A loose straw bale burns readily, but a plastered straw bale wall has been tested to 2-hour fire-resistance ratings, better than conventional wood-frame construction, because the dense plaster starves the compressed straw of oxygen. The rating depends on there being no exposed straw anywhere, especially around windows, doors and where the wall meets the roof.
Part of Straw Bale Construction
The plaster coat is the fire protection of a straw bale wall, so continuity of the plaster is the whole safety question.
The Barrier Is Only As Good As Its Gaps
Compressed straw burns slowly because little oxygen reaches it, but loose straw, or a bale with an opening to the air, burns fast. The plaster keeps oxygen out and flame off. The parent article says plastered bales have been tested to 2-hour ratings, and that the key is no exposed straw after plastering. It does not state the plaster type, coat thickness or test conditions behind that rating, so do not assume that a thin, cracked or partly finished coat reaches it.
The places where straw gets exposed are the places where the plaster stops: the reveals of windows and doors, the junction of wall and roof, the top plate, any pipe or flue that passes through the wall, and the base. Close every one of them with plaster or a non-combustible cover.
What You Need
- Complete three-coat plaster; see Application Technique.
- Plaster or other non-combustible material to close reveals, the roof junction and penetrations.
- Flues and stovepipes kept clear of straw, with the clearance your local practice requires for combustible walls.
- A regular inspection of the plaster for cracks that expose straw.
Method: Inspect For Exposed Straw
Step 1: Walk the building looking for any place where straw shows: openings, eaves, penetrations, base, cracks.
Step 2: Close each gap with plaster pressed into the straw and keyed to the surrounding coat.
Step 3: Treat penetrations such as electrical boxes and pipes by sealing around them with plaster.
Step 4: Keep heat sources away. A stove, flue or lamp should not touch or lean on the plaster.
Step 5: Repair cracks as soon as they show.
Common Mistakes
- Leaving straw showing at a window reveal.
- Assuming “plastered” means “finished” while the roof junction is still open.
- A flue through the wall with no gap.
- Cracked plaster left unrepaired.
Safety
Unplastered straw burns fast
During construction, bales are unprotected: no smoking, open flames, hot work, grinding or welding sparks near stacked or trimmed bales, and keep a bucket of water and a shovel at hand. After plastering, any exposed patch is a path for fire into the wall, so close it. In a fire, a smouldering bale can keep burning inside a wall that looks intact: watch the wall for hours afterward.
What’s Next
- Learn next: Bale Moisture Monitoring
- Combine with: Waterproofing and Fire Safety — the whole set of wall protections