Greenhouses are very popular for extending the growing season, but heating them in cold climates is expensive. In contrast, pit greenhouses — greenhouses built into the earth — use free solar energy from the sun for heating. The temperature of the earth (usually around 55-58 degrees Fahrenheit) helps stabilize the temperature in the greenhouse.
One option for pit greenhouses is to use earthbag walls with about 2’ (60cm) of scoria on the outside for insulation. This design would be more energy efficient than uninsulated walls of earth and should maintain growing temperatures year-round. Watch over the greenhouse and provide adequate ventilation to avoid overheating.
Walipini
Underground greenhouse with geothermal heating system
Low cost greenhouse in Texas using hoops of “cattle panels”
Mike Oehler’s earth-sheltered greenhouse
I always thought that when they talk about the ground temperature they mean below the frost line.For instance here in Montana when they put in a Geo thermal heat pump heating system they bury the lines below the 4 foot level which is the frost line.At that depth they are utilizing the earths warmer temperatures as a starting point and only have to raise the temps 20 to 30 degrees to be comfortable.
That sounds right.
My latest blog post over at http://blog.greenhomebuilding.com/2010/10/earth-sheltered-solar-greenhouse.html reviews a book by Mike Oehler about his design for a pit greenhouse that can produce food year round in Idaho, near the Canadian border, with nothing but the sun and earth for heat. It would be easy to duplicate his concept with earthbags for walls.
I think this is a marvelous idea. I may try this on my own here in Texas.
Cody
I completely agree that this is a widespread misconception. However, since it is intimately involve in the potential success of any ground-coupled living structure, as always the individual design should take the site-specific details into account.
I strongly applaud your efforts to introduce this method of building to people who can actually afford to build a home in this method who might never be able to own their own home if they don’t adopt such a method. It is also fantastic for folks who might want to work a bit less hard, or who are saving for retirement, to put children through school, or so many other purposes that are precluded when being slaves to a mortgage.
However, putting a huge amount of work into a home which is not comfortable or, in some cases, even healthy without costly additions of energy is most unfortunate, when the alternatives can be much more simply done at low cost when planned for in advance.
Even those who gather wood for heating often are at a severe economic disadvantage–and the denuding of the forests in many areas has serious consequences. Thus, if we can make a dwelling that requires minimum additional heat that is a huge win for everyone.
In some areas, it might even be well worth adding sub-grade insulation where ground temperatures are lower than the comfort zone. While this might increase the cost of building initially, it could dramatically reduce the lifecycle costs of living in the home through lower energy costs over time.
Stable ground temperatures in the Continental U.S. vary between the low 70s (in extreme South Florida and a small bit of Arizona, IIRC) and the 30s in the extreme Northern Great Plains. It is simply untrue that there is any sort of uniform “55 to 58 degree” average for many places–although it is a common fallacy.
Furthermore, the temperatures near the surface vary widely depending upon season, mostly with various air temperatures and levels of insolation although it may also be influenced by other factors such as radioactive decay and the presence or absence of subsurface water flow.
Still, a pit greenhouse can work in nearly all U.S. locales as you have indicated, keeping temperatures above freezing.
There are various soil temperature maps available to serve as a rough guideline, but for any given location perhaps the best guide is the temperature of the water coming from a nearby well.
Most sources cite the ground temperature at 55-58 degrees. I think what they’re doing is just giving an average figure — a ballpark average.