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When an interior and exterior skin of rigid plaster is applied to the dense insulated core of straw bales, a wall system is created that is more than the sum of its parts. A plastered bale wall creates what engineers call a stressed skin panel or sandwich panel, and it has impressive structural capabilities. As leading straw bale engineer Bruce King says: “It is essential to understand that, once plaster is applied directly to either or both bale surfaces,with or without reinforcing mesh, the structure is now a hybrid of straw and plaster. Effectively, any further loading — snow, people, wind, earthquakes — will go mostly, or entirely, into the plaster skins. So the assembly consists of strong, brittle, thin ‘concrete walls’ braced by, and somewhat elastically connected by, the straw bale core.”

4.1: The unplastered bales are capable of carrying the roof loads (left side of building), but once the plaster skin is applied, the plaster does most of the work.

Pioneering bale engineer Bob Platts continues this thought: “The proven Nebraska [load-bearing] structure is not, despite appearances, a stacked block structure. At most, the straw bales take little more than the dead loads only, while the rigid skins — stucco or plaster, chicken wire reinforced — must accept all of any live loads. Any imposed in-plane loads will be taken by the relatively high modulus skins with very little further deflection; such small further strains will scarcely be resisted by the straw. In fact, the still yielding straw does finally adjust to take little of the dead loads either, after a week or two have passed. The rigid skins prevent further settling overall; the straw ‘creep’ manifests itself as stress relaxation; the dead loads are passed from the straw to the unyielding skins.As commonly built, the straw bale house is a stressed skin structure, of the structural sandwich type, in which the straw is simply the core which stabilizes the skins against buckling under load, takes the shear loads, and provides thermal insulation.”

So as you read on, remember that all references to the performance of straw bale walls include the inherent characteristics of the entire structural sandwich, and not just a stack of bales.

Fun Facts: Scientific answers to the most common straw bale questions

Q: Won’t the big bad wolf be able to blow this house down?

A: “Both walls tested withstood the maximum static air pressure that was applied, representing a significant wind of over 134 mph (60m/s).” — ASTM E72 transverse load testing of load-bearing straw bale walls, Building Research Centre of the University of New South Wales, Australia, 1998

Q: Are these walls strong enough to hold up a roof? What about a second story?

A: “Two-string bale walls average an ultimate strength of 6156 pounds per lineal foot, exceeding ASTM E72 requirements.” — ASTM E72 compression test of plastered straw bale walls, University of Colorado at Boulder, 1999

Q: Won’t these walls be easy to burn?

A: “Bale walls withstood temperatures up to 1,850°F for two hours.” — Fire safety tests, National Research Council of Canada

“The bale panel was tested for over two hours and withstood temperatures that reached 1942°F. The temperature rise on the unheated side averaged less than 10°F.” — ASTM E-119 Fire Test, SHB AGRA, New Mexico, USA, 1993 ☞

Q: What is the insulation value of a straw bale wall?

A: The R-value varied from 30 to 40. The R-value of the straw bale walls is in the range of super efficient homes. — Canadian Society of Agricultural Engineers, Halifax, NS, Canada

Q: Won’t a bale home be likely to rot or mold?

A: Straw bale walls do not exhibit any unique propensity for moisture retention. It is clear that straw bale walls can function, without incorporating an interior vapor barrier, in northern climates. — Strawbale Moisture Monitoring Report, submitted to the Canada Mortgage and Housing Corporation (CMHC) by Rob Jolly, 2000

Q: Can straw bale techniques really make a difference to the environment?

A: The embodied energy for the conventional frame house was 509,000 KBtus. The embodied energy for the low-impact straw bale house is 41,000 KBtus, or about one twelfth that of the frame house. — Investigation of Environmental Impacts, Straw Bale Construction, by Ann V. Edminster, University of California, Berkeley, 1995.

Q: Will a bale home be less expensive to heat?

A: Straw bale construction, along with appropriate building conservation technologies and simple passive solar design, could provide up to a 60 percent reduction in building heating loads over current practice. — US Department of Energy (DOE) straw bale assessment program, 1995