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

Overview

This guide covers the Earthaus BUILD Plaster System over wood fiberboard insulation for exterior applications. Wood fiberboard insulation boards (such as STEICO, GUTEX, and Hempitecture PlantPanel X) are increasingly used as exterior continuous insulation in high-performance wall assemblies. The Earthaus BUILD system provides a durable, breathable lime plaster exterior finish directly over these boards, with no metal lath or traditional stucco preparation.

A note on building codes: this system is not prescriptive code stucco (ASTM C926) and is not EIFS; in the United States and Canada it is typically approved per project under the code's alternative materials provisions. Talk with your building official before purchasing materials, and see Earthaus Technical Bulletin TB-05: Code Pathways for Lime Plaster over Wood Fiberboard Insulation for the approval pathway and supporting documentation.

Key Properties:

Bond Plaster bonds mechanically and chemically to the wood fiber surface; alkali-resistant fiberglass mesh embedded across the full field reinforces the entire surface, including board joints, and resists cracking from thermal and structural movement.
A breathable system that allows the wall assembly to dry to the exterior, maintaining the vapor-open performance of the wood fiberboard insulation.
Slow and steady lime curing allows flexibility during early structural movement and reduces cracking.

Exterior System (Wood Fiberboard):

Layer	Product	Thickness	Coverage
Bond Coat (with fiberglass mesh)	Bond Plaster + fiberglass mesh	1/4" notch	50 ft²
Brown Coat	PHL Binder (2:1 sand to binder)	2 lifts to 1/2" ground	85 ft²
Finish option 1	Lime Stucco Coarse	1/4" (1 coat)	50 ft²
Finish option 2	Lime Stucco Fine	1/8" (1 coat)	100 ft²
Finish Color	Mineral Pigment Packs	1 pack/bag	–
Sealer (optional)	Breathable Sealer	1 coat	400/1600 ft²

Total system thickness: 5/8" (Fine Finish) or 3/4" (Coarse Finish). Working conditions for all coats: apply between 40°F and 85°F, and do not apply if temperatures may reach freezing within 72 hours of application.

Accessories

The exterior system requires stucco accessories with a 1/2" ground. Standard EIFS accessories (1/16" ground only) are not suitable.

Casing bead — metal casing bead with 1/2" ground at all stucco terminations: window and door perimeters, wall-to-roof transitions, and material changes. Example product: ClarkDietrich SFCB or equivalent.
Weep screed — 1/2" ground weep screed at the base of all exterior plaster walls per local building codes. Maintain minimum 4" clearance to finished grade, or 2" above paved surfaces.
Outside corner reinforcement — straight corner bead at all outside corners; pre-coat the flange with Bond Plaster before the full Bond coat to embed and partially fill the bead profile. Example product: Structa V-Truss Straight Corner Bead (ClarkDietrich) or equivalent.

Wood Fiberboard Substrate Preparation

Inspect the Boards — Check for defects, damage, or gaps; joints should be tight. The surface must be free of dust, dirt, or contaminants. Brush or vacuum to remove debris.
Verify Board Installation — Confirm boards are properly fastened per manufacturer guidelines with no movement or deflection. Verify the weather-resistive barrier (WRB) behind the boards is properly lapped.
Install Accessories — All casing beads, weep screeds, and corner beads must be installed before plaster application. Tack with T-50 staples or screws for initial placement.
Substrate Compatibility Test — Test per the Bond Plaster Installation Guide. Apply a small area and allow to dry 48 hours. Dry Bond Plaster should resist pulling away when pried with a putty knife.
Protect Surrounding Areas — Mask windows, doors, wood trim, and adjacent materials.

Bond Plaster Installation Over Wood Fiberboard

Bond Plaster is applied by hand with hawk and trowel; a 1/4" x 1/4" x 1/4" square notch trowel is required for field application. See the Bond Plaster Installation Guide for additional reference.

Set Accessories with Bond Plaster — Bed all accessories with Bond Plaster before the full field coat, pressing firmly through mesh and perforated flanges into the board. For V-Truss corners, pre-coat the flange to partially fill the bead profile. Allow to dry minimum 24 hours.
Apply Bond Plaster with the Notch Trowel — Work in horizontal sections approximately 3 feet tall, corresponding to the width of the mesh and the working reach from scaffolding. Using the 1/4" square notch trowel, apply Bond Plaster to one section in even, horizontal passes. The notch trowel creates a uniform 1/4" bed for embedding the mesh. Coverage: approximately 50 ft² per bag.
Embed Fiberglass Mesh — Immediately embed alkali-resistant fiberglass mesh (minimum 4.5 oz/yd²) into the wet section, running the mesh horizontally. Press into the ridges with a flat trowel, collapsing the ridges and fully burying the mesh; it should not be visible. Overlap mesh joints minimum 4". At outside corners, lap the field mesh onto the corner bead flanges; the corner bead provides the corner reinforcement. Do not wrap field mesh around the corner.
Apply the Second Notch-Trowel Pass — With the mesh fully buried and the section still wet, apply a second pass with the 1/4" square notch trowel over the embedded mesh, then move on to the next section. This creates the scarified texture (mechanical key) for the brown coat; no additional scarification is needed.
Dry — Allow the Bond Plaster to dry minimum 24 hours. Protect from direct sun, wind, and freezing.

PHL Binder Brown Coat Installation

The brown coat fills to the 1/2" accessory ground and provides the base surface for the Lime Stucco finish.

Mixing: PHL Binder with local sand at 2 parts sand to 1 part PHL Binder by volume. See the PHL Binder Application Guide for sand selection and mixing. Coverage: approximately 85 ft² per bag.
Application: two wet-on-wet lifts with hawk and trowel. The first lift fills the notch-trowel ridges and builds an initial base; the second fills to the 1/2" ground. Use accessory edges as screed guides.
Low suction: Bond Plaster has low suction due to its acrylic-modified formula, so the brown coat stays wetter longer than on a traditional scratch coat. A gentle sponge-float pass after the second lift helps open the surface and promote initial set.
Float out: allow the brown coat to firm up before floating. At approximately 65°F, expect 8–12 hours; timing is temperature-dependent. Use a wet sponge float for a uniform, open texture.
Finish prep: for Lime Stucco Coarse Finish, sponge float to a sandy, open texture. For Lime Stucco Fine Finish, sponge float, then knock down with a trowel to embed sand grains. Flat and tight, but not burnished.
Cure 10 days before the Lime Stucco finish coat.

Applying the Finish Coat

Apply Lime Stucco Coarse Finish (1/4") or Lime Stucco Fine Finish (1/8") over the cured brown coat. Pigment color and the optional Breathable Sealer are listed in the system table above. See the Lime Stucco Application Guide for comprehensive finish application instructions.

Calculate How Much You Need

Bond Plaster: 1 bag per 50 ft² (no calculator; order by wall area)
PHL Binder (brown coat): PHL Binder Calculator
Exterior finish: Lime Stucco Fine Finish Calculator or Lime Stucco Coarse Finish Calculator

Technical Support

Product questions: email [email protected]
Technical consultation: Earthaus technical consultation