Appendix BJ Strawbale Construction

This appendix provides prescriptive and performance-based requirements for the use of baled straw as a building material. Other methods of strawbale construction shall be subject to approval in accordance with Section R104.2.2 of this code. Buildings using strawbale walls shall comply with this code except as otherwise stated in this appendix.

Strawbale wall systems include those shown in Figure BJ101.2 and approved variations.

In flood hazard areas established in Table R301.2, buildings using strawbale wall systems shall meet the requirements of Section R306.

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

Bales shall be rectangular in shape, except for partial bales made to fill nonrectangular spaces in accordance with Section BJ103.6.

Bales shall have a height and thickness of not less than 12 inches (305 mm), except as otherwise permitted or required in this appendix. Bales used within a continuous wall shall be of consistent height and thickness to ensure even distribution of loads within the wall system. See Figure BJ103.2 for approximate dimensions of common straw bales.

Bales shall be confined by synthetic fiber, natural fiber or metal ties sufficient to maintain required bale density. Ties shall be not less than 3 inches (76 mm) and not more than 6 inches (152 mm) from the two untied faces and shall be spaced not more than 12 inches (305 mm) apart. Bales with broken ties shall be retied with sufficient tension to maintain required bale density.

The moisture content of bales at the time of application of the first coat of plaster or the installation of another finish shall not exceed 20 percent of the weight of the bale. The moisture content of bales shall be determined with a moisture meter designed for use with baled straw or hay, equipped with a probe of sufficient length to reach the center of the bale. Not less than 5 percent and not fewer than 10 bales shall be randomly selected and tested.

Bales shall have a dry density of not less than 6.5 pounds per cubic foot (104 kg/cubic meter). The dry density shall be calculated by subtracting the weight of the moisture in pounds (kg) from the actual bale weight and dividing by the volume of the bale in cubic feet (cubic meters). Not less than 2 percent and not fewer than five bales shall be randomly selected and tested on-site.

Partial bales made after original fabrication shall be retied with ties complying with Section BJ103.3.

Bales shall be composed of straw from wheat, rice, rye, barley or oat. The dry stems of other cereal grains or similar crops shall be acceptable where approved by the building official. Bales shall not be composed of hay.

Straw bales shall be placed laid flat, on-edge or on end in accordance with this appendix.

Finishes applied to strawbale walls shall comply with this section and with Chapters 3 and 7 unless stated otherwise in this section.

Strawbale walls shall be finished so as to provide mechanical protection, fire resistance and protection from weather and to restrict the passage of air through the bales, in accordance with this appendix and this code. Vertical strawbale wall surfaces shall receive a coat of plaster not less than 3/8 inch (10 mm) thick, or greater where required elsewhere in this appendix, or shall fit tightly against a solid wall panel or dense-packed cellulose insulation with a density of not less than 3.5 pounds per cubic foot (56 kg/m³) blown into an adjacent framed wall. The tops of strawbale walls shall receive a coat of plaster not less than 3/8 inch (10 mm) thick or be tightly covered by gypsum board or a roof-bearing assembly.

Class I and II vapor retarders shall not be used on a strawbale wall, nor shall any other material be used that has a vapor permeance rating of less than 5 perms, except as permitted or required elsewhere in this appendix.

Plaster applied to bales shall be any type described in this section, and as required or limited in this appendix. Plaster thickness shall not exceed 2 inches (51 mm).

Strawbale walls shall be designed and constructed in accordance with this section and with Figures BJ105.1(1) through BJ105.1(4) or an approved alternative design. Strawbale structural walls shall be in accordance with the additional requirements of Section BJ106.

Buildings using strawbale nonstructural walls shall be subject to the following limitations and requirements:

1. 1.

   Number of stories: not more than one, except that two stories shall be allowed with an approved engineered design.

2. 2.

   Building height: not more than 25 feet (7620 mm), except that greater heights shall be allowed with an approved engineered design.

3. 3.

   Wall height: in accordance with Table BJ105.4.

4. 4.

   Braced wall panel lengths: in accordance with Section R602.10.3, with the additional requirements that Table R602.10.3(3) shall apply to all buildings in Seismic Design Category C, and the minimum total length of braced wall panels in Table R602.10.3(3) shall be increased by 60 percent for buildings in Seismic Design Categories C, D₀, D₁ and D₂.

Sill plates shall be installed in accordance with Figure BJ105.1(1) or BJ105.1(2). Sill plates shall support and be flush with each face of the straw bales above and shall be of naturally durable or preservative-treated wood where required by this code. Sill plates shall be not less than nominal 2 inches by 4 inches (51 mm by 102 mm) with anchoring complying with Section R403.1.6 and the additional requirements of Table BJ105.4, where applicable, and Sections BJ106.13.2 and BJ106.13.3 for strawbale braced wall panels.

Strawbale walls shall employ a method of out-of-plane load resistance in accordance with Table BJ105.4 and comply with its associated limits and requirements.

Light-frame walls perpendicular to, or at an angle to, a strawbale wall assembly shall be fastened to the bottom and top wood members of the strawbale wall in accordance with requirements for wood or cold-formed steel light-frame walls in this code, or the abutting stud shall be connected to alternating strawbale courses with a 1/2-inch diameter (12.7 mm) steel, 3/4-inch-diameter (19.1 mm) wood or 5/8-inch-diameter (15.9 mm) bamboo dowel, with not less than 8-inch (203 mm) penetration.

Strawbale walls shall be protected from moisture intrusion and damage in accordance with Sections BJ105.6.1 through BJ105.6.9.

The building official shall inspect the following aspects of strawbale construction in accordance with Section R109.1:

1. 1.

   Sill plate anchors, as part of and in accordance with Section R109.1.1.

2. 2.

   Mesh placement and attachment, where mesh is required by this appendix.

3. 3.

   Pins, where required by and in accordance with Section BJ105.4.

Voids between bales and between bales and framing members shall not exceed 4 inches (102 mm) in width, and such voids shall be tightly stuffed with flakes, loose straw or straw-clay before application of finish.

Plastered strawbale walls shall be permitted to be used as structural walls in accordance with the prescriptive provisions of this section.

Buildings using strawbale structural walls shall be subject to the following limitations and requirements:

1. 1.

   Number of stories: Not more than one, except that two stories shall be allowed with an approved engineered design.

2. 2.

   Building height: Not more than 25 feet (7620 mm), except that greater heights shall be allowed with an approved engineered design.

3. 3.

   Wall height: In accordance with Table BJ105.4, BJ106.13(2) or BJ106.13(3), as applicable, whichever is most restrictive.

4. 4.

   Braced wall panel lengths: The greater of the values determined in accordance with Tables BJ106.13(2) and BJ106.13(3) for buildings using strawbale braced wall panels, or in accordance with Item 4 of Section BJ105.2 for buildings with load-bearing strawbale walls that do not use strawbale braced wall panels.

Live and dead loads and other limitations shall be in accordance with Section R301. Strawbale wall dead loads shall not exceed 60 psf (2872 N/m²) per face area of wall.

Foundations for plastered strawbale walls shall be in accordance with Chapter 4, Figure BJ105.1(1), Figure BJ105.1(2) or an approved engineered design.

Bales in strawbale structural walls shall be laid flat or on-edge and in a running bond or stack bond, except that bales in structural walls with unreinforced plasters shall be laid in a running bond only.

Plaster on load-bearing walls shall be in accordance with Table BJ106.12. Plaster on shear walls shall be in accordance with Table BJ106.13(1).

Plaster on strawbale structural walls shall be straight, as a function of the bale wall surfaces they are applied to, in accordance with all of the following:

1. 1.

   As measured across the face of a bale, straw bulges shall not protrude more than 3/4 inch (19.1 mm) across 2 feet (610 mm) of its height or length.

2. 2.

   As measured across the face of a bale wall, straw bulges shall not protrude from the vertical plane of a bale wall more than 2 inches (51 mm) over 8 feet (2438 mm).

3. 3.

   The vertical faces of adjacent bales shall not be offset more than 3/8 inch (9.5 mm).

Strawbale structural walls shall not have a membrane between straw and plaster, or shall have attachment through the bale wall from one plaster skin to the other in accordance with an approved engineered design.

Mesh in plasters on strawbale structural walls, and where required by Table BJ105.4, and where used to resist wind uplift in accordance with Section BJ106.14, shall be installed in accordance with Sections BJ106.9.1 through BJ106.9.4.

Plaster skins on strawbale structural walls shall be continuously supported along their bottom edge. Acceptable supports include: a concrete or masonry stem wall, a concrete slab-on-grade, a wood-framed floor in accordance with Figure BJ105.1(2) and an approved engineered design or a steel angle anchored with an approved engineered design. A weep screed as described in Section R703.7.2.1 is not an acceptable support.

Where plastered strawbale walls are used to support superimposed vertical loads, such loads shall be transferred to the plaster skins by continuous direct bearing in accordance with Figure BJ105.1(3) or by an approved engineered design. Where plastered strawbale walls are used to resist in-plane lateral loads, such loads shall be transferred to the reinforcing mesh from the structural member or assembly above in accordance with Figure BJ105.1(3) or BJ105.1(4) and to the sill plate in accordance with Figure BJ105.1(1) or BJ105.1(2) and with Table BJ106.13(1).

Bearing capacities for plastered strawbale walls used as load-bearing walls in one-story buildings to support vertical loads imposed in accordance with Section R301 shall be in accordance with Table BJ106.12.

Plastered strawbale walls used as braced wall panels for one-story buildings shall be in accordance with Section R602.10 and Tables BJ106.13(1), BJ106.13(2) and BJ106.13(3). Wind design criteria shall be in accordance with Section R301.2.1. Seismic design criteria shall be in accordance with Section R301.2.2. An approved engineered design in accordance with Section R301.2.1 shall be required where the building is located in a special wind region or where wind design is required in accordance with Figure R301.2(2) and Section R301.2.1.1, respectively.

Plaster mesh in skins of strawbale walls that resist uplift forces from the roof assembly, as determined in accordance with Section R802.11, shall be in accordance with all of the following:

1. 1.

   Plaster shall be any type and thickness allowed in Section BJ104.

2. 2.

   Mesh shall be any type allowed in Table BJ106.13(1), and shall be attached to top plates or roof-bearing elements and to sill plates in accordance with Section BJ106.9.2.

3. 3.

   Sill plates shall be not less than nominal 2-inch by 4-inch (51 mm by 102 mm) with anchoring complying with Section R403.1.6.

4. 4.

   Mesh attached with staples at 4 inches (51 mm) on center shall be considered to be capable of resisting uplift forces of 100 plf (1459 N/m) for each plaster skin.

5. 5.

   Mesh attached with staples at 2 inches (51 mm) on center shall be considered to be capable of resisting uplift forces of 200 plf (2918 N/m) for each plaster skin.

Post-and-beam with strawbale infil systems shall be in accordance with Figure BJ105.1(4) and Items 1 through 7, or be of an approved engineered design.

1. 1.

   Beams shall be of the dimensions and number of members in accordance with Table R602.7(1), where the space between posts equals the span in the table.

2. 2.

   Beam ends shall bear over posts not less than 1 1/2 inches (38 mm) or be supported by a framing anchor in accordance with Table R602.7(1).

3. 3.

   Discontinuous beam ends shall be spliced with a metal strap with not less than 1,000-pound (454 kg) wind or seismic load tension capacity. Where the wall line includes a braced wall panel, the strap shall have not less than a 4,000-pound (1814 kg) capacity.

4. 4.

   Each post shall equal NJ + 1 in accordance with Table R602.7(1), where the space between posts equals the span in the table.

5. 5.

   Posts shall be connected to the beam by an approved means.

6. 6.

   Roof and ceiling framing shall be attached to the beam in accordance with Table R602.3(1), Items 2 and 6.

7. 7.

   Posts shall be supported by the sill plate of the bale wall in accordance with BJ105.3 or BJ106.13.2, with fastening in accordance with Table R602.3(1), Item 16, or shall be supported and fastened at their base by an approved means.

Strawbale walls shall not be considered to exhibit a fire-resistance rating, except for walls constructed in accordance with Section BJ107.1.1 or BJ107.1.2. Alternately, fire-resistance ratings of strawbale walls shall be determined in accordance with Section R302.

Openings and penetrations in bale walls required to have a fire-resistance rating shall satisfy the same requirements for openings and penetrations as prescribed in this code.

Strawbale surfaces adjacent to fireplaces or chimneys shall be finished with not less than 3/8-inch-thick (10 mm) plaster of any type permitted by this appendix. Clearance from the face of such plaster to fireplaces and chimneys shall be maintained as required from fireplaces and chimneys to combustibles in Chapter 10, or as required by manufacturer’s instructions, whichever is more restrictive.

The unit R-value of a strawbale wall with bales laid flat is R-1.55 for each inch of bale thickness. The unit R-value of a strawbale wall with bales on-edge is R-1.85 for each inch of bale thickness.

Straw bales meet the requirements for insulation materials in Section R302.10.1 for flame spread index and smoke-developed index as tested in accordance with ASTM E84.

See Table BJ109.1 for standards that are referenced in various sections of this appendix. Standards are listed by the standard identification with the effective date, the standard title and the section or sections of this appendix that reference the standard.