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Section AS105 Strawbale Walls—General

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AS105.1 General
Strawbale walls shall be designed and constructed in accordance with this section. Strawbale structural walls shall be in accordance with the additional requirements of Section AS106.
AS105.2 Building Requirements for Use of Strawbale Nonstructural Walls
Buildings using strawbale nonstructural walls shall be subject to the following limitations and requirements:
  1. Number of stories: not more than one, except that two stories shall be allowed with an approved engineered design.
  2. Building height: not more than 25 feet (7620 mm).
  3. Wall height: in accordance with Table AS105.4.
  4. Braced wall panel length, and increase in Seismic Design Categories C, D0, D1 and D2: the required length of bracing for buildings using strawbale nonstructural walls shall comply with Section R602.10.3 of this code, with the additional requirements that Table R602.10.3(3) shall be applicable to buildings in Seismic Design Category C, and that the minimum total length of braced wall panels in Table R602.10.3(3) shall be increased by 60 percent.
AS105.3 Sill Plates
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 Tables AS105.4 and AS106.6(1), where applicable.
AS105.4 Out-of-Plane Resistance and Unrestrained Wall Dimensions
Strawbale walls shall employ a method of out-of-plane resistance in accordance with Table AS105.4, and comply with its associated limits and requirements.
OUT-OF-PLANE RESISTANCE AND UNRESTRAINED WALL DIMENSIONS
METHOD OF OUT-OF-PLANE RESISTANCEaFOR WIND DESIGN SPEEDS (mph)FOR SEISMIC DESIGN CATEGORIESUNRESTRAINED WALL DIMENSIONS, HbMESH STAPLE SPACING AT BOUNDARY RESTRAINTS
Absolute limit in feetLimit based on bale thickness Tc in feet (mm)
Nonplaster finish or unreinforced plaster≤ 100A, B, C, D0H ≤ 8H ≤ 5TNone required
Pins per Section AS105.4.2≤ 100A, B, C, D0H ≤ 12H ≤ 8TNone required
Pins per Section AS105.4.2≤ 110A, B, C, D0, D1, D2H ≤ 10H ≤ 7TNone required
Reinforcedc clay plaster≤ 110A, B, C, D0, D1, D2H ≤ 10H ≤ 8T0.5 (H ≤ 140T0.5)≤ 6 inches
Reinforcedc clay plaster≤ 110A, B, C, D0, D1, D210 < H ≤ 12H ≤ 8T0.5 (H ≤ 140T0.5)≤ 4 inchese
Reinforcedc cement, cement-lime, lime or soil-cement plaster≤ 110A, B, C, D0, D1, D2H ≤ 10H ≤ 9T0.5 (H ≤ 157T0.5)≤ 6 inches
Reinforcedc cement, cement-lime, lime or soil-cement plaster≤ 120A, B, C, D0, D1, D2H ≤ 12H ≤ 9T0.5 (H ≤ 157T0.5)≤ 4 inchese
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.
  1. Finishes applied to both sides of stacked bales. Where different finishes are used on opposite sides of a wall, the more restrictive requirements shall apply.
  2. H = Stacked bale height in feet (mm) between sill plate and top plate or other approved horizontal restraint, or the horizontal distance in feet (mm) between approved vertical restraints. For load-bearing walls, H refers to vertical height only.
  3. T = Bale thickness in feet (mm).
  4. Plaster reinforcement shall be any mesh allowed in Table AS106.16 for the matching plaster type, and with staple spacing in accordance with this table. Mesh shall be installed in accordance with Section AS106.9.
  5. Sill plate attachment shall be with 5/8-inch anchor bolts or approved equivalent at not more than 48 inches on center where staple spacing is required to be ≤ 4 inches.
AS105.4.1 Determination of Out-of-Plane Loading
Out-of-plane loading for the use of Table AS105.4 shall be in terms of the design wind speed and seismic design category as determined in accordance with Sections R301.2.1 and R301.2.2 of this code.
AS105.4.2 Pins
Pins used for out-of-plane resistance shall comply with the following or shall be in accordance with an approved engineered design. Pins shall be external, internal or a combination of the two.
  1. Pins shall be 1/2-inch-diameter (12.7 mm) steel, 3/4-inch-diameter (19.1 mm) wood or 1/2-inch-diameter (12.7 mm) bamboo.
  2. External pins shall be installed vertically on both sides of the wall at a spacing of not more than 24 inches (610 mm) on center. External pins shall have full lateral bearing on the sill plate and the top plate or roof-bearing element, and shall be tightly tied through the wall to an opposing pin with ties spaced not more than 32 inches (813 mm) apart and not more than 8 inches (203 mm) from each end of the pins.
  3. Internal pins shall be installed vertically within the center third of the bales, at spacing of not more than 24 inches (610 mm) and shall extend from top course to bottom course. The bottom course shall be similarly connected to its support and the top course shall be similarly connected to the roof- or floor-bearing member above with pins or other approved means. Internal pins shall be continuous or shall overlap through not less than one bale course.
AS105.5 Connection of Light-Framed Walls to Strawbale Walls
Light-framed walls perpendicular to, or at an angle to a straw bale 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-framed walls in this code, or the abutting stud shall be connected to alternating straw bale 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.
AS105.6 Moisture Control
Strawbale walls shall be protected from moisture intrusion and damage in accordance with Sections AS105.6.1 through AS105.6.8.
AS105.6.1 Water-Resistant Barriers and Vapor Permeance Ratings
Plastered bale walls shall be constructed without any membrane barrier between straw and plaster to facilitate transpiration of moisture from the bales, and to secure a structural bond between straw and plaster, except as permitted or required elsewhere in this appendix. Where a water-resistant barrier is placed behind an exterior finish, it shall have a vapor permeance rating of not less than 5 perms, except as permitted or required elsewhere in this appendix.
AS105.6.2 Vapor Retarders
Wall finishes shall have an equivalent vapor permeance rating of a Class III vapor retarder on the interior side of exterior strawbale walls in Climate Zones 14 and 16, as referenced in the California Energy Code. Bales in walls enclosing showers or steam rooms shall be protected on the interior side by a Class I or Class II vapor retarder.
AS105.6.3 Penetrations in Exterior Strawbale Walls
Penetrations in exterior strawbale walls shall be sealed with an approved sealant or gasket on the exterior side of the wall in all climate zones, and on the interior side of the wall in Climate Zones 14 and 16, as referenced in the California Energy Code.
AS105.6.4 Horizontal Surfaces
Bale walls and other bale elements shall be provided with a water-resistant barrier at weather-exposed horizontal surfaces. The water-resistant barrier shall be of a material and installation that will prevent water from entering the wall system. Horizontal surfaces shall include exterior window sills, sills at exterior niches and buttresses. The finish material at such surfaces shall be sloped not less than 1 unit vertical in 12 units horizontal (8-percent slope) and shall drain away from bale walls and elements. Where the water-resistant barrier is below the finish material, it shall be sloped not less than 1 unit vertical in 12 units horizontal (8-percent slope) and shall drain to the outside surface of the bales wall's vertical finish.
AS105.6.5 Separation of Bales and Concrete
A sheet or liquid-applied Class II vapor retarder shall be installed between bales and supporting concrete or masonry. The bales shall be separated from the vapor retarder by not less than 3/4 inch (19.1 mm), and that space shall be filled with an insulating material such as wood or rigid insulation, or a material that allows vapor dispersion such as gravel, or other approved insulating or vapor dispersion material. Sill plates shall be installed at this interface in accordance with Section AS105.3. Where bales abut a concrete or masonry wall that retains earth, a Class II vapor retarder shall be provided between such wall and the bales.
AS105.6.6 Separation of Bales and Earth
Bales shall be separated from earth by not less than 8 inches (203 mm).
AS105.6.7 Separation of Exterior Plaster and Earth
Exterior plaster applied to straw bales shall be located not less than 6 inches (102 mm) above earth or 3 inches (51 mm) above paved areas.
AS105.6.8 Separation of Wood and Plaster
Where wood framing or wood sheathing occurs on the exterior face of strawbale walls, such wood surfaces shall be separated from exterior plaster with two layers of Grade D paper, No. 15 asphalt felt or other approved material in accordance with Section R703.7.3.
Exceptions:
  1. Where the wood is preservative treated or naturally durable and is not greater than 11/2 inches (38 mm) in width.
  2. Clay plaster shall not be required to be separated from untreated wood that is not greater than 11/2 inches (38 mm) in width.
AS105.7 Inspections
The building official shall inspect the following aspects of strawbale construction in accordance with Section R109.1:
  1. Sill plate anchors, as part of and in accordance with Section R109.1.1.
  2. Mesh placement and attachment, where mesh is required by this appendix.
  3. Pins, where required by and in accordance with Section AS105.4.

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