This Help Center article is a summary of the official Earthaus Hempcrete Wall System Guide. Download the full Hempcrete Wall System Guide (PDF).

Overview

The Earthaus BUILD Pozzolanic Hydraulic Lime (PHL) Plaster System is designed for hempcrete construction. It is standardized but versatile enough to adapt for unique project needs.

Key Properties:

High-plasticity lime + pumice pozzolan for workability, adhesion, and durability.
Breathable plaster system: allows hempcrete to release moisture, reducing mold and structural risks.
Slow, steady curing provides flexibility during early structural movement.

Exterior System (Hempcrete Walls):

Layer	Product	Thickness	Coverage
Scratch Coat	PHL Binder	1 coat: ⅜"	65 ft²
Brown Coat	PHL Binder	1 coat: ⅜"	75 ft²
Exterior Finish option 1	Lime Stucco — Coarse	1 coat: ¼"	50 ft²
Exterior Finish option 2	Lime Stucco — Fine	1 coat: ⅛"	100 ft²
Finish Color	Mineral Pigment: Bulk or Pigment Packs	–	–
Sealer (optional)	Breathable Sealer	–	TBD

Interior System (Hempcrete Walls):

Layer	Product	Thickness	Coverage
Scratch Coat	PHL Binder	1 coat: ¼"	100 ft²
Brown Coat	PHL Binder	1 coat: ⅜"	75 ft²
Interior Finish option 1	Sand Finish	1 coat: ⅛"	100 ft²
Interior Finish option 2	Stone Finish	1 coat: 1/16"	250 ft²
Finish Color	Mineral Pigment: Bulk or Pigment Packs	–	–
Sealer (optional)	Olive Oil Soap Sealer	–	700 ft²

Sand Aggregate:

PHL Binder requires the addition of local sand aggregate, which improves structural integrity, increases plaster volume, and helps reduce shrinkage during drying (how to source and select local sand is provided in the PHL Binder Installation Guide).

Scratch Coat: one bag of PHL Binder + three 5-gallon buckets of sand aggregate
Brown Coat: one bag of PHL Binder + five 5-gallon buckets of sand aggregate

Hempcrete Substrate Preparation

Preparation is key to successful BUILD Plaster application on hempcrete walls. Every project will have unique considerations; consider the following when preparing for PHL Binder application:

Inspect the Walls — Thoroughly inspect hempcrete surfaces for defects such as cracks, holes, or uneven areas that might affect adhesion or appearance. The surface must be free from dust, oils, or any substances that could impair the bond. Light brushing or vacuuming removes surface debris; stubborn contaminants may require gentle cleaning with a damp cloth.
Fully Cured Hempcrete — The hempcrete must be completely dry and fully cured. Curing duration varies with wall thickness, ambient temperature, and humidity. Applying PHL plaster too early can lead to adhesion issues and possible damage to the hempcrete structure.
Dampen Walls Prior to Plaster Application — Hempcrete is porous and can rapidly absorb water from the plaster, leading to poor adhesion. Pre-dampen the hempcrete immediately before plastering with a fine mist of water, leaving the surface moist without visible standing water or runoff.
Plaster Stops and Lath — Use adequate plaster stops where plaster terminates or meets other materials. Protect dissimilar materials like wood with felt paper covered with expanded metal lath to prevent cracking. Recommended: embed fiberglass reinforcing mesh in the brown coat to reduce cracking.
Protect Surrounding Areas — Cover and protect windows, doors, and any other elements that should not be plastered.

PHL Binder Installation Over Hempcrete

The scratch and brown coats can be applied by hand (hawk and trowel) or by spray application. A plaster sprayer is preferred for the scratch coat as it embeds the plaster deeper into the hempcrete. With a hawk and trowel, "massage" the plaster into the hempcrete to ensure adequate adhesion. See the PHL Binder Installation Guide for comprehensive guidelines.

Scratch Coat:

Standard thickness: ⅜". While still wet, scratch horizontally ¼" deep with a plaster scarifier to maximize bonding with the brown coat.
The hempcrete should be completely covered with no hempcrete showing through.
Cure 10 days before applying the brown coat.

Brown Coat:

Standard thickness: ⅜". The brown coat applies the plaster to its final shape and builds/evens out the wall as needed.
Exterior finish: sponge float the brown coat for Lime Stucco Coarse; sponge float and smooth for Lime Stucco Fine.
Interior finish: sponge float and smooth the brown coat for Sand and Stone Finishes.
Cure 10 days before applying a finish plaster. See the Lime Stucco Installation Guide and Interior Finishes Installation Guide for finish plaster guidelines.

Applying the Finish Coat

After the brown coat has cured (10 days), apply your selected finish from the system table above:

Exterior finishes (Lime Stucco Coarse or Fine): see the Lime Stucco — Application Guide.
Interior finishes (Sand or Stone): see the Interior Sand & Stone Finishes application guide.

Pigment color and optional sealer are listed in the system table above.

Calculating Quantities & Pricing

Material quantities and pricing are calculated through the Earthaus website calculators (PHL Binder, Lime Stucco Fine, Lime Stucco Coarse, Sand Finish, and Stone Finish calculators). Quantities are based on standard coverages and thicknesses; coverage may vary with thicker or thinner coats and substrate condition.

Use the Earthaus calculators to estimate materials:

PHL Binder (scratch & brown coats): PHL Binder Calculator — select your wall system in the calculator.
Exterior finish: Lime Stucco Fine Calculator or Lime Stucco Coarse Calculator.
Interior finish: BUILD Sand Finish Calculator or BUILD Stone Calculator.

Technical Support

Product questions: email support@earthausplaster.com
Technical consultation: Earthaus technical consultation