Interior earth plaster for straw bale walls is about 1 part clay-rich subsoil to 2-3 parts sharp sand with chopped straw for reinforcement, applied in three coats: a scratch coat 15-20 mm thick, a brown coat 10-15 mm thick, and a finish coat 3-5 mm thick. The first coat cures for 1-2 weeks at minimum, and a test panel should show the mix sticks without cracking as it dries.
Part of Straw Bale Construction
Earth plaster is a clay, sand and straw mortar that lines the inside of a straw bale wall.
Earth Plaster Breathes, Bonds To Straw, And Can Be Mended With The Same Mud
Earth plaster absorbs and releases moisture, which helps regulate indoor humidity. It bonds well to straw, uses free materials (subsoil clay and sand) and is repairable with the same material. The sand gives bulk and limits shrinkage; the clay binds; the straw reinforces. For testing clay see Clay Testing, and for a comparable mud mix see Adobe Mix.
What You Need
- Clay-rich subsoil, sharp sand and chopped straw.
- Water, a mixing surface or tub, and trowels.
- A notched trowel to score the first coat.
- A sprayer for dampening the wall.
- A trimmed, dry wall; see Surface Preparation and Mesh.
Method: Mix, Test, Three Coats
Step 1: Mix about 1 part clay-rich subsoil to 2-3 parts sharp sand, plus chopped straw.
Step 2: Make test panels of a few mixes on spare bale faces, and let them dry. Pick the mix that sticks without cracking as it dries.
Step 3: Trim the bale faces flat and even, then mist the wall.
Step 4: Scratch coat, 15-20 mm. Press it firmly into the straw, then score the surface with a notched trowel so the next coat grips.
Step 5: Cure for 1-2 weeks minimum.
Step 6: Brown coat, 10-15 mm, built up to a flat, even surface.
Step 7: Finish coat, 3-5 mm. It can be polished smooth or textured.
Worked Example
How much plaster does one wall take per coat? Take a wall 3 m long and 2.4 m high (assumed example), area 3 x 2.4 = 7.2 m².
- Scratch coat: volume = area x thickness = 7.2 m² x 0.015 m = 0.108 m³ up to 7.2 x 0.020 = 0.144 m³.
- Brown coat: 7.2 x 0.010 = 0.072 m³ up to 7.2 x 0.015 = 0.108 m³.
- Finish coat: 7.2 x 0.003 = 0.0216 m³ up to 7.2 x 0.005 = 0.036 m³.
These are volumes of finished plaster per coat; add allowance for wastage and for the wall face being rougher than a flat plane, and subtract any window and door area.
Common Mistakes
- Too much clay. The coat cracks as it dries.
- Too little clay. It dusts off.
- Skipping the test panels.
- Applying to a dry wall. It dries too fast; mist first.
- Putting a new coat on an uncured one.
Safety
Trimming and dust
Trimming bale faces with a chainsaw, hedge trimmer or shears throws straw dust and risks kickback or cuts. Wear eye protection and a dust mask, and keep the cutter away from your body. Dry clay and sand dust irritate lungs; wet the pile before mixing.
What’s Next
- Learn next: Application Technique
- Combine with: Lime Plaster (Exterior)