Fire Resistivity
Note: the flame spread index values are from tests conducted at various laboratories based on ASTM standards.
How resistive to fire are straw-bale walls compared to more conventional wall systems?
Surprisingly, unplastered straw-bale walls have a lower flame spread index than unfinished conventional walls. When examining finishes, drywall has a significant flame spread index when compared to earth and lime plasters, which have no flame spread index because earth and lime plasters are not flammable. A straw-bale wall assembly with earth or lime plasters therefore clearly offers superior fire resistance when compared to more conventional wall systems.
Why do straw-bale walls have such a low flame spread index?
A straw-bale is so dense that very little air is available to support combustion—it's like trying to burn a phone book.
Unlike baled straw, loose straw is a potential fire hazard. Care should be taken on the construction site to minimize this hazard—prevent activities that produce sparks or open flames, post "No Smoking" signs, have fire extinguishers and/or water readily available, and sweep up loose straw regularly.