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Light Straw Clay Appendix for IRC



The provisions contained in this appendix are not mandatory unless specifically referenced in the adopting ordinance.

Section AR101 GENERAL

AR 101.1 Scope. This appendix shall govern the use of light straw-clay as a non-bearing building material and wall infill system in Seismic Design Categories A and B..



AR102.1. General. The following words and terms shall, for the purposes of this appendix, have the meanings shown herein. Refer to Chapter 2 of the International Residential Code for general definitions.


CLAY. Inorganic soil with particle sizes less than 0.00008 in. (0.002 mm) having the characteristics of high to very high dry strength and medium to high plasticity.


CLAY SLIP. A suspension of clay soil in water.


CLAY SOIL. Inorganic soil containing 50% or more clay by volume.


INFILL. Light straw-clay that is placed between the structural members of a building.


LIGHT STRAW-CLAY. A mixture of straw and clay compacted to form insulation and plaster substrate between or around structural and non-structural members in a wall.


NON-BEARING. Not bearing the weight of the building other than the weight of the light straw-clay itself and its finish.


STRAW. The dry stems of cereal grains after the seed heads have been removed.


VOID. Any space in a light straw-clay wall in which a 2-inch (51 mm) sphere can be inserted.



AR103.1  General. Light straw-clay shall be limited to infill between or around structural and non-structural wall framing members.


AR103.2  Structure.  The structure of buildings using light straw-clay shall be in accordance with the International Residential Code or shall be in accordance with an approved design by a registered design professional.


AR103.2.1 Number of stories.  Use of light straw-clay infill shall be limited to buildings that are not more than onestory above grade plane.


Exception: Buildings using light straw-clay infill that are greater than one-story above grade plane shall be in accordance with an approved design by a registered design professional


AR103.2.2  Bracing.  Wind bracing shall be in accordance with Section R602.10 and shall use Method LIB.  Walls with light straw-clay infill shall not be sheathed with solid sheathing.


AR103.2.3  Weight of light straw-clay. Light straw-clay shall be deemed to have a design dead load of 40 pounds

per cubic foot (640 kg per cubic meter) unless otherwise demonstrated to the building official.


AR103. 2.4  Reinforcement of light straw-clay.  Light straw-clay shall be reinforced as follows:


  1. Vertical reinforcing shall be a minimum of nominal 2-inch by 6-inch (51 mm by 152 mm) wood members at a maximum of 32 inches (813 mm) on center where the vertical reinforcing is nonload-bearing and at 24”(610mm) on center where it is load-bearing. The vertical reinforcing shall not exceed an unrestrained height of 10 feet (3,048 mm) and shall be attached at top and bottom in accordance with Chapter 6 of the International Residential Code. In lieu of these requirements, vertical reinforcing shall be in accordance with an approved design by a registered design professional.


  1. Horizontal reinforcing shall be installed in the center of the wall at not more than 24 inches (610 mm) on center and shall be secured to vertical members. Horizontal reinforcing shall be of any of the following: ¾ inch (19 mm) bamboo, ½ inch (13 mm) fiberglass rod, 1-inch (25 mm) wood dowel or nominal 1-inch by 2-inch (25 mm by 51 mm) wood.


AR103.3  Materials. The materials used in light straw-clay construction shall be in accordance with Sections AR103.3.1 through AR103.3.4.


AR103.3.1  Straw.  Straw shall be wheat, rye, oats, rice, or barley, and shall be free of visible decay and insects.


AR103.3.2  Clay soil.  Suitability of clay soil shall be determined in accordance with the Figure 2 Ribbon Test or the Figure 3 Ball Test of the Appendix to ASTM E2392/E2392M.


AR103.3.3  Clay slip. Clay slip shall be of sufficient viscosity such that a finger dipped in the slip and withdrawn remains coated with an opaque coating.


AR103.3.4  Light straw-clay mixture.  Light straw-clay shall contain a minimum of 65% and a maximum of 85% straw, by volume of bale-compacted straw to clay soil. Loose straw shall be mixed and coated with clay slip such that there is no more than 5% uncoated straw.


AR103.4  Wall Construction. Light straw-clay wall construction shall be in accordance with the requirements of Sections AR103.4.1 through AR103.4.7.


AR103.4.1  Light straw-clay maximum thickness.  Light straw-clay shall be not more than 12 inches (305 mm) thick, to allow adequate drying of the installed material.


AR103.4.2  Distance above grade. Light straw-clay and its exterior finish shall be not less than 8 inches (203 mm) above exterior finished grade.


AR103.4.3  Moisture barrier.  An approved moisture barrier shall separate the bottom of light straw-clay walls from any masonry or concrete foundation or slab that directly supports the walls. Penetrations and joints in the barrier shall be sealed with an approved sealant.


AR103.4.4  Contact with wood members. Light straw clay shall be permitted to be in contact with untreated wood members.


AR103.4.5  Contact with non-wood structural members.  Non-wood structural members in contact with light strawclay shall be resistant to corrosion or shall be coated to prevent corrosion with an approved coating.


AR103.4.6  Installation.  Light straw-clay shall be installed in accordance with the following:


  1. Formwork shall be sufficiently strong to resist bowing when the light straw-clay is compacted into the forms.


  1. Light straw-clay shall be uniformly placed into forms and evenly tamped to achieve stable walls free of voids. Light straw-clay shall be placed in lifts of no more than 6 inches (152 mm) and shall be thoroughly tamped before additional material is added.


  1. Formwork shall be removed from walls within 24 hours after tamping, and walls shall remain exposed until moisture content is in accordance with Section AR103.5.1. Any visible voids shall be patched with light straw-clay prior to


AR103.4.7  Openings in Walls. Openings in walls shall be in accordance with the following:


  1. Doors and windows. Rough framing for doors and windows shall be fastened to structural members in accordance with the -International Residential Code. Windows and doors shall be flashed in accordance with the International Residential Code.


  1. Window sills. An approved moisture barrier shall be installed at window sills in light straw-clay walls prior to installation of windows.


AR103.5  Wall Finishes. The interior and exterior surfaces of light straw-clay walls shall be protected with a finish in accordance with Sections AR103.5.1 through AR103.5.5.


AR103.5.1  Moisture content of light straw-clay prior to application of finish.  Light straw-clay walls shall be dry to a maximum moisture content of 20% at a depth of 4 inches (102 mm), as measured from each side of the wall, prior to the application of finish on either side of the wall. Moisture content shall be measured with a moisture meter equipped with a probe that is designed for use with baled straw or hay.


AR103.5.2  Plaster finish. Exterior plaster finishes shall be clay plaster or lime plaster. Interior plaster finishes shall be clay plaster, lime plaster, or gypsum plaster. Plasters shall be permitted to be applied directly to the surface of the light straw-clay walls without reinforcement, except that the juncture of dissimilar substrates shall be in accordance with Section AR103.5.4. Plasters shall have a thickness of not less than 1/2 inch (13 mm) and not more than 1 inch (25 mm) and shall be installed in no less than 2 coats. Exterior clay plaster shall be finished with a lime-based or silicatemineral coating.


AR103.5.3 Separation of wood and plaster.  Where wood framing occurs in light straw-clay walls, such wood surfaces shall be separated from exterior plaster with No.15 asphalt felt, grade D paper, or other approved material except where the wood is preservative-treated or naturally durable.


Exception: Exterior clay plasters shall not be required to be separated from wood.


AR103.5.4  Bridging  across dissimilar substrates. Bridging shall be installed across dissimilar substrates prior to the application of plaster. Acceptable bridging materials include: expanded metal lath, woven wire mesh, welded wire mesh, fiberglass mesh, reed matting, or burlap. Bridging shall extend not less than 4 inches (102 mm), on both sides of the juncture.


AR103.5.5  Exterior siding. Exterior wood, metal, or composite material siding shall be spaced a minimum of 3/4 inch (19 mm) from the light straw-clay such that a ventilation space is created to allow for moisture diffusion. The siding shall be fastened to wood furring strips in accordance with manufacturer’s recommendations. Furring strips shall be spaced not more than 32 inches (813 mm) on center, and shall be securely fastened to the vertical wall reinforcing or structural framing. Insect screening shall be provided at the top and bottom of the ventilation space. An air barrier consisting of 3/8” thick, maximum, clay plaster or lime plaster shall be applied to the light straw-clay prior to application of siding.




AR104.1  R-value. Light straw-clay, when installed in accordance with this appendix, shall be deemed to have an Rvalue of 1.6 per inch.



The purpose of the proposed code change is to include Light Straw Clay as a nonload-bearing building material and wall infill system into the IRC because no such section currently exists.

Light straw-clay construction has proven to be a viable, ecologically sound, and energy efficient building method.  To date, permitting of light straw-clay construction has generally been left to the discretion of individual building officials on a case-by-case basis. Two exceptions are the State of New Mexico and the State of Oregon. Since 1998 the State of New Mexico has successfully permitted straw-clay construction using its standard permitting process when a project complies with its “Clay Straw Guidelines”.

The proposed light straw-clay section of the IBC is derived from and builds upon the fourteen years of success of New Mexico’s Clay Straw Guidelines. In October of 2011 the Oregon Reach Code (ORC) was amended to include light straw-clay construction. Inclusion in the IBC would make proven provisions accessible to more designers and builders interested in using this environmentally beneficial material and to building officials who will be evaluating and enforcing its proper use.

The proposed mixture of clay and straw as a monolithic non-load bearing building enclosure has been successfully used in the United States since 1990 and since 1950 in Europe. Prior to this a heavier form of clay, straw, and woven wood construction known as wattle and daub was in common use throughout Europe, Africa, Asia, and North and South America. Many thousands of existing structures dating back 300-400 years have been continuously occupied, attesting to the durability of these natural materials.   In the United States residential and non-residential structures using straw-clay have been completed in 17 states, and most of those have been constructed with full permits and inspections.                 The centuries old European predecessors and light straw clay buildings built to date in North America have all been constructed without the use of a moisture barrier. The proposed light straw clay materials are vapor permeable and do not require a moisture barrier. Code precedents for vapor permeable construction exist for adobe construction, log construction and half-timber construction. In these systems as in light straw clay construction there is sufficient hygric capacity to hold and re-release moisture without damage to structural members or degradation of the wall due to weather related moisture fluctuations. Furthermore for exterior siding applications, with ventilated space and rain screen a water resistive barrier is not necessary.

Through The EcoNest Company, and as a licensed architect for over 25 years, I have been involved in the design and/or construction of over 50 buildings utilizing light straw-clay construction.  In 2005 I co-authored, with my husband and business partner Robert Laporte, the book “Econest, Creating Sustainable Sanctuaries of Clay, Straw and Timber”.

Official guidelines for straw-clay construction have been in effect in New Mexico since 1998. At least 20 residential structures have been successfully permitted and built since that time in New Mexico following these guidelines. Other building officials in surrounding States have also permitted straw-clay construction in their jurisdictions based on these guidelines.

In 2004 the Canada Mortgage and Housing Corporation (CMHC) funded a study to explore the material characteristics of Straw Light Clay

(SLC) construction. The proposed section for the IBC uses this study as well as the many years of experience of our company and other practitioners of light straw-clay construction as its basis. The CMHC study includes issues of thermal performance, fire-resistance, moisture, and vapor permeability. The CMHC study and other supporting documentation is available for viewing and download at:  EcoNest’s numerous projects utilizing light straw-clay construction can be viewed at



2011 Oregon Reach Code (Section 1307) (Based on 2012 International Green Construction Code)

Baker-Laporte, Laporte (2005) Econest, Creating Sustainable Sanctuaries of Clay, Straw and Timber, Gibbs Smith Publishers (This book is available only by purchase. See

  1. Thornton (2004) Initial Material Characterization of Straw Light Clay Canada Mortgage and Housing Corporation

State of New Mexico Construction Industries Division (2001) Clay Straw Guidelines

Richard Duncan PE, Resistance to Out-Of -Plane Lateral Forces of Light Straw Clay Wall Infill

Cost Impact: The code change proposal will not increase the cost of construction.


Public Hearing: Committee:                   AS                                AM                   D









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