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Contrary to western movie images of bale-filled barns bursting into flames, straw bales are in fact very fire resistant. Combustion requires oxygen and a plastered straw-bale wall just does not contain enough air to keep a fire going. Fire safety tests by sponsored by the National Research Council of Canada showed that plastered straw-bale walls withstood temperatures up to 1850°F for 2 hours before a small crack appeared in the stucco. In laboratory fire tests conducted at the Richmond California Field Station in 1997 by students at UC Berkeley a straw-bale wall at passed the ASTM E-19 test for a 1-hour wall. Similar testing in New Mexico showed only 2” of charring into the straw where the plaster had cracked and a temperature rise of only 12° F after two hours on the far side of the wall panel. Even unplastered bale wall panels are fire resistant. In New Mexico testing, a burning through a panel took 34.5 minutes with temperatures over 1550° F. Loose or exposed straw, however, is vulnerable to fire and can combust. A few straw bale buildings have burned during the construction phase, so builders should take precautions to maintain a clean site, avoid heat/spark producing activities, protect the site against arson, and provide a water supply adequate to fight a fire should it occur. Some builders have taken the extra precaution of using a commercial fire retardant or a simple spray-on solution of borax and boric acid. Borax has the added advantage of fighting fungal infestations and some insects. |
