Strawbale Construction Takes a Leap Forward with IRC Appendix S

Tue, 12/27/2016 - 10:17
by California Straw Building Association (CASBA) members: Lesley Christiana - Advisory Board, David Arkin - Architect and CASBA Director, Martin Hammer - Architect and Lead Author of Appendix S

The International Code Council (ICC) has officially recognized strawbale construction with the inclusion of Appendix S in the 2015 International Residential Code (IRC). This is great news for those who build, live in, or insure this durable and safe form of building, as the IRC is the basis for the residential code in virtually every jurisdiction in the United States.

With the trend towards zero net energy and non-toxic materials, and concerns about pollution and deforestation, it is a very timely development. Straw (not hay) is an abundant agricultural by-product that was typically burned in the field, causing seasonal air quality problems as well as releasing carbon into the atmosphere. Used as a building material instead, straw bales, stacked and then plastered, form an extremely well-insulated wall system that greatly reduces heating and cooling needs. Also, framing in strawbale walls is reduced, conserving lumber.

Properly constructed straw buildings are very fire and earthquake resistant. Straw bales are quite dense, and when covered with plaster, the lack of available oxygen prevents combustion; unlike the voids in wood framed walls, which can act as many chimneys. Strawbale walls tested with earth plaster and cement-lime plaster earned 1- and 2-hour fire ratings respectively, per fire test standard ASTM E119.

In an earthquake, strawbale walls demonstrate tremendous shock absorbing qualities. Homes near quakes ranging from 6.0 in California, to 7.8 in New Zealand, have shown that plaster may crack but structures remain sound. In a 2009 shake table test at the University of Nevada, in conjunction with the Center for Civil Engineering Earthquake Research (CCEER), a full-scale model of a home designed for an earthquake recovery project withstood twice the peak ground acceleration of the Northridge, CA 1994 earthquake.

The earliest strawbale structures were built in the state of Nebraska in the late 1800's. A renewed interest in this form of building began in the 1980's when it was found that those that had been maintained were still in good condition and in use 100 years later. The revival steadily gained momentum and there are now strawbale buildings in all 50 U.S.. states and in over 50 countries worldwide.

The California Straw Building Association (CASBA) has been at the forefront of this movement since 1996. CASBA members have worked to develop best practices for straw building by exchanging practical experience, research, and testing with the global straw building community. CASBA members - including architects, engineers, contractors, and homeowners - have educated many building and fire officials, and have developed strawbale building codes to fully gain acceptance of this method of construction. The presence of Appendix S in the IRC is expected to give the insurance industry the confidence it needs to provide insurance coverage as more people seek to build and insure strawbale homes. We look forward to serving as a resource for this purpose.

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