A straw bale wall stays dry and fire-resistant when it has good boots, a good hat and an unbroken plaster skin: a raised stem wall with a moisture barrier and drainage at the base, a roof overhang of at least 45 cm and preferably 60 cm or more at the top, and plaster that leaves no straw exposed. Loose straw burns readily, but a plastered bale wall has been tested to a 2-hour fire-resistance rating.
Part of Straw Bale Construction
Water and fire are the two threats to a bale wall, and both are controlled by the same details at the base, the top and the surface.
Two Threats, One Set Of Details
Water rots straw from inside the wall; fire burns loose straw fast. The wall defends itself with details, not with a single product:
- Good boots: an elevated stem wall, a moisture barrier and drainage away from the building; see Foundations and Stem Walls.
- Good hat: roof overhangs of at least 45 cm and preferably 60 cm or more; see Roof Overhang and Rain Protection.
- Unbroken plaster: lime outside, earth inside, no exposed straw; see Plaster as Fire Barrier.
- Moisture watch: because the failure is hidden, see Bale Moisture Monitoring.
With those protections plus proper lime plaster, the parent article says straw bale walls last indefinitely.
What You Need
- A stem wall, moisture barrier and drained ground at the base.
- Eaves that project beyond the plaster face.
- Complete plaster, including at openings and the roof junction.
- A way to check moisture.
Method: Check The Three Zones
Step 1: Base. Is the first bale clear of the ground by the stem wall height, and is the barrier in place?
Step 2: Top. Does the roof project far enough to keep rain off the plaster?
Step 3: Surface. Is any straw visible, especially around windows, doors and the roof junction?
Step 4: After the first wet season, look again at each zone.
Common Mistakes
- Insufficient roof overhang. Rain on plaster walls eventually penetrates.
- Cement plaster outside. It traps moisture.
- Gaps in plaster at openings. Fire and water find them.
Safety
Loose straw is fuel
An unplastered or cracked-open straw bale wall burns readily. Keep fires, welding and smoking away from bales until they are plastered, and repair any plaster that exposes straw.
What’s Next
- Learn next: Roof Overhang and Rain Protection
- Combine with: Plaster as Fire Barrier