Fire Resistivity
In order to achieve a Class A fire resistance rating, a building material needs to pass the ASTM E84 test with a Flame Spread Index of 0-25, and a Smoke Developed Index of 0-450. The building materials in the following table are all appropriate choices for Passive House applications, and all have a Class A fire rating…
* a building material with both FSI and SDI values of 0 is considered noncombustible ** test data is not available for straw/clay, but FSI and SDI values will be less than those of straw bales because the straw fibers in straw/clay are coated with noncombustible clay
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 these materials 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?
The density of a straw-bale wall is such that very little air is available to support combustion — it's like trying to burn a phone book.
Unlike baled straw, loose straw is highly flammable! Care should be taken on the construction site to minimize fire hazards from loose straw — 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.