Lego Blocks from Straw
For those who loved playing with Lego blocks as kids, Stak Block, from the Goleta, California, company Oryzatech, may be a dream come true. The company is still looking for funding to build a factory but has developed and tested its product: a straw-composite, structural building block made from 96% rice straw. An underutilized waste agricultural product, rice straw has become a disposal problem since burning straw in fields was banned in California and elsewhere. According to the October 2006 issue of Ethanol Producer magazine, California produces 1.35 million tons of rice straw annually from the 600,000 acres (240,000 ha) in rice production, and only 3%–4% of that straw is used commercially.
Stak Blocks are 12" x 12" x 24" (305 x 305 x 610 mm) and weigh 30 pounds (14 kg), with a density of 15 lbs/ft3 (240 kg/m3)—about twice that of typical straw bales. The straw fibers are held together with a polyurethane binder (MDI), used at a concentration of about 2% by weight, according to company president Jay Ruskey. The blocks have a high enough density to create load-bearing walls with simple stacking. (With strawbale construction, load-bearing applications require special measures to prevent cracking of plaster as bales compress over time.) Stak Blocks interlock with molded bumps and dimples (think Lego), leaving a channel that is used for a threaded rod to secure the top plate to the foundation. The cavities could also be filled with re-bar and concrete to create a sort of insulated concrete form, though the diameter of the concrete columns would be fairly small.
Thermal (R-value) performance was measured by Justin Monell, a student at California Polytechnic State University (Cal Poly), as a class assignment in the materials engineering department. He used a guarded hot box to measure the steady-state heat flux (R-value per inch) of prototype Stak Blocks with various densities, ranging from 10 to 15 lbs/ft3 (160–240 kg/m3). The measured R-values per inch ranged from 3.89 per inch to 4.14 per inch. Oryzatech has used the 15 lbs/ft3 block data to arrive at an insulating value of R-48 for the 12-inch-thick, 30-pound blocks. This is roughly three times the R-value of a 2x6 wood-frame wall.
Structural engineer Bruce King, P.E., who has worked with Oryzatech on product development and sits on its board, was surprised that the R-value came out as high as it did and said that some colleagues in the strawbale world have questioned the results. He told EBN that a top priority of the company will be to carry out more comprehensive thermal testing at Oak Ridge National Laboratory or another accredited testing facility. But he’s convinced that the bottom-line conclusions will be strong. “Even if we really only have (say) R-30, that is huge and puts our thermal performance among the very best wall systems that are in widespread use.”
As for structural properties, early testing shows that Stak Block walls outperform 2x4 stud walls in shear and lateral strength, according to Ruskey. Due to the binder and straw density, the blocks also provide relatively good screw-withdraw properties, allowing interior walls to be finished with drywall, and exterior walls to be sheathed and finished like frame walls. In addition, the walls “are extremely fire resistant, even better than strawbale” according to Ben Korman, the cofounder and chief technology officer at Oryzatech. The high level of silica in rice straw (up to 14%) contributes to the excellent fire resistance.
Korman told EBN that the company is seeking about $4 million in funding to build a first plant. Based on its financial models, the company expects that blocks would sell for $7.50 to $8.50, resulting in walls with cost per square foot similar to that of 2x6 frame walls with insulation.
The company is also promoting Stak Block construction as a way to offset carbon emissions. Each of the 30-pound (14 kg) blocks sequesters about nine pounds (4 kg) of carbon directly (the carbon contained in the straw). But indirectly, if used in developing countries (a major market for the technology) each Stak Block offsets significantly more, according to Korman. Some of that additional benefit assumes that the straw being used to produce the block would otherwise be burned in the fields (common practice in China, India, and elsewhere). The technology also reduces energy that would be used to heat conventional (poorly insulated) homes.
EBN rarely reviews products that are not in production, but we made an exception in this case because of the unique properties and potential of Oryzatech’s Stak Block. King believes that Stak Block is in many ways an ideal building material. “I think we’ve succeeded in taking the benefits of strawbale construction and turning it into something that’s relevant to home builders, big and small,” he told EBN.
– Alex Wilson
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