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Plastering and Finishing Straw Bale Walls

2 min read

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Plastering & Finishing
Sub-Topics
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Earth Plaster (Interior)
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Lime Plaster (Exterior)
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Surface Preparation & Mesh
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Application Technique

Plaster is what makes a straw bale wall a building, protecting the bales from moisture, fire, rodents and insects. Use earth plaster on the inside and lime plaster on the outside, never cement, in three coats, over trimmed bales whose moisture content is below 20%.

Part of Straw Bale Construction

Plastering and finishing covers the bale wall with breathable earth and lime coats that protect it and stiffen it.

Plaster Is Protection, Not Decoration

Without plaster, bales are vulnerable to moisture, fire, rodents and insects. With proper plaster they are protected from all four. The coat choice is about vapour: interior earth plaster absorbs and releases moisture and bonds to straw; exterior lime plaster sheds rain but is breathable, so moisture in the wall can escape. Cement plaster on the outside traps moisture and causes hidden rot.

The topics in this group are:

For plaster in general see Plaster and Rendering.

What You Need

  • A dry wall (below 20% moisture) with the roof and overhangs in place.
  • Clay-rich subsoil, sharp sand and chopped straw for earth plaster.
  • Lime putty and sharp sand for exterior plaster.
  • Trimming tools, water sprayer, trowels and a hawk.

Method: The Order Of Work

Step 1: Check the wall is dry, below 20% moisture; see Bale Moisture Monitoring.

Step 2: Trim the faces flat and even.

Step 3: Dampen the wall so plaster does not dry too fast.

Step 4: Apply the first coat, press into the straw and score. The full system is three coats: scratch, brown and finish.

Step 5: Let it cure, then build the second and finish coats.

Step 6: Leave no exposed straw, particularly around windows, doors and the roof junction.

Common Mistakes

  • Cement plaster outside. It traps moisture.
  • Plastering wet bales. Sealed-in moisture causes rot.
  • Insufficient roof overhang. Rain on plaster eventually penetrates.

Safety

Lime burns and trimming tools

Lime is strongly alkaline; wear goggles and gloves, and if it gets in an eye flush with clean water for at least 30 minutes and seek care. Chainsaws and hedge trimmers used to trim bales throw dust and can kick back: wear eye and ear protection and a dust mask.

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