Why Would You Want a Solar Greenhouse?
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.
Resources:
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