Wall Infill: Light Straw-Clay

Why Light Straw-Clay?

Traditional cob (dense clay-sand-straw mixture) presents challenges when used as infill between eucalyptus poles that shrink 5-8% as they dry.

The Problem with Cob Infill

Cob is rigid and heavy (~1,800 kg/m³). When eucalyptus poles shrink and shift during drying, cob cracks at the pole junctions — sometimes severely.

Light straw-clay solves this by reversing the ratio: instead of clay with straw reinforcement, it’s straw with clay coating. The result is a lightweight (~400 kg/m³), flexible infill that:

• Absorbs frame movement internally rather than cracking
• Dries faster (4-6 weeks vs 2-3 months for cob)
• Easier to repair (stuff more straw into gaps)
• More forgiving to mix (less skill required)
• Reduces load on frame (1/4 the weight of cob)

The Recipe

Ingredient	Description	Purpose
Straw	Loose, long-fiber (wheat, rye, or rice straw)	Bulk material — creates insulation and structure
Clay slip	Clay + water to thick cream/yogurt consistency	Coating — binds straw, provides fire resistance

No sand. Unlike cob, light straw-clay uses pure clay slip without aggregate.

Making Clay Slip

1. Soak Clay Overnight

   Put clay soil in a container, cover with water. Let sit 12-24 hours to fully hydrate.

2. Mix to Yogurt Consistency

   Stir vigorously. Add water until the slip coats your hand and drips off slowly — like thick yogurt.

3. Strain if Needed

   If your clay soil has stones, strain through a coarse screen.

Coating the Straw

1. Toss Straw in Slip

   Work in manageable batches. Toss loose straw into slip and turn until all fibers are lightly coated. The straw should glisten but not drip.

2. Don’t Saturate

   More slip is NOT better. Saturated straw takes longer to dry and loses insulation value.

Packing the Wall Cavity

1. Work between wattle poles in 15-20cm lifts
2. Pack coated straw firmly into the cavity
3. Tamp with a flat board or your fist
4. Don’t over-compress (you want insulation value)
5. Continue until cavity is filled to the next wattle level
6. Final wall thickness: 35-45cm

Wall cavity cross-section: straw-clay infill between wattle poles

Tamping Technique

Tamp firmly but not aggressively. The packed wall should feel springy, not solid.

Drying Time

• Per 15cm depth: 1-2 weeks to dry through
• Total wall drying: 4-6 weeks
• Test for dryness: Push a thin stick into the wall — if it comes out damp, keep waiting
• Good ventilation: Essential during drying

Fire Resistance

Clay-coated straw is fire-resistant — the clay coating prevents the straw from igniting easily. In fire tests, light straw-clay chars on the surface but doesn’t sustain flame.

Repairs

• Small gaps from settling: Stuff with more clay-coated straw
• Gaps at pole junctions: Pack additional straw around poles after they’ve finished shrinking (typically after first year)
• Cracks in render: Standard lime render repair