Greenhouses

A greenhouse is a weather protecting structure for your garden plants. It could be as simple as a cold frame (a low wooden box covered with glass) to a tube frame, plastic covered high tunnel, to a stud framed glass house. With a greenhouse it is easy to extend your growing season a month earlier in spring, and a month later in fall. Depending on your greenhouse, and your crops, you can extend your harvest into winter.


Uses

• Seed starter greenhouse
• Forth season production
• Flower / cash production

Important concepts

• base temperature
• optimum temperature
• temperature integration
• photo period
• irradiance
• poly sun degradation

Design considerations

The ground warms and cools with the surrounding temperatures. As we cover the ground with the greenhouse, the earth under the house can offer stored heat. Utilize this heat battery to your advantage. In sunny areas, the solar input of energy will be more significant than earth heat radiation. If you are using glass for your glazing, then you will want to orient the angle of the glass to the perpendicular to the elevation of the sun during the winter. This angle is your latitude at the winter solstice. You want the angle of incidence to be low so that most of the light will completely penetrate the glass, and not loose some of it through reflection.

Using a GPS, property map or Google Earth, determine the latitude of your location. Add between 10 and 20 degrees to your site latitude to determine the orientation of your greenhouse glazing. For instance, if your latitude is 35 degrees north, your glazing should be angled at a 45 to 55 degree angle.


Heat storage / sources

• Water stored inside the greenhouse, in black containers to absorb as much daytime heat as possible. This heat would then be radiated at night to moderate temperatures. One friend of ours has used 55 gallon metal drums full of water in the center of their greenhouse with planter boxes on top of the drums. Another has water containers on the backside of the greenhouse. Another greenhouse we reviewed had a small pool of water within and under the floor.
• Rock mass as a thermal heat sink. Another greenhouse we have reviewed has a rock wall on the north side of the lean-to greenhouse. Another example is a solarium made with massive amounts of rock as thermal mass.
• Earth-air tubes are tubes buried deep in the ground and air passed through the tube to take the constant heat from the ground into the air and then into the greenhouse. Electricity is needed to move the air.
• Below ground radiant heat, with heat being added from a boiler or solar heater.
• Wood stove heater within the greenhouse.
• Greenhouse lowered into the surrounding ground, to encourage more earth-heat coupling.
• Cold sink trench within the greenhouse to hold coldest air down within the soil.

Resources

• Improving Greenhouse Production Efficiency (pdf)
• Greenhouse Information (pdf)
• Planning and Building a Greenhouse (web)
• Solar panel optimum angle (web)
• Greenhouse structures (overview pdf)
• The Solar Greenhouse That's Right for You (web)