the sound isolation of a building member between two acoustically separated chambers, with the test sample placed in an aperture between the chambers. Although we were aware of the limitations of the test facility for testing a wall system, we endeavored to make the test as accurate and as representative as possible. The size of the aperture (ISO standard) is 1.88m2 (18 sq. ft.). The tested bale wall section had the following configuration:
• two-string bales laid flat (density 120-130kg/m3)
• earth/clay straw plaster between 25mm and 35mm (1 – 1.5 in.) thick (intentionally asymmetrical plasters)
• no reinforcing plaster netting or mesh or any form of pinning
The chosen sample structure was to be representative of a normal earth/clay plastered bale wall structure, as used by experienced builder Rob Kaptein of RAMstrobouw, who was also responsible for manufacturing the test sample. The graph and table summarize the test result.
The result can be expressed as 55 decibel (dB) A-weighting, which approximates human hearing sensitivity. This result might seem low, but in fact it is very good. Most conventional wall systems — including a brick cavity wall with much higher mass — have a lower performance. Specifically interesting to note is the 2 – 3dB better performance at very low frequencies of the bale test sample when compared to conventional brick cavity walls. Heavy mass like a meter of concrete is still necessary for very low frequencies, i.e., less than 60 Hz.
A recipe for good acoustic isolation with a straw bale wall is: besides mass, low stiffness with sufficient mass, and acoustic decoupling. The relatively low stiffness of a bale wall with earthen plasters is ideal. The fact that the cavity between the two outer plaster shells is filled with straw provides excellent acoustic damping. Care must be taken to fill all cavities and voids with very light straw/clay. Avoid any direct mechanical contacts between the inner and outer plaster shells, as these will seriously degrade sound-damping performance. Contrary to what you might expect, loosely packed bales will perform better than very tightly packed bales (rice straw, due to its floppy nature, is ideal). Pay a lot of attention to all openings and edge details; these are the weak points. An air leak of only 1mm2 will seriously degrade performance. Door openings and windows are literally acoustic holes in the wall. These need special detailing and attention to even remotely approach the performance of the walls.
Here are some simple rules of thumb about room acoustics, depending on the type of acoustics you want to create. Soft acoustic instruments require a live room. Loud amplified sounds are better in a dampened room. The single most important parameter is the reverberation time and level. The harder the surfaces, the livelier the sound. A tiled bathroom is lively, hence your desire to sing (even if you can’t!). Standing on top of a snowbound hillock gives the opposite effect. The bigger and harder the room, the longer the reverberation time. An oblong box approaches the ideal relative room dimensions, preferably the dimensions relate to each other at the ratio of approximately 2:3:5. This ratio will avoid the formation of predominant harmonic resonances and standing waves. The exact ratios depend on the size and acoustic reflectivity. I personally prefer rooms without parallel surfaces, thus avoiding standing waves. I think if you finish a room with earth/clay stucco on bale walls, with wooden flooring and a well-pitched ceiling, you will have quite acceptable acoustics for acoustic performances.
In conclusion, I would like to stress the following: Due to the nature of a bale wall (homogenic continuous surface), the wall itself won’t be a problem acoustically, but the connections between the wall and all other structures, incorporated or surrounding, require proper detailing and careful execution.
— René Dalmeijer
Estimated embodied energy (production only) of some common materials:
• baled straw = 0.24 MegaJoules per kilogram (MJ/kg)
• fiberglass = 30.3 MJ/kg
• expanded polystyrene plastic (EPS) = 117 MJ/kg
• cement = 7.8 MJ/kg
• virgin steel = 32 MJ/kg
• recycled steel = 10.1 MJ/kg
• virgin aluminum = 191 MJ/kg
• recycled aluminum = 8.1 MJ/kg
(Source: Andrew Alcorn, Embodied Energy Coefficients of Building Materials, 2nd ed.)
As the list above shows, it can be quite an eye-opener to see the amount of energy industry expends on creating and supplying building materials. The numbers on this list show MegaJoules of energy consumed per kilogram of material, hence the higher numbers for the lighter materials and lower numbers for the heavier ones. But straw is both a lightweight material and it has a low embodied energy figure.
1.5: Other natural building materials, including tile and wood, can perfectly compliment the feel of plastered bale walls, and lower the potential toxicity of a home.
Environmental Benefits that Don’t Cost More ...
Many new green building products (many are patented and proprietary) are being introduced on the market, and most of them are higher priced than their conventional competitors. Straw bales are an environmentally friendly alternative that does not require sacrifices in terms of costs, appearance, or availability. Straw bale building provides that elusive, and increasingly necessary, alternative to wasteful, unsustainable modern practices.
... and Just Might Be Better for You
Straw bale walls offer a potential solution for those who find that the paints, chemicals, glues, and toxins embedded in manufactured building materials negatively affect their health. Organically grown straw coated with earth-based and/or lime plasters have received positive feedback from environmentally sensitive people.
Bales Aren’t Magic
Although the benefits of using straw bales are many, straw is not actually a magical material. This cheap and abundant cellulose fiber just happens to get packaged into conveniently sized rigid bundles that are suitable for building.When the mystique of building with bales has been stripped away, the truth is that straw bales simply allow for the creation of very thick walls without consuming the quantity of resources that would be needed to make equally thick walls of wood, fiberglass, or other materials.Any conventional building method, if used to build walls of the same thickness as a bale wall, would provide similar levels of performance, but at a much greater financial and environmental cost. Bales work — cheaply and sustainably!
If Bales Are So Good, Why Doesn’t Everybody Use Them?
The obvious advantages of building with bales gives rise to this common question. Passive resistance to bale construction comes from two sources: homeowners and the building industry.
Who Wouldn’t Want a Bale Home?
Big financial commitments like the building of a house usually inspire conservatism in even the most adventurous spirits. Conventional frame houses are widely accepted as the safest financial choice. Even those who are willing to invest their money in new ideas can face significant resistance from spouses, family, friends, lenders, architects, building inspectors, building supply yard employees, and a host of other cautious types. It takes a lot of spirit and resolve to overcome such personal obstacles, and many people do not pursue their ideals in the face of such resistance.
There’s No Money in Them There Straw Houses
The building industry has not yet embraced straw bale construction