When we started our adventure to build (well, more like rebuild) a 4 season greenhouse back in 2014, we were sure that heating would play the most important role in keeping the greenhouse from freezing in our cold zone 3 winters.
Guess what, it is not!
Are you surprised? Well, we did not expect that either. But let’s start at the beginning.
In our first winter at this home, the old garage was a freezing cold place. I tried to do some work in there, but it was too cold. In fact, many days it was colder inside than it actually was outside in the sun.
In the summer we got busy turning that old garage into a food production place. You can read all about it here. The greenhouse we are talking about is an alternative to a passive solar greenhouse since we were working with an existing building rather than starting from zero.
Summer was too short to finish up the whole project, so winter caught me still working on the rocket stove heater. I was desperate to get it done, thinking that it was priority number one.
While spending hours tinkering and trying to get the heater going it dawned on me that maybe I also should finish the insulation. We used R20 insulation, and I was almost done anyway but completing it made a huge difference. Which brings me to the first conclusion:
First Thing to Keep a Greenhouse From Freezing in the Winter is Insulation
Most heat loss in a room happens by means of conduction through the windows and through the poorly insulated walls/ceiling. The same process that heats the inside of a frying pan when the flame is on the outside. All materials conduct heat (or cold) by conduction, that is why we use insulation inside the walls. Insulation has a very low thermal transfer rate, it conducts very little heat/cold by conduction.
As soon as I had finished insulating the ceiling I spent less time heating up the room. And since we just want to keep the temperature above freezing we go many days and nights without heating at all. There are easily 12 degrees Celsius difference to the outside temperature at night.
If you want to keep a greenhouse from freezing, insulate, insulate, insulate! But insulation alone would not have such a big effect, there is a second thing.
Second Thing to Keep a Greenhouse From Freezing in the Winter is Thermal Mass
A passive solar greenhouse for winter growing in cold climate always works with thermal mass. Since we are working with an old building, I knew there was no way to install that kind of thermal mass.
We have a water collection system in place that holds 2000 liters (528 US gallons) of water. In the fall that fact was stressful for us. What if the water freezes? Not to imagine the mess. Well, it never froze, in fact, it kept the greenhouse from freezing.
We have had some frost in the greenhouse. It got down to -5 °C (23°F). On a very cold night, we made an interesting observation. We had two buckets of water sitting on the floor, one next to the water tanks and the other one next to the stove. The stove was being heated for a few hours in the evening.
Guess which bucket with water was solid frozen in the morning?
The one which was further away from the thermal mass next to the now cold stove was frozen, the bucked with water next to the water tank was not.
From this experience, we can conclude, that the water storage was radiating heat, and even though it was not much, it kept the surrounding area warmer than the rest of the space. It was more useful than the stove which cools down too quickly.
Sure, in zone 3 climate, with temperatures going down to -40, we do need a heating system in place. But with good insulation and lots of thermal mass, it turns out to be more like just backup heating that is needed for winter vegetable growing. Besides, most winter vegetables do not mind at all if it goes a bit below the freezing point.
This puts things into a different perspective for us. We are working on adjusting to these important facts.
Update 5 Years later
Since our humble beginnings, we have added even more insulation to the room. Here an example of a sunny day in the indoor garden room. It’s -13C(7F) outdoors versus 25C(77F) indoors. A well-insulated room is key.
A passive solar greenhouse usually works with thermal mass in the floor/ground. A warm floor makes a huge difference to the room temperature. Our old garage had a very cold concrete floor. To change that we raised the floor a few feet. The building was high enough to do that.
After raising the floor we also enclosed the area, making the growing room smaller and cozier.
The two water totes have since been moved to the outside for harvesting rainwater for the garden. The smaller room with the raised floor had no room for them anymore. And we don’t miss them. Since we mostly grow in hydroponics, there is lots of water in the room and an abundance of plants that them self also work as thermal mass.
We supplement heat during the night, keeping the room above 10C(50F). During most days the temperature rises well above 10C without any additional heat.
If you have been following us you know that we gave up on the rocket stove very quickly. Heating is only needed during the night and blizzard conditions, and it is no fun to be in the greenhouse during those times heating the stove. I’m still working on a perfect solution, but with the current set up we just use electric heater.
If you have space and means to build a greenhouse for winter growing, a passive solar greenhouse is a nice structure to have.
A great video from a greenhouse in the rocky mountains in Canada is: ‘Heating Your Glasshouse. Passive solar greenhouse: A way to produce more local food and use less energy to do it’.
We had the privilege to visit that greenhouse, you can read about it here.
However, if you just want to grow some edible plants for personal use, a grow room like ours in the back of the garage is a great way to do it. It was relatively easy to build and gives us a great reward.
All you need are some big windows, insulation and preferably a thermal floor.
Inspiring books for winter gardening: