1: How durable are StrawCore buildings compared to standard US construction?
We are designing StrawCore buildings to last hundreds of years with a minimal amount of maintaince. While it is possible for stud and drywall buildings to have similar life spans, the maintenance costs eventually become unmanageable. With conventional construction many aspects of the building need to be replaced on a regular basis. Roofs, gutters, siding, paint, even insulation and drywall eventually need to be replaced. Rodents make their way into wall cavities, termites devour the structure, and wooden buildings are a fire hazard. The fire insurance costs for a period of several hundred years alone render a typical US home unsustainable.
With StrawCore construction, there are no wall cavities, buildings will not burn, there is no wood for termites, and virtually everything that would normally need to be replaced is integrated into the structure. Conventional houses are composed of thousands of pieces that are susceptible to degradation over time, while StrawCore houses, on the other hand, would be composed of a few dozen monolithic components.
2: Don't plant fibers like straw deteriorate?
Yes, most straw will rot under the right conditions, but the same can said for wood. Wheat straw is the most problematic of the various straw varieties that we have tested. We would prefer not to use wheat at all, but because it is so abundant we are working on several measures to compensate for the decay problem.
- We are developing building component designs that prevent the straw from being exposed to moisture.
- One of the research teams is testing additives that will prevent decay.
- Most of our effort in this regard is devoted towards using plant materials that are not so susceptible to decay. The first houses that we build will be made out of flax straw. For places like the tropics where decay issues are extreme, we plan to use palm fronds and bamboo.
- We are recommending wheat straw for
- Temporary refugee housing
- Use in arid climates
- Interior walls where its excellent sound attenuation and pleasant aroma are a plus
3: But doesn't straw and plant fiber burn?
Yes straw burns, but StrawCore panels do not. There two reasons why they will not burn;
- The plasters that are used throughout the panels have a high mineral content which render them nonflammable.
- Unlike conventional construction there are no wall cavities, which would otherwise facilitate combustion inside the wall.
As part of a recent contract with the EPA, we were required to perform a burn test. To comply with the test we built a kiln with one wall composed of StrawCore. The kiln needed to be kept above 1,000°F for 2 hrs with the temperature to peak at 2,000°F. In order to pass the test, the temperature of the outside surface of our sample could rise no more than 250 F. We let the test run for 3 hours, and at the end of that time the temperature on the outside of our sample rose 5.4°F.
4: If the plaster on the outside of the building is some sort of adobe, won't that erode over time?
In order to formulate the plasters and the matrix material (the internal mortar) we drew upon a wide range of sources, from ancient adobe techniques to modern cement, fiber, and resin technologies. Various formulations are intended for different surfaces to provide the required protection. The plaster for the roof is waterproof and UV resistant. The plaster for the exterior walls is weatherproof and breathable, while the plaster intended for interior walls is slightly water soluble so a fractional amount can be washed off periodically to expose a fresh plaster surface.
5: What kinds of fasteners are used with this kind of construction?
Bamboo pins are used to help bind the layers together, and the panels themselves are joined together using hardware cloth imbedded in the plaster. Screws work well to hold cabinets and other internal fixtures.
6: Have any houses been built yet?
We have only built wall sections and other components for engineering tests. We are planning to build the first houses this summer.
7: Do other fibers work as well as wheat straw?
Almost every plant fiber that we have tested works better than wheat straw. The only reason that we are still using wheat at all is because it is so abundant. Some of the other plants that we have tested are many times stronger than wheat.
8: Would a StrawCore building be as strong as a wood framed structure?
A StrawCore structure would be substantially stronger than a typical wood framed structure because of the nature of composite construction. With composite systems, loads are distributed across the entire panel rather than the point loading that occurs with stud framing. Due to the extreme low cost of this type of construction, it’s entirely feasible to add additional layers to obtain desired structural strength and/or thermal insulation.
9: What is the R value of StrawCore?
The only fiber that we have tested so far is wheat straw, and the R value for that equals about 1.5 per inch. We are hoping for higher numbers from some other fibers. As insulation goes, 1.5 is not spectacular and that is why the walls are as thick as they are, but there is another factor working in our favor, and that is thermal mass. The high mineral content of the plaster and matrix material result in a high thermal mass quotient. In hot climates the thermal mass quotient of StrawCore can more than double the effective R value.
10: How expensive would a StrawCore house be?
We expect StrawCore houses will be built at a fraction of the cost of conventional construction. The bulk of the raw materials, (straw and soil) are virtually free, construction does not require skilled labor, and we are working hard to keep processing costs to a minimum.
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