building
project intro 
Design for Straw Bale building. An excellent book containing a lot of good technical information but written for the the average person. (click for larger image) |
The exterior walls of the building will be constructed with straw bales. When combined with a seperate supporting structure (ie. post-and-beam) this is called 'infill'. If the bales are used to support the roof and other floors, it is called 'load-bearing' or Nebraska-style from where it originated in the late 19th century.
The difference between straw and hay is that hay still contains the seeds, whereas straw is only the residual stalks of the plant. This is important because the seeds in hay cause two problems: they attract animals which can damage the walls, and they will begin to rot causing decomposition. Straw is very similar to regular wood in composition.
The difference between straw and hay is that hay still contains the seeds, whereas straw is only the residual stalks of the plant. This is important because the seeds in hay cause two problems: they attract animals which can damage the walls, and they will begin to rot causing decomposition. Straw is very similar to regular wood in composition.
Straw bale walls prior to plastering.
Advantages...
The benefits of using straw bales in construction are numerous, and include:
Building with straw does have some disadvantages. Foremost amongst them is the need to ensure that the bales remains dry both during construction, and throughout the life of the building.
Moisture is the 'Achilles heal' of straw bales. Levels above 20% will support the growth of mold which will gradually turn the straw into compost. However, this is not the same as humidity. Sustained humidity levels can be as high as 75 - 80% at 70 degrees F without causing a problem for bale walls.
As long as the climate is not excessively humid, the dangers of moisture can be successfully avoided by careful design and construction.
Another hurdle to building with straw in many parts of the world is the lack of building codes, and the resultant expense of having to get all designs approved by an engineer.
I hope to install moisture and temperature monitors into the structure, in order to record the long-term performance of the various elements of the building. I also want to work closely with my local building inspectors and, hopefully, various other organisations such as local colleges, the Canadian National Research council (NRC), and the Canadian Mortgage and Housing Council (CMHC). In this way we may be able to gather useful data which can be used to help generate applicable building code.
Construction process...
For the purposes of building, two sizes of bales exist: 2-string and 3-string. Typical 2-string bales are about 18 inches wide, 14 inches thick and between 36 to 40 inches long. 3-string bales tend to be a couple of inches bigger in all dimensions. The length of the bales can be varied by an adjustment on the baling machine, as can the amount of compression. The higher the compression, the better for building.
It is recommended that the bales should have a minimum density of 7 - 7.5 lbs per cubic foot. This has been adopted as the standard for code-approval in several US states.
Some people favour 3-string bales for load-bearing walls, since they are slightly wider and a little more stable, but there is little difference between them in terms of performance. I prefer 2-string bales as they are a little easier to carry and work with.
Similarly, some people suggest that certain types of straw are better than others. Oats, wheat, rice and barley are all commonly used, but you also find flax, hemp, timothy grass and all sorts of other materials used, including paper! Flax and hemp are supposedly more durable than other types of straw, but there is a price to pay, since they are also heavier and much rougher to handle.
The best advise that I've received is to use whatever is local and most available. This reduces the cost and carbon footprint of transportation.
Bales can be stacked flat or on edge but flat-laid bales are more stable and have a higher insulation value. My design will require seven or eight courses of bales, laid flat. This should result in about 500 bales for the main construction. A certain amount of extra straw may be used as fibre in the plaster and floor.
Since the walls are not load-bearing, they will be finished with a lime plaster.
It has been a challenge finding a source of standard square (14" x 18") bales within B.C. It seems we generate little straw in the province and much of the production is in the form of the large round bales. However, I believe I have found a few farmers who can supply our needs.
References...
As mentioned earlier, straw bale walls have been shown to be remarkably fire-proof. This is because there is insufficient oxygen within the walls to sustain rapid combustion. Walls tested in accordance with ASTM 119 05a for fire resistance have received an official rating which significantly exceeds that of conventional stud walls. Depending on the type of plaster used, the straw bale walls received one-hour (for earthen plaster) or two-hour (for cement plaster) ratings.
The benefits of using straw bales in construction are numerous, and include:
- exceptional thermal insulation. With an average R-30 when stacked flat, straw bale houses are considered to be 'super-insulated' and require significantly less energy to heat or cool;
- highly environmentally-friendly. Straw is a waste product much of which gets burned or ploughed back into the ground each year; (enough straw is burned each year in the USA to build five million houses)
- excellent fire resistance. Straw bale walls have a fire rating of between one and two hours (depending on the type of plaster used) which far exceeds the 20-minute rating for conventional 'stick frame' construction;
- excellent acoustic insulation. Straw bale houses are very quiet inside due to the construction of the walls (plaster-straw-plaster) which absorbs high levels of noise;
- very healthy air quality when plastered with natural materials;
- impressive seismic strength. Recent tests have shown that straw bale walls can withstand twice the g-forces recorded in the most powerful earthquakes;
- relatively easy to work with. Straw bale construction requires little technical knowledge or skill. However, as with any system, a sound understanding of the construction methods is advisable;
- relatively cheap. In north America bales can be bought for about $3.00 each. When compared to the cost of conventional materials required to build to the same level of thermal insulation, this represents a significant saving;
- bio-degradable. At the end of the building's life the straw can be returned to the earth as compost, and;
- solid. The bale walls are almost two feet thick, providing a sense of solidity and nice wide window alcoves in which to sit.
Building with straw does have some disadvantages. Foremost amongst them is the need to ensure that the bales remains dry both during construction, and throughout the life of the building.
Moisture is the 'Achilles heal' of straw bales. Levels above 20% will support the growth of mold which will gradually turn the straw into compost. However, this is not the same as humidity. Sustained humidity levels can be as high as 75 - 80% at 70 degrees F without causing a problem for bale walls.
As long as the climate is not excessively humid, the dangers of moisture can be successfully avoided by careful design and construction.
Another hurdle to building with straw in many parts of the world is the lack of building codes, and the resultant expense of having to get all designs approved by an engineer.
I hope to install moisture and temperature monitors into the structure, in order to record the long-term performance of the various elements of the building. I also want to work closely with my local building inspectors and, hopefully, various other organisations such as local colleges, the Canadian National Research council (NRC), and the Canadian Mortgage and Housing Council (CMHC). In this way we may be able to gather useful data which can be used to help generate applicable building code.
Construction process...
For the purposes of building, two sizes of bales exist: 2-string and 3-string. Typical 2-string bales are about 18 inches wide, 14 inches thick and between 36 to 40 inches long. 3-string bales tend to be a couple of inches bigger in all dimensions. The length of the bales can be varied by an adjustment on the baling machine, as can the amount of compression. The higher the compression, the better for building.
It is recommended that the bales should have a minimum density of 7 - 7.5 lbs per cubic foot. This has been adopted as the standard for code-approval in several US states.
Some people favour 3-string bales for load-bearing walls, since they are slightly wider and a little more stable, but there is little difference between them in terms of performance. I prefer 2-string bales as they are a little easier to carry and work with.
Similarly, some people suggest that certain types of straw are better than others. Oats, wheat, rice and barley are all commonly used, but you also find flax, hemp, timothy grass and all sorts of other materials used, including paper! Flax and hemp are supposedly more durable than other types of straw, but there is a price to pay, since they are also heavier and much rougher to handle.
The best advise that I've received is to use whatever is local and most available. This reduces the cost and carbon footprint of transportation.
Bales can be stacked flat or on edge but flat-laid bales are more stable and have a higher insulation value. My design will require seven or eight courses of bales, laid flat. This should result in about 500 bales for the main construction. A certain amount of extra straw may be used as fibre in the plaster and floor.
Since the walls are not load-bearing, they will be finished with a lime plaster.
It has been a challenge finding a source of standard square (14" x 18") bales within B.C. It seems we generate little straw in the province and much of the production is in the form of the large round bales. However, I believe I have found a few farmers who can supply our needs.
References...
As mentioned earlier, straw bale walls have been shown to be remarkably fire-proof. This is because there is insufficient oxygen within the walls to sustain rapid combustion. Walls tested in accordance with ASTM 119 05a for fire resistance have received an official rating which significantly exceeds that of conventional stud walls. Depending on the type of plaster used, the straw bale walls received one-hour (for earthen plaster) or two-hour (for cement plaster) ratings.
A video of the fire testing process can be found at: http://www.ecobuildnetwork.org/firemovie.htm
A very informative article discussing the subject of moisture and straw bale walls can be found at The Last Straw.
An article on the recent (March '09) earthquake test of straw bale walls can be found at The Science Daily.
Links... (hover over links for info)
The Living Building Challenge
Fotoprint
John Gower - Gower design group
Chelsey Braham - C D B Design
Earth Futures
BCSEA
SIPDistribution.ca
Chemicallime.com
Perlite.com
Strawbale.com
HomesteadHouse.ca
Eco-Sense.ca
Building Limes Forum
Straw bale fire test movie
Straw bale earthquake test movie
Susanne Dannenberg - visual artist
Goodshepherdwool.com
Elke Cole