Fire Testing Strawbale Wall Systems - Results
Over the past two decades, strawbale wall construction has transitioned from a niche practice to a recognized and sustainable building method, particularly praised for its fire resistance.
Early tests in the mid-2000s (read more below on this historic moment!) demonstrated that plastered strawbale walls could withstand fire exposure for up to two hours without structural compromise in a furnace test.
More recent studies have reinforced these findings, showcasing the reliability and safety of strawbale walls in fire scenarios. witpress.com
Here’s some updates from around the world to add to our communal knowledge of strawbale wall performance. (Do you know of more research sources? Please share in the comments to share with the strawbale community!)
Updated Research Supporting Fire Resistance
Fire Resistance of Strawbale Walls (2017): This study evaluated both load-bearing and non-loadbearing strawbale walls, both plastered and un-plastered. The findings indicated that such walls could endure temperatures exceeding 1000°C for two hours, aligning with international building codes. researchgate.net
Safety of Strawbale Walls in the Event of Fire (2020): Focusing on UK construction standards, this report detailed fire tests on strawbale walls with lime and clay plaster coatings. The results showed that these walls achieved fire resistance ratings between 120 to 135 minutes without failure, meeting the REI 120 to 135 classification. strawworks.co.uk
Fire Hazard of Compressed Straw as an Insulation Material (2020): This study investigated the fire hazards associated with compressed straw used as thermal and acoustic insulation within wood-framed structures. The research aimed to document fire hazards of compressed straw when used as thermal and acoustic insulation within wood-framed structures. onlinelibrary.wiley.com
Fire Test on Straw Bale Walls (2004): Conducted by the Czech Technical University in Prague, this research assessed various strawbale wall assemblies. The study confirmed that load-bearing strawbale structures could achieve significant fire resistance, withstanding standard fire exposure without losing structural integrity. CTU in Prague slide deck
Factors Contributing to Fire Resistance
Several inherent characteristics of strawbale walls contribute to their impressive fire resistance:
Density and Compression: Tightly packed straw bales contain minimal oxygen, hindering combustion.
Protective Plaster Coatings: Applying lime, clay, or cement plasters adds a fire-resistant barrier, enhancing the wall's ability to withstand fire exposure.
Low Flame Spread: The compact nature of strawbale walls limits flame propagation, reducing the risk of rapid fire spread.
Back in 2006…
Andrew Morrison received an email from Bill Christensen, the proprietor of GreenBuilder.com. It held some very exciting news on recent fire testing of strawbale wall assemblies. Of course, those involved in building strawbale houses had known that they were extremely resistant to fire; however, this was new proof to back it up!
Here is what Bill had to say:
“It’s my pleasure to announce that plastered straw bale walls have just passed two important fire tests.
Two walls were constructed at a building materials testing lab in San Antonio earlier this summer - one was built with poly-tied bales on edge and covered with two coats of hand-troweled cement/lime plaster over stucco mesh, intended to approximate an infill wall in a commercial or institutional (or residential) setting. The second wall was stacked flat, put under 600 pounds per linear foot load, and shot with two coats of clay plaster, approximating load-bearing residential construction. No mesh was used on this wall.
The cement/lime plastered wall easily passed the ASTM E-119 2-hour fire and hose stream test. Lab personnel told us they felt it probably would have made another hour easily.
Due to some cracking on the exterior (non-fire) side of the clay plastered wall, which occurred while moving the wall up to the furnace, and having seen the fireside plaster on the cement/lime wall basically being held on only by the wire mesh for the majority of the second hour, and considering that a 2-hour rating is rarely required for residential construction, we chose to do the clay plastered wall for just a one hour test. It, too, passed.
The official test reports will be available in about two weeks on the www.ecobuildnetwork.org and www.DCAT.net websites. They will also be included in the appendix of Bruce King’s forthcoming book, Design of Plastered Straw Bale Structures (due out in time for the ISBBC in Ontario in late September).
Next time some insurance company gives you a hard time about straw bale being a fire hazard, you have an answer for them.
Congrats to the team: Bruce King, David Eisenberg, Matts Myrhman, Ben Obregon, Frank Meyer, Kindra Welsh, Baron Cougar, Mike Atkinson, Paul Taylor, and yours truly.
Thanks also to CASBA, COSBA, SBAT, and the individual donors for their financial contributions, as well as to whatever foundation it was that Bruce King managed to get the grant from.”
Conclusion
The research continues to evolve and, over the past two decades, has solidified the understanding that strawbale walls are not only sustainable and energy-efficient but also possess remarkable fire resistance.
These findings provide confidence to builders, homeowners, and regulators in the safety and viability of strawbale construction.
I’m looking forward to seeing what further testing reveals to continually inform and improve best practices for building strawbale walls!
Happy baling,
Timbo
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Updated 03/2025