Well, it fell below freezing last night. Our overnight low was 30° F. My greenhouse did quite well considering, I haven’t caulked all the air leaks, yet. The low temperature in the greenhouse was 40°. The greenhouse has mostly single paned glazing. On the plus side I put about 140 gallons of water in there, which is almost one gallon per square foot of floor space. Considering I am in the Northwest, I think the greenhouse will perform just fine. I will be fixing the leaks and perhaps increasing the thermal storage, just to see how well it can do.
The main issue in our neck of the woods is getting enough light in the Winter months, so I will not decrease the glazing to increase the heat retention. Light seems to be good so far. We have plenty of Mustards and Swiss Chard growing like crazy.
This is the kind of innovation I like to see. A greenhouse that is insulated by soap bubbles. This greenhouse is in the UK, but the original one is in Canada. Who said Canadians never invent anything? See more on the Canadian greenhouse here.
For small greenhouses, my favorite insulation on the south facing glazing is foil faced rigid insulation panels. You can design it so when they open up, they reflect more light into the greenhouse.
We have been in our present house for two and a half years now, and this Summer we finally got our greenhouse built. We built our greenhouse on the south side of our house, for the insulation and because it’s one of the best spots in our garden for sunshine. It really is not a solar greenhouse, but I did incorporate some solar principles into it. I will post on how it performs and what changes I make, this Winter.
The south wall of the greenhouse is tilted to 60% to maximize Winter sunlight. I wanted to use old windows to make the greenhouse, but because of the angle, I thought they should be tempered glass for safety. So I found some used patio doors; some for cheap, some for free. Because we live in a fairly mild climate, I did not bother to double glaze. The ends have old windows and storm doors. The roof is coroplast. The greenhouse is about 7 feet by 22 feet. I have two 50 gallon barrels of water for thermal mass. That is a little low, so I may be adding more. There are currently a lot of air leaks in our greenhouse, which I may try to stop up as Winter progresses.
I am cheating a bit in terms of solar design. There are two windows from the house opening into the greenhouse and the basement bathroom fan blows into the greenhouse. In the Winter, I plan to only open the windows when it is too hot in the greenhouse.
I will soon be buying a minimum/maximum thermometer, for the greenhouse, so I can give you some real data on the performance. I look forward to sharing with you how it goes, and I really look forward to having fresh Winter veggies
When you are building a solar greenhouse, one of the important considerations is insulation. Many people have questions about greenhouse insulation. What to use, how much, and where? There are also special considerations when integrating insulation into a greenhouse, that are different from other buildings.
Ideally, especially in colder climates, only the south wall of a solar greenhouse should be glazed. The other three walls and the ceiling should be insulated. The type of insulation and the amount can be the same as for regular buildings. Check with your local building code for R-values for attic and wall insulation. If the north wall of the greenhouse is cement, which is good for thermal storage, insulate it with rigid insulation panels on the outside of the wall. If you have a perimeter foundation, insulate it with rigid panels, on the outside as well. The foundation insulation should go below the frost line, even if the footing does not. Extending the insulation a few inches below the frost line keeps the earth beneath the greenhouse significantly warmer.
One important difference between greenhouses and other buildings is the amount of humidity inside. For this reason it is very important to have a good vapor barrier between the insulation and the inside of the greenhouse. Otherwise you will have problems with mildew and rot. It is also important to insulate the glazing at night. Solar greenhouses should have double glazing, as well as insulation to prevent heat loss.
You can use insulating curtains or shutters. The inside of the shutters or curtains should be reflective to increase their heat retaining capacity. The rigid foam boards with foil on one side make great shutters. For a curtain, aluminized mylar attached to thinsulate works very well. Make sure your curtains are very durable, as they will get a lot of use, especially if they are on the outside of the greenhouse. Also make sure you have a tight seal between the greenhouse and the shutters or curtains. A lot of cold air can escape through a small gap.
Getting the design of your solar greenhouse is not too difficult, and planning it well can give you a greenhouse that does not need supplemental heat through even the coldest Winters.
I wrote a post about solar greenhouses earlier. A theme I will be coming back to many times in this blog. In my first post I only talked about passive solar greenhouses. Passive solar technology is very simple, no moving parts. Active solar uses fans to move hot air or pumps to move hot water, and more complex storage systems. Usually, they cheat a bit by using electricity, but they can greatly improve efficiency.
A company in Isreal has become, I believe the first, to manufacture active solar greenhouse equipment for commercial greenhouses. This will be a big leap forward in the commercial food industry becoming more green. To learn more about them see this post over at the Green Prophet.
What’s the difference between a solar greenhouse and a regular one? Don’t they both rely on the sun? Well, yes, but a solar greenhouse stores the heat it collects from the sun during the day, to keep itself warm over night.
If you live where the Winters get too cold for growing vegetables, and you are serious about eating from your garden throughout the year, you have two choices, a solar greenhouse or a greenhouse with supplemental heaters. The latter can get costly and are not so good for the environment. We want more greenhouses, not more greenhouse gases.
So how is a solar greenhouse different from a regular one? They are designed to absorb lots of solar radiation without losing much heat. And they have a way of storing the heat.
Absorbing solar radiation
The majority of the glazing on the greenhouse should be oriented to face the Winter noonday sun. That means the south wall is tilted at an angle or the south facing roof comes almost to the ground. The angle of that glazing depends on your latitude. If you live at 45% latitude, the noon sun on December 21 is at about 22% above the horizon. On February 21 it is about 34% above the horizon. Typically, it is colder later in the Winter, so you would want to orient your glazing toward late January or February sun. You want the sun to hit your windows perpendicularly, therefore you might choose an angle of 60% (90%-30%=60%). Having done the math, I can now tell you that a good rule of thumb is to add 15% to your latitude, and that will give you the angle your greenhouse glazing should be tilted at.
Of course you want your greenhouse to be oriented so that most of the glazing is facing south, or near to it.
Another thing to consider is how much glazing you need. About one square foot of glazing for every square foot of floor space in your greenhouse is ideal.
Retaining Heat
There are three aspects to retaining heat in a solar greenhouse: glazing, insulation, and curtains.
Greenhouse Glazing
For a solar greenhouse you need double or triple glazing. If you are using glass, that can get heavy and expensive. If you are using glass, I suggest low-e double glazed units. That will give you the best heat retention, without going to three layers. Most modern greenhouses are glazed with plastics. In the past decade, there have been great improvements in weatherability and longevity of plastic greenhouse glazings. If you aren’t dead set against using plastics they will give you the best bang for your buck. I recommend triple walled polycorbonate. Make sure your glazing is designed for greenhouses, otherwise it may not have the UV resistance to give it a long life.
Next you have to make sure that your glazing is installed well. Anybody who has lived in a house with leaky windows knows how much heat they lose. One disadvantage of some plastics is their tendency to expand and contract with temperature changes. Good mounting technique can deal with this problem.
Insulation
The north wall of your greenhouse would not absorb much light, so it should not be glazed. Instead, it should be well insulated. Insulate it to the amount recommended in your area for house walls. In many cases, you will want to insulate, instead of glaze, your east and west walls, as well. Insulate the foundation walls on the outside, so that the cement in the foundations can act as part of the heat storage. Some people will use earth berms to insulate. You can build you greenhouse into the side of a south facing slope. If you are on relatively flat ground, the greenhouse can be partially underground. There are some good plans for these type of solar greenhouses. Besides insulating the walls, make sure you weather strip the doors and vents.
Curtains
Insulating curtains on the inside or outside of the glazing can greatly reduce night time heat loss. Most people only bother on the very coldest night. If you have a small greenhouse, even throwing a tarp or blanket over it will help. Well designed insulating curtains are worth it if you live in a long cold Winter and you want a productive greenhouse.
Storing Heat
You need a large quantity of material that can absorb the heat from the sun and slowly release it during the night. Probably the most common heat storage method in greenhouses is barrels full of water. They work well for seedling benches, too. Just make sure the sun hits them full on, most of the day. Rocks, bricks and tiles can also be used. You need a lot of them, though. In cold climates, you will need 3 gallons or more of water to absorb the heat that enters through each square foot of glazing. Approximately three square feet of four-inch thick brick or cinder block wall is required for each square foot of south-facing glass. If you have a ten by twenty foot greenhouse with two hundred square feet of glazing, you need about six hundred gallons of water or six hundred square feet of four inch thick bricks. I’d rather go with the twelve oil drums full of water, than all of those bricks. If you don’t live in the coldest climate, of course you can get away with less.
Check out this short clip of a solar greenhouse:
You won’t find a greenhouse kit to build a solar greenhouse. You will need a good solar greenhouse plan, that explains modifications, depending on location. Or learn more and design your solar greenhouse yourself.
