More Straw Bale Building. Peter Mack
hold up to 520 well-stacked bales, which is usually enough for houses up to 2000 square feet. Special bale trailers (used by hay brokers) hold up to 720 bales, are low to the ground, and are accessible on three sides for easy unloading.
In the previous version of this book, we gave instructions for storing bales on your building site. Creating a proper bale stack, one that will stay dry and stable, is more work than we’d recommend taking on unless you have no other option. Outdoor stacks must be elevated from the ground with a moisture barrier under-neath.
3.6: A transport trailer is a great way to move and store bales. The trailer can be rented and left on your site until you need the straw, eliminating one round of loading and unloading.
3.7: From the back of a transport trailer onto an elevator and immediately into the wall: the best way to do it!
The stack should be made with a peak at the top to encourage water runoff, and well-secured, waterproof coverings must be in place. We have yet to see an outdoor stack that hasn’t lost some bales to moisture, and they have definitely been responsible for some builders losing sleep on windy rainy nights.
Post and beam builders may be able to stack their bales under the completed roof of their building. Remember that bales must still be elevated and protected from water and dampness on or in the floor.
Purchase More than You Need
Plan to have extra bales on hand to compensate for broken, poorly tied, or damp bales in the stack. Extra bales can come in handy around the building site as step stools, makeshift work benches, boot scrapers, and comfortable seats during breaks.
Handling Bales
You should take some simple precautions when working with bales. Lift by their strings and wear gloves to avoid pinched or sliced fingers. Longsleeved shirts cut down on straw scratch to your arms. Remember to use proper lifting techniques to avoid hurting your back. Depending on how and where they were harvested and stored, your bales may be quite dusty; if so, use adequate breathing masks (proper respirators, not flimsy dust masks).
In all our time building with bales, it has been very rare to come across bales that aren’t suitable for building. So don’t worry too much. Focus instead on good relations with the farmer, and proper transportation and storage schemes.
The color, size, shape, and smell of bales make them unlike any other building material. They’re fun to play with, so go ahead and play.
CHAPTER 4
The Red Flag Questions
What about fire? What about moisture? What about insects and pests? What about mortgages and insurance? What about longevity? These are the most frequently asked questions about straw bale construction. They all have answers that should alleviate your concerns.
How Straw Bale Walls Work
Before addressing common concerns about straw bale walls, we need to explain how straw bale wall systems actually work. To be accurate we should really use the term plastered straw bale walls. The plaster — and there are many different kinds — is what seals the bales against fire, rain, wind, bugs, and big bad wolves. The plaster also gives the wall much of its rigidity and strength. Stacked bales alone can hold up 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