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Natural Composite Architecture: Building Without the Use of Lumber, Concrete, Steel, or Petroleum Products

Over the past five years the International Resource Institute (IRI) has been working to find ways to replace all of the lumber, concrete, steel, and petroleum products used in new home construction with materials that have a decidedly lower environmental impact. Our most recent sustainable architecture concept house uses a composite bentonite clay/cellulose fiber/straw-bale wall and roof system that eliminates the need for all structural lumber or steel.

One of the key building components used to replace much of the lumber, steel, portland cement and lime masonry products is a natural composite material being developed by IRI. A composite is a structural material typically made from two components: the first, a binder or adhesive, called a matrix, and the second, a fibrous material of some type. The combination of the two materials, a viscous adhesive and a non-viscous fiber, mixed in the proper proportions and configurations, when dry, results in some of the highest performing materials known.

IRI's approach to composite structures is in sharp contrast with what is happening throughout the rest of the composite industry. Whereas most composite manufacturers rely almost exclusively on expensive, high-tech carbon fibers and toxic epoxy resins to form their high performance composites, IRI has focused on using some of nature's most abundant, therefore inexpensive, and non-toxic structural materials. Chief among these is microscopic and macroscopic cellulose fiber (straw, for example) and expansive clays.

When an appropriate combination of cellulose fiber and expansive clay is left to dry, the excessive shrinkage caused by the loss of water forces the clay molecules to pull down around the cellulose fibers, creating a very tough composite material. Using an Iowa field clay, in our first year we were able to achieve breaking strengths (modulus of rupture) for these materials of around 1400 psi. This contrasts with five-bag concrete at modulus of rupture of 56 psi and construction lumber starting around 10,000 psi and up.

Over the next few years we learned how to work with more expansive clays like bentonite and hectorite, which gave greater shrinkage on drying and therefore greater strength. Our latest tests have since resulted in breaking strengths approaching 6,000 psi. And by extruding our composite mix through a reducing extruder to give better parallel alignment to the cellulose fibers (much the same as in a tree) we expect to begin achieving approximately the same modulus of rupture as construction lumber.

We have found ways to use the cellulose/clay composite for virtually every part of a building's structure from the foundation to structural wall components, structural skins, millwork and cabinetry. For foundations we start with a cellulose/clay composite mix and add sand and aggregate to form a pourable "concrete-like" footing material. Currently, for the walls and roof we use a thick paste-like mixture of the cellulose/clay material to glue straw bales together to form a catenary dome.

We then give the structure a high-performance structural skin through a three-coat composite application. The first coat is comprised of the thick paste-like mix that was used to glue the straw bales together. It is applied to both the inside and outside of the exterior walls of the structure. Then while it is still wet we apply a long-straw fill mix made of the same cellulose/clay composite material cut about six times with water and with a relatively high percentage of long straw fibers mixed in. This is used to fill and level any unevenness and to provide good cross-bale surface bonding. Once this coat dries we apply a mixture of the same cellulose/clay composite cut about 3 times with water and with short, 1"-3", straw mixed in. This is used as exterior and interior putty coats to provide a smooth, tight finish. We have also recently experimented with variations of this three-coat process to build cabinets, shelves and for flooring, all with great success.

The primary disadvantage to our natural composite material is that it is water soluble. It will take up moisture if exposed to water and that moisture will begin to degrade the structural qualities of the material. This has necessitated that we take certain precautions in our building approach. Structures are built on small hills, and to further protect from moisture wicking up through surface soils we separate our buildings from the ground with a 6" gravel pad. To protect the exterior skin from moisture we are developing a linseed oil/chalk permanent waterproof coating that is applied as a thick caulk, in much the same way as you would ice a cake. So far, our efforts in this regard look highly promising, although additional long-term testing work still needs to be done.

Considerable additional research must be done before our natural composite approach can become commercially viable. Some of our challenges center around finding ways to get our materials to dry faster, especially in cold and damp climates. We have recently started adding gypsum (low-temperature-kilned) plaster to our mixes to accelerate the drying process. This has worked quite well, although we do experience some loss of strength and increased materials cost in the process.

We are also looking to use our cellulose/clay composite material for the struts in space-frame construction. We believe that the advanced used of natural composites will someday lead to the construction of buildings that are inherently beautiful, low cost, non-toxic, superinsulated, super environmentally friendly, and virtually indestructible.

This article was originally published in The Last Straw Issue #17 and is reprinted with permission. The Last Straw, The Grassroots Journal of Straw Bale and Natural Building, HC66 Box 119, Hillsboro NM 88042; ph 505-895-5400, fax 505-895-3326;,


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