Here's the dirt on a prefabricated plastic earth sheltered home design you can buy off the shelf

community earth sheltered homes
© Green Magic Homes

No, this isn't Hobbiton or a scene from Teletubbies. It's a rendering of a community of earth-sheltered houses built from a new prefab building system. I once wrote that from Hobbiton to Tatooine, earth sheltered houses make sense all over the universe; now you can just order one up.

Earth-sheltered houses have long been known to be very energy-efficient, with the thermal mass of all that dirt keeping the temperature relatively even all year. However they have often been expensive to build and difficult to make totally waterproof. Now a Florida company, Green Magic Homes, has designed a prefabricated system of fiber-reinforced polymers (FRP) where you can roll your own earth sheltered house at a reasonable price (US $41 per square foot for the shells). Their system addresses the problems of weight and cost:

installed home© Green Magic Homes

The Green Magic Homes system has addressed these problems in an entirely new way, using the age-old methods of building with earth in conjunction with the space-age technology of composite materials. The inner shell of the buildings is very strong, light, waterproof, and modular, and the earth covering is constructed in such a way that it collaborates structurally with the shell because of its layered construction and the vaulted geometry of the system.

in factory© Green Magic Homes

The FRP components are made in their factory....

assembly© Green Magic Homes

Assembled on site with glue and stainless steel screws through those flanges sticking up,

covering with plants© Green Magic Homes

Then covered with soil and planting. According to their FAQ, there can be 8 inches of soil on the top, and they are designed to handle about 44 pounds per square foot of live load on top of that. However they are inconsistent, they also make some claims about the R value of soil and how it is different than insulation:

The R value or heat transfer resistance value of a GREEN MAGIC HOME, is approximately 1 per every 10 cm of Earth. A typical GREEN MAGIC HOME has an average of 60 centimeters between walls and deck, which would give an R factor of 6. However, the thermal character of the mass of the Earth is quite different from the materials designed and used principally for the resistance to the transfer of heat (R value) such as polystyrene or polyurethane foam. When changing from soil to a less massive "insulating" material, you should understand the design performance with regards to heat capacity of the building and/or soil mass (K-value). A massive soil wall or roof can store heat energy to even out the temperature swings of day--lightweight insulation does not perform this way. The thermal value you get from 18 inches of soil far surpasses the 4.5 R-value (0.25 per inch). The use of earth as a large capacity heat storage makes it possible not only to reduce such buildings’ demand for heating and cooling energy, but also helps to preserve the local microclimate.

Green magic homes roof installation© Green magic homes roof installation

Now this is totally confusing because they are mixing metric dimensions with American R values. They say there is 18 inches when before they said 8 inches was on top. They are also not taking into account that in northern climates, frozen soil has not much of an R value at all; dirt is a lousy insulator. In fact, judging from the installation photos, these would be useless in cold climates without a lot more insulation and a lot more dirt on top. However it appears that most of these are going into warmer climates where the thermal mass of dirt can be very useful at keeping the place cool.

components© Green Magic Homes

Then there is the issue of the fiber-reinforced plastic panels, made in a manner similar to a modern fiberglass boat. This is not exactly the greenest of technologies, though much depends on the resin that is being used. The common phenol formaldehyde resins emit high levels of formaldehyde during manufacture, endangering workers, and can outgas for some time after manufacture. It also is dangerous in a fire, emitting serious poisons.

domes in plan© Green Magic Homes

However the usual alternative material in earth sheltered housing is usually reinforced concrete, which is then needs expensive plastic waterproofing, bigger foundations and a lot more material. This is certainly more efficient and logical.

Malcolm Wells, the pioneer of earth sheltered housing, wrote:

A building should consume its own waste, maintain itself, match nature's pace, provide wildlife habitat, moderate climate and weather and be beautiful. That's a series of pass/fail evaluation criteria.

I am not sure what he would think of these. Calling FRP shells green is a matter for debate; there is a lot of plastic in this, and the green consensus is that we should be trying to eliminate plastic from our buildings. And it certainly isn't magic, but the Green Magic Home is certainly an interesting, quick and more affordable way of doing an earth sheltered home.

Tags: Green Building


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