The two most common questions we encounter from individuals exploring straw bales as a building option are: “Do they pose a fire hazard?” and “Are they going to be a haven for mice and vermin?” This is a cursory overview of both issues which we will be dealing with in much more detail in future postings.
Straw bales are naturally fire resistant. The dry straw that makes up the bale is very combustible when loose, but compressed in the form of a bale, the straw does not trap enough air to allow easy or rapid combustion. Building-grade bales will burn and char on the surface but their density resists combustion. Straw bales are never left exposed on any sides in a properly designed straw bale building. There is a layer of cement-lime stucco on both sides of the walls along with a fire-stop assembly at the top of each wall section, between floors and roof systems.
A properly designed, constructed and stuccoed bale wall will give a fire rating that can far exceed a fire rated frame wall. Fire safety tests performed by the Research Council of Canada in 1984 withstood temperatures up to 1,850 F for two hours*. The Canadian result was confirmed by ASTM E-119-05a tests in New Mexico in 1993 and Texas in 2006 easily passed to obtain a two hour fire rated wall. The two hour fire rating is double the time required for residential structures and is the standard for commercial buildings in Canada and the U.S.A. The construction techniques and attention to detail on our projects is also a testament to the lasting fire retardant characteristics of a well-design, well-built straw bale building.
Straw bale walls are no more susceptible to an infestation of mice or other vermin than a standard exterior frame wall filled with fiberglass, foam or cellulose insulation. Making sure the walls are properly constructed and not providing holes or entry points are the best defense against the invasion of these creatures or other unwanted pests. Another common misconception is that straw bales provide a food source for mice. Straw is the dried stems from grain crops, and combined and baled properly they do not contain any food source or for rodents.
It is properly designed structural systems, attention to detail during baling, and application of cement-lime plasters that give straw bale buildings their superior fire ratings and make them impervious to rodents or other pests.
*Magwood, Chris, Peter Mack and Tina Therrien. More Straw Bale Building: A Complete Guide to Designing and Building With Straw. First Edition. Gabriola Island: New Society Publishers, 2005. Page 32.