greenhouse blog is one of the most read blogs on our website, so I thought it was time for an update! This post will cover the benefits of our greenhouse, the current improvements I’m working on as well as what I would do differently for the next version. There’s also a video so you can see the greenhouse in action!
Features I Love
Overall the greenhouse has been a great addition to the garden. It has meant ripe tomatoes from June until November, early greens in late winter, a beautiful space to work in, and a much needed boost to our annual crop. The elements that I love about this greenhouse include:
Its ability to capture and hold heat: This building captures enormous amounts of heat and stores a lot of it. Having said this, with a subterranean heating system, it would perform even better. We will be writing more about these in the near future.
Its ability to shed snow and protect from hail: Living in the world’s most extreme hail belt presents challenges. Growing in a greenhouse means that we always have a yield regardless of hail, cool temperatures and extreme wind.
Ease of watering of the wicking beds: Unfortunately, our greenhouse is built on a concrete pad which means that we are stuck growing in raised beds. To make watering these beds easy, we decided to experiment with wicking beds. The beds need to be watered about once per week and we have noticed that they benefit from an occasional drying which lets the soil pores open up to let air in.
While these beds are great, the soil sure takes a beating because the plants in our greenhouse grow for three seasons instead of one and they don’t slow down growth at night because it does not go below 10 degrees C. In order to keep things healthy, prevent disease, and prevent nutrient deficiency, we are constantly having to add worm castings, base fertilizer mix and compost tea all season long.
We have been experimenting with putting red wriggler worms into the beds which keeps things aerobic, but it is hard to keep on top of feeding all the beds all the time. Instead we have started experimenting with a commercial worm composter in our garage which will keep the castings coming in copious amounts all season long.
The weakest thermal link in any greenhouse after infiltration losses is always going to be glazing. This is because as the glazing insulative value (R value) increases, the amount of light transmitted (transmissivity) decreases. Plants need more than 70% of the sunlight to penetrate the glazed surface in order to thrive, which places a considerable limit on how insulative the glazing can be. The good news is that on sunny days, high quality glazing will let in more energy than it loses. Therefore the main design constraint for greenhouses is how to manage heat loss at night, which is where thermal curtains come into play. Thermal curtains are curtains which can be closed to cover the glazing and have been designed to minimize radiative, convective and conductive heat loss. For greenhouses, they need to be mould resistant and easy to deploy. There is a whole spectrum of thermal curtains possibilities, from high-end automated systems from greenhouse manufacturers to what we have done, which is cheap, practical and works like a hot dam!
There are three things that we want to accomplish with these thermal curtains:
- Reduce internal radiation losses
- Reduce convective losses
- Reduce conductive losses
Radiation is lost through the glazing as it is transparent to radiation (letting sunlight in during the day also means that heat can escape). The best way to stop radiation is with a reflective surface like tin foil or those NASA space blankets. If you don’t have access to either of those resources, a material that is opaque such as an insulated construction tarp will work great!
Energy is transferred when air moves in vertical cells called convection cells. As air heats up, it gets lighter and moves higher in the building, and as it cools down, it sinks. At night, a series of convection cells start at the back of the greenhouse moving the air up, at which point the air will cool along the front (glazing) of the greenhouse and sink onto the growing beds. It is more important to seal the top and bottom of the glazing with a thermal curtain than the sides because convection currents move vertically not laterally.
Conduction between air molecules is actually what drives convection, but there is conduction that occurs in addition. This will be reduced by the use of a thermal curtain.
The glazing on our greenhouse is approximately R 2 which is dramatically less than the walls at R 20. The construction tarp that we are going to use will have an R value of about 2 as well, which seems puny, but by using the tarp we are doubling the R value of the weakest link which is a huge achievement.
Not only will this keep the temperature in the greenhouse much more reasonable at night, it will also allow the rocket mass heater to keep the space warmer longer which means that we need to fire it much less and thus use less fuel and labour.
This year the auto louvres will be going in. These devices are levers that open and close with heat activated cylinders. The cylinders are filled with an expanding wax that apply a torque to a mechanism which can open and close louvres. This will help us to maintain a more consistent temperature year round.
In addition to the louvres, we will likely be adding additional ventilation to the structure as we have found the greenhouse does not have enough. This is something that is often overlooked in greenhouse design and construction. It seems you can not over do ventilation.
See the Solar Vent Works website for more information
The interior of the greenhouse is light grey. To reduce overheating in the middle of summer we installed an overhang on the greenhouse which helps to keep the heat down but also reduces the light entering the space to the detriment of the plants. To increase light and reflection we will be painting all of the interior surfaces white this year with a low VOC bathroom latex paint. We will report how it affects plant growth later this year.
Changes to the design for the next greenhouse
The best part of this greenhouse was all of the knowledge that I have gained and continue to gain by using it. Over the past three years, I have created quite a list of things that I would like to see done differently in the next greenhouse. Some of the changes were things that I wanted to do from the start but the site did not allow for it.
While the shape of the existing greenhouse maximizes vertical space, it is a complicated shape and there are some inherent disadvantages. The future greenhouse will have a simple shed-style roof slanting to the south but with a much lower angle to allow for a knee wall which will push the ventilation wall further up. This will eliminate the back roof and the main glulam beam going through the middle of the greenhouse. Having the vent wall so low to the ground has impaired the ventilation and made it tough to build raised beds in the greenhouse. This is especially bad in our greenhouse as we are not planted in the ground but on top of a concrete foundation. It turns out that a shallower angle on the glazing will have a very small impact on solar gain making the increase in usability inside and better ventilation worth while.
I under-designed the ventilation in this greenhouse, which is one of the most common mistakes people make. In the future design I will add 10% -20% more ventilation on the top louvres than I have on the bottom louvres as air expands when it rises. This should eliminate the need for fans.
An air lock or vestibule is one of the most important and inexpensive features on all northern temperature controlled structures. It is a crying shame that every home in Canada does not have a vestibule to enter the home. A vestibule is an intermediate room at the entrance of a building which forces people to pass through two doors. This prevents large amount of warm air leaving the building which saves energy and ultimately labour. In a greenhouse setting, this will keep your plants happier, use less fuel to heat and provide additional storage for tools.
In our greenhouse I used wicking beds which was likely the best option we could have pursued since our greenhouse is built on a concrete pad. While the ease of watering has been nice with these beds, the soil burns out more quickly and water stress (both too much and too little water) is still a major concern. Water stress and heat stress (from lack of ventilation) are the main causes to plant disease in our greenhouse. The best place to grow a plant is in the ground connected to the earth! The next greenhouse will be built into the earth with a frost wall so that we don’t have to garden in containers.
Frost Wall Planted Greenhouse:
As mentioned above, having a greenhouse with an insulated footing provides countless benefits to the grower. Greenhouses that are built into the ground are biologically and thermally easier to manage.
- It is possible to use a subterranean heating systems.
- Frost walls are designed to slow down the migration of frost into the envelope.
- Soil temperatures will remain higher all season long.
- The earth helps to regulated air temperature and humidity, both important conditions inside a greenhouse. Well controlled heat and humidity help to prevent heat stress and the diseases that result from it.
Plant pests and pathogens are always around; you can’t get rid of them. The conventional approach to them is to kill everything. I won’t go into why that does not work because you probably already know. The permaculture approach is improve plant health so that the plant has the ability to fight off these problems and grow lots of flowers so that the beneficial insects come along with the pests. Keeping the plants healthy is done by building soil and ensuring that the soil food web has the nutrients and minerals that it needs. One of the main sources of minerals is subsoil which is not available to plants growing in containers. For this reason a greenhouse that is planted in the ground will produce better, healthier, more resilient crops.