Light Straw-Clay Code

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Light Straw-Clay is a local natural wall system !  Click here for a description of the process: Building a Light Straw-Clay House. 

Appendix R Light Straw-Clay Construction

Michigan Residential Code 

International (U.S.) Residential Code:


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

Section AR101 GENERAL

AR101.1 Scope

This appendix shall govern the use of light straw-clay as a nonbearing building material and wall infill system.


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 of less than 0.00008 inch (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 percent 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 nonstructural members in a wall.

NONBEARING. 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 nonstructural 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 one story 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 and seismic bracing shall be in accordance with Section R602.10 and shall use Method LIB. The required length of bracing shall comply with Section R602.10.3, with the additional requirements that Table 602.10.3(3) shall be applicable to buildings in Seismic Design Category C, and that the minimum total length of bracing in Table R602.10.3(3) shall be increased by 90 percent. In lieu of these prescriptive requirements, wind and seismic bracing shall be in accordance with an approved design by a registered design professionalWalls 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 shallbe not less than nominal 2-inch by 6-inch (51 mm by 152 mm) wood members at not more than 32 inches (813 mm) on center where the vertical reinforcing is nonload bearing and at 24 inches (610 mm) on center where it is load bearing. The vertical reinforcing shall not exceed an unrestrained height of 10 feet (3048 mm) and shall be attached at top and bottom in accordance with Chapter 6 of the this code. In lieu of these requirements, vertical reinforcing shall be in accordance with an approved design by a registered design professional.
  2. Horizontal reinforcing shallbe 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 (19.1 mm) bamboo, ½-inch (12.7 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 not less than 65 percent and not more than 85 percent straw, by volume of bale-compacted straw to clay soil. Loose straw shall be mixed and coated with clay slip such that there is not more than 5 percent 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 strawclay shall be permitted to be in contact with untreated wood members.

AR103.4.5 Contact with nonwood structural members

Nonwood structural members in contact with light strawclay shallbe 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 shallbe sufficiently strong to resist bowing where the light straw-clay is compacted into the forms.
  2. Light straw-clayshall be uniformly placed into forms and evenly tamped to achieve stable walls free of voids. Light straw-clay shall be placed in lifts of not more than 6 inches (152 mm) and shall be thoroughly tamped before additional material is added.
  3. Formwork shallbe removed from walls within 24 hours after tamping, and walls shall remain exposed until moisture content is in accordance with Section5.1. Visible voids shall be patched with light straw-clay prior to plastering.

AR103.4.7 Openings in walls

Openings in walls shall be in accordance with the following:

  1. Rough framing for doors and windows shallbe fastened to structural members in accordance with theInternational Residential Code. Windows and doorsshall be flashed in accordance with the International Residential Code.
  2. An approved moisture barrier shallbe 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 moisture content of not more than 20 percent 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 AR103.5.3. Plasters shall have a thickness of not less than ½ inch (12.7 mm) and not more than 1 inch (25 mm) and shall be installed in not less than two coats. Exterior clayplaster shall be finished with a limebased or silicate-mineral 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 not less than ¾ inch (19.1 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 the manufacturer’s instructions. 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 not more than ⅜-inch-thick (9.5 mm) clay plaster or lime plaster shall be applied to the light strawclay prior to the application of siding.


AR104.1 R-value

Light straw-clay, where installed in accordance with this appendix, shallbe deemed to have an R-value of 1.6 per inch.



ASTM E2392/
E2392M- 10
Standard Guide for Design of Earthen Wall

Building Systems………………..AR103.3.2