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The distinguishing factor that identifies Earthship houses, such as the topic of this case study, the Potter House, is the basic fact that these buildings are made out of used materials, such as used car tires stacked one on top of another. The construction materials and methods used to build the Potter House are an important part of this analysis as they are central to the technological and ecological discourse we have set up for this building.
Used Tires as Building Blocks
The construction of the Potter House began with a delivery of used car tires, for which the Potter's received a payment of one dollar a piece. The tire walls were then made by staggering the tires in courses, much the way that bricks or concrete blocks would be layered. Each tire, as it was set on the tire wall, was filled with compacted earth, "so that it becomes a rammed earth brick encased in steel belted rubber." Once compacted, each tire weighed approximately 150kg, therefore, all tires were pounded in their intended place within the wall structure.
The 'U' Module
As is illustrated in the plan of the Potter House, the structural system consists of a row of consecutive "U" modules. The scope of the project, the "U" module, was the basic shape in which the tires were set. Each "U" module was structured independently from the other. These U-shaped walls bear the mass of the building and serve as the very foundation of the building. Therefore, they serve a dual function of bearing the load of the building, while at the same time, they act as mass walls, an aspect of passive solar heating which will be discussed later in the section on mechanical systems. The load bearing and mass bearing foundation walls of conventional housing in comparison to Earthship housing is illustrated here.
Thermal Mass
Thermal mass is achieved simply because a large quantity of earth was contained in the rubber tires. The heavy structure that makes up the building is heavy enough to be durable and resistant enough to handle the seismic loads created by earthquakes. The Potter House is buried at the perimeter, therefore, the bottom of the mass wall is below the frost line. As a result, the house is not in danger of being affected by the freeze-thaw movements of the earth.
Dense and massive materials were used in the construction of this Earthship in order so that temperature is stored. As a result, the house itself serves as a sort of battery for storing energy.
Framing of the Roof
The roof structure was constructed with the use of two-foot wide beams extending horizontally across the "U" module, in an east-west direction. This configuration effectively transfers the loads to the load bearing tire walls. As a result of this distribution, no point loads are created.
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The Greenhouse
The Potter House is flanked with a beautiful expansive greenhouse on the south side of the building, as seen in the section and plan. The greenhouse is a structural component in and of itself, requiring support somewhat separate from the massive U structure. The greenhouse is a "Lean-to" figure, which gains its support by resting on the southernmost beam. Posts, approximately 12" in diameter, made out of tree trunks eliminate the need for structural uprights for the greenhouse.
Interior Partition Walls
As a result of the tire walls bearing the load of the structure, partition walls need not be structural walls. They were simply used by the Potters to enclose specific rooms, such as bathrooms. Since partition walls only carry their own load, and do not bear that of the rest of the structure, they do not need to be set on deep foundations. Therefore, these interior walls were set on foundations as deep as they were thick. Half inch (1/2") steel reinforcement bars are utilized to hold the interior walls together.
Exterior Partition Walls
Exterior partition walls are similar to interior partition walls in that they do not carry any load whatsoever. However, the exterior partition walls are different because they need to be insulated, and as such, need to be thicker, leading to the need for a second reinforcement bar.
Can Infill
Used aluminum cans were used to fill the spaces between beams and the adjacent blocking that holds the beams in place. The aluminum cans were set in mortar, whereby they acted as the spacers within a perforated concrete network. The cans do not provide structural support in and of themselves. This is why aluminum cans set in mortar are also used to construct interior partition walls.
Floors
There are two types of floors in the Potter House, flagstone and concrete. Both of these were laid directly on top of the earth subfloor. A 6 mil plastic vapour barrier was set on the earth subfloor prior to setting the concrete and the flagstone, to prevent the possibility for dampness.
Vapour Barriers
A vapour barrier, made of 6 mil plastic, was applied across all the wood on the exterior surface of the roof structure. On this first layer of plastic, felt roofing is applied on top. This felt provides a vapour barrier on the interior of the roof insulation. A second layer of plastic was applied on top of the felt to protect it in turn.
Roof Insulation
Rigid insulation was achieved through the use of foam insulation board, which, in conjunction with the roofing materials, provides sufficient protection from the elements.
Exterior Plaster
Exterior tire walls were packed smooth with mud, after which time, they were covered with a 6 mil plastic stapled on. This was then covered with an expanded metal lath, which was nailed on, prior to the application of cement plaster. Finally, a "floated brown coat" was applied in order to attain a surface that looks like stucco.
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