a roof and keep out the weather, at least for a while, but when the walls are plastered, they take on many impressive properties.
It’s a Sandwich, But Don’t Eat It
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
Fire
Straw bale walls have been tested for fire resistance. In these tests, straw bale walls have proved themselves far superior to standard wood framed walls.
Why Don’t They Burn?
Straw bale walls are naturally fire resistant.While the dry straw that makes up a bale is easily combustible when loose, the compact nature of a bale does not trap enough air to support combustion. A good analogy can be drawn by comparing the combustibility of a single sheet of newsprint to that of an entire telephone directory. The single sheet will ignite and burn quickly, but if you drop a phone book on your fire, you’ll probably put out the fire. The plaster coating effectively seals the already fire-resistant bales inside a noncombustible casing. A fire would have to burn through the plaster in order to reach the straw. When plaster is combined with the thickness of a bale wall, fire resistance is enhanced. Far from being an issue of concern, the fire testing performed around the world shows the fire-resistant nature of bale walls to be another impressive advantage of the system!
Don’t Ignore the Real Fire Hazard
All of the above is true of baled, plastered straw. Unbaled, loose straw, however, is extremely combustible, and the large amounts of loose straw that accumulate during construction are a serious fire hazard. Smoking, welding, grinding, or any other spark- or flame-producing activities should not be undertaken in the vicinity of loose straw. During construction, always be sure to have fire extinguishers and enough water available to deal with potential fires in your loose straw. Also, keep loose straw raked away from the walls to minimize the risk of any accidental fire spreading into the unplastered walls.
This is not just an idle warning. Several bale homes have burned to the ground while the straw was exposed and the site covered in loose straw, ignited most often by work site activity like soldering pipes and metal grinding.
4.2: This single, plastered bale shows how the plaster penetrates into the bale, gripping the straw. The two “columns” of plaster carry most of the loads imposed on the finished walls.
Loose straw (especially organic straw) makes excellent garden mulch. Rake it up, keep it under control,
