7 Useful Features You Need in a Passive Solar Greenhouse
Want to save on greenhouse heating costs? Wondering how to build a passive solar greenhouse? With these 7 features you can keep the heat in and cold out.
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.
I’ve included 7 features in my passive solar greenhouse attached to my house that helps do this. Some features had to be built into the greenhouse when I designed it but others can be retrofitted in an existing greenhouse with a bit of work.
And there was even one “problem” that turned into a benefit that I hadn’t thought about.
Insulated Greenhouse 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.
Since the most effective insulation is opaque, it will block light which is not good for your plants or for heat gain during the day when the sun is shining. So keeping insulation at the bottom of the greenhouse is key.
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. My one raised bed is located away from the walls so the plants I grow in it get enough light.
The solution here is to build insulated knee walls.
Essentially these are built similar to a house wall. Basically they look like a sandwich where wood cladding is the bread and insulation 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 until later. Once I insulated the walls and added the inside siding, 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).
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 easy 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.
Greenhouse Roof Rain Collection
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 solar heat sink 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).
Insulated Greenhouse North Wall
In many passive solar greenhouse designs you’ll notice that the whole greenhouse is not glazed. In other words not every wall has windows that let in light.
Usually the north side of the greenhouse is where you will find a solid wall (at least here in the Northern Hemisphere – it would be the south side in the Southern Hemisphere).
After all there is no point in putting glazing where there is no direct sun coming in. Usually the north side of the greenhouse is where you will find a solid wall (at least here in the Northern Hemisphere – it would be the south side in the Southern Hemisphere).
In my case the north wall is actually the wall of my garden shed underneath my second floor deck so I wasn’t able to glaze it anyways. Instead I’ve insulated the floor of the deck overhang above.
This is one less area where I lose heat especially with the cold winds from the north we often