Section BL104 — FINISHES

The interior and exterior surfaces of hemp-lime walls shall be protected with a finish in accordance with Section BL104. Finishes shall have a vapor permeance rating of 5 perms [285 mg/(s × m [2] × Pa)] or greater tested in accordance with Procedure B of ASTM E96.

Hemp-lime infill shall have an average moisture content of no more than 20 percent at a depth of 1½ inches (38 mm), as measured from the face of the wall to which the finish will be applied for each wall. Moisture content shall be measured with a probe-style wood moisture equivalent (WME) meter.

Exterior plaster shall be lime plaster, clay plaster in accordance with Section BL104.3.6.3, or other approved plaster. Interior plasters shall be any plaster permitted in Sections BL104.3.1 through BL104.3.9. Plasters shall be permitted to be applied directly to the surface of the hemp-lime infill without reinforcement, except that the juncture of dissimilar substrates shall be in accordance with Section BL104.5. Plasters shall have a thickness of not less than [1] / 2 inch (13 mm) on the interior and [3] / 4 inch (19 mm) on the exterior and shall be installed in not less than two coats, or per binder manufacturer’s instructions. Not less than [3] / 8 inch (10 mm) exterior plaster is permitted behind exterior cladding in accordance with Section BL104.6.

BL104.3.1 Membranes. Membranes are prohibited between plaster and hemp-lime, except where a membrane is allowed or required elsewhere in this appendix.

BL104.3.2 Lath and mesh for plaster. The surface of the hemp-lime functions as lath, and other lath or mesh shall not be required, except as required in Section BL104.5.

BL104.3.3 Plaster additives. Additives shall be permitted to increase plaster workability, durability, strength or water resistance. Additives shall not reduce the plaster vapor permeance rating to less than 5 perms [285 mg/(s × m [2] × Pa)]. Additives containing polymers are prohibited.

BL104.3.4 Plaster reinforcing fibers. Reinforcing fibers shall be permitted in plaster. Acceptable reinforcing fibers include hemp fiber, chopped straw, sisal, animal hair and fiberglass. BL104.3.5 Lime plaster. Lime plaster is any plaster with a binder primarily composed of calcium hydroxide [Ca(OH) [2] ], including Type N or S hydrated lime, hydraulic lime, natural hydraulic lime or slaked quicklime. Hydrated lime shall comply with ASTM C206. Hydraulic lime shall comply with ASTM C1707. Natural hydraulic lime shall comply with ASTM C141 and CEN EN 459. Quicklime shall comply with ASTM C5. Lime plaster shall contain sufficient lime to fully bind the sand or other aggregate and shall be permitted to contain pozzolans.

BL104.3.6 Clay plaster. Clay plaster shall be any plaster having a clay or clay subsoil binder. Such plaster shall contain sufficient clay to fully bind the sand or other aggregate.

BL104.3.6.1 Clay subsoil requirements. The suitability of clay subsoil shall be determined in accordance with Figure 2, Ribbon Test, and Figure 3, Ball Test, in the appendix of ASTM E2392/E2392M. BL104.3.6.2 Thickness and coats. Clay plaster shall be not less than [3] / 4 inch (19 mm) thick and shall be applied in not less than two coats.

BL104.3.6.3 Rain-exposed. Clay plaster, where exposed to rain, shall be finished with an approved erosion-resistant finish.

BL104.3.6.4 Prohibited finish coat. Plaster containing Portland cement shall not be permitted as a finish coat over clay plasters.

BL104.3.7 Clay-lime plaster. Clay-lime plaster shall be composed of refined clay or clay subsoil, sand and lime.

BL104.3.8 Hemp-lime plaster. Hemp-lime plaster shall be composed of hemp hurd and lime and shall be permitted to contain sand or other aggregate, and pozzolans.

BL104.3.9 Hemp-clay plaster. Hemp-clay plaster shall be composed of hemp hurd and clay or clay subsoil and shall be permitted to contain sand or other aggregate.

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

APPENDIX BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

Wood framing at the exterior surface of hemp-lime walls shall be separated from exterior plaster with Grade D paper or other approved material, except where the wood is naturally durable.

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

Bridging shall be installed onto and across dissimilar substrates prior to the application of plaster on the interior or exterior. Acceptable bridging materials include expanded metal lath, woven wire mesh, welded wire mesh, fiberglass mesh, reed mat, burlap or other approved materials. Bridging shall extend not less than 3 inches (76 mm) on both sides of the juncture.

BL104.5.1 Returns on recessed openings. Plaster or other exterior finish returns at recessed windows and doors shall require an approved design that prevents the intrusion of moisture.

Nonplaster exterior cladding shall be spaced not less than 1 inch (25 mm) from the face of the water-resistive barrier or air barrier to the back of the cladding to allow for ventilation. The ventilation space shall be open at the top and bottom and be provided with insect screening.

BL104.6.1 Water-resistive and air barriers. Water-resistive barriers and air barriers, when vapor permeable, are permitted to be applied directly to the hemp-lime when exterior cladding is installed in accordance with Section BL104.6.

Exterior hemp-lime walls enclosing showers or steam rooms shall be lined on the interior side with ceramic tiles on an approved tile backer board, ceramic tiles on a lime plaster, or a tadelakt finish.