When I was designing my greenhouse, one of the questions I had was: how to build a passive solar greenhouse?
The proper insulation and glazing and adding ways to capture and hold onto solar energy will yield a passive solar greenhouse that doesn’t cost too much more to build compared to a traditional greenhouse.
There was even one “problem” that turned into a benefit that I hadn’t thought about.
Adding Knee Walls
The feature that will provide the best benefit in your greenhouse is insulation on parts of the walls that don’t need to be glazed.
In my case I mainly grow in pots on a bench, so I really don’t need light coming in at the bottom of the walls of the greenhouse.
And typically cold air settles at the lowest point so towards the bottom of the greenhouse walls will be the coldest.
The solution here is to build insulated knee walls. And essentially these are built similar to a house wall. Basically they look like a sandwich where wood cladding is the bread and insulation of some time is the peanut butter and jelly.
Here is a diagram showing the construction:
I originally built the greenhouse with these knee walls but held off insulating the inside. Once I did that I noticed the minimum temperature in winter time went up. Less cold air is coming into the greenhouse from the bottom and less heat is escaping through the knee walls (although heat typically rises, so most of the heat is lost through the roof).
And if you can afford it, go with the thickest knee walls that you can. I used 2×4 and 4×4 cedar for the wall framing and so my wall thickness is 3.5” (90mm). Using 2×6 and 6×6 lumber would have been better as that would have given me a 5.5” (140mm) thick wall, which would have meant more insulation. However it would have increased the cost of my greenhouse, which already was expensive to build.
It’s not that convenient to add knee walls to an existing greenhouse, especially if you have glass or polycarbonate panels going all the way to the bottom. They can however be retrofitted by building up a knee wall on both sides of the existing wall, adding insulation and sealing well around the glass or polycarbonate with a flexible sealer.
Adding A Water Barrel (or two)
Where it is legally allowed, you should collect rain water from your greenhouse roof and then use the water to water your plants.
Rainwater is much better for plants. It doesn’t contain the chemicals that municipalities have to put in the water to make it safe to drink. And rainwater is softer and is usually warmer than tap water.
However in wintertime there is another use for the barrel of rainwater that you might not have thought about.
If you can place the barrel inside the greenhouse, you provide a great way to capture the solar heat of the day and store it to be released at nighttime. It works best if your barrels are a dark colour. You can either buy them that way or paint them.
The compromise here is that you have to take up valuable growing space. However you might be able to use the barrels as plant stands to elevate your plants. Larger greenhouses have water walls where they have literally stacked up rain barrels along the back wall of the greenhouse (usually the north wall since that wall typically is solid with no glazing).
Insulating The North Wall
In many passive solar greenhouse designs you’ll notice that there isn’t glazing everywhere. Usually on the north side of the greenhouse is where you will find a solid wall.
After all there is no point in putting glazing where there is no direct sun coming in, which happens on the north side (assuming you live in the northern hemisphere of course).
In my case the north wall is actually the wall of my garden shed underneath my second floor deck.
However I also have a section of ceiling underneath the overhang of my deck which presented another opportunity to insulate to keep the heat in and the cold out.
Adding A Heat Absorbing Floor
The floor of the greenhouse can be used as a heat sink to absorb the passive solar energy of the day and release it at night.
However it has to be the right material.
The best material for a greenhouse floor is some kind of stone or concrete. The thermal mass of the stone will absorb and then release some of the solar energy created during the day.
The thicker the better of course. You can build up an existing floor with large concrete pavers like I did or if you are building a new greenhouse have a concrete pad poured.
Dirt and wood floors don’t provide much solar gain, so avoid those unless your budget doesn’t allow for a concrete or stone floor. But always keep it as an option in the future when spare funds allow. It will make a significant difference.
Glazing The Roof Properly
The trick is to collect as much solar passive energy during the day to heat up the greenhouse so that heat can help keep the greenhouse warmer longer into the beginning of the night.
Glass can generate the best solar gain as it is the most transparent of all glazing materials. However glass also isn’t a good insulator and so will allow heat to escape at nighttime. Double-pane or even triple-pane glass is better, but unless you can find a free source like I did, it can be quite expensive. And multi-pane glass cannot be cut to size after it is manufactured, so you may need to custom order it.
The alternative to glass is twin-wall polycarbonate. This is a panel that can easily be cut to size, has a better R-value than single pane glass and is lighter, meaning you don’t need that sturdy of a roof frame as with glass. It also is safer if you have a lot of hail storms as it will absorb direct hits very well.
Ideally though in a very cold climate, it helps to provide insulation at night time. There are special curtains that you can lower over the glazing (either from the outside or inside) that helps insulate at night time. During the daytime you open the curtains and allow the sun in to warm up the greenhouse again. There are even automatically controlled ones that will eliminate you having to go out and lower and raise the curtains every day.
Adding A Temperature Sensor
This doesn’t actually provide passive heating to your greenhouse but will allow you to monitor any improvements you make to see if they are working.
I have the remote sensor hanging in my greenhouse where it doesn’t get direct sunlight (that is key otherwise you will get some abnormal readings when the sun is shining on it). And the receiver is in my kitchen where I can monitor the temperature easily without having to go outside.
I especially check it in the early evening to see if temperatures are getting too low. Luckily our winters are not cold so we have very few nights where temperatures go below freezing. If it will get too cold I can always add an electric heater in the greenhouse to at least keep the temperature above freezing at nighttime.
The sensor also has a minimum and maximum record temperature feature, so I can look back to see how low the temperature went or how high.
The sensor also comes in handy in summer to monitor the temperature to make sure that the greenhouse doesn’t get too hot. I have automatic roof vents and a solar-powered ventilation system installed that help to keep the temperatures reasonable. But vents can fail to open or the ventilation system could fail, so checking daily once the greenhouse heats up could catch an issue before it becomes a problem.
Running A Dryer Vent Through A Passive Greenhouse: A Problem That Provided An Unexpected Benefit
Warning: never vent a gas dryer into an enclosed space!
I happen to have my laundry room on the inside of a shared wall with my lean-to style greenhouse.
During the build of the greenhouse I had to find a way to route my clothes dryer’s vent hose through a wall to the outside. I had to use a few elbows to get it around the door to my shed and then outdoors. Not ideal but sometimes solutions are not perfect to a problem.
However what seemed to be a problem also provided a solution I at first didn’t think of. The residual heat as the air from the dryer goes through the metal vent pipe actually provides some additional heat to the greenhouse.
We can see a slight increase (maybe 1 to 2 degrees) when the dryer is running. It’s almost like a heat exchanger.
So wouldn’t it work better to simply direct the warm air into the greenhouse for even more heating?
That would be a big mistake! Don’t be tempted to do this. With a gas dryer this is extremely dangerous as carbon monoxide gas will accumulate in the greenhouse. Not good for your plants and extremely dangerous for anyone working in the greenhouse while the dryer is running.
But even with an electric dryer like I have, this is a bad idea. All that moist air going into the greenhouse will cause problems. You will end up with mildew and other moisture problems if you do this.
So always, always have the vent on the outside.
There are many ways to capture greenhouse passive heating energy and retain it. It may require design decisions at the time of planning or some retrofitting after having lived with a greenhouse for a while. But in the end any improvements you can make will help to keep temperatures in your greenhouse at a more consistent level without large fluctuations.
Also be sure to check out the video that inspired this post.
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Tranquil Garden Urban Homestead, Victoria, BC