Strawbale History & Facts
This summary of straw-bale building is part of a project by Nikki Brookman and Catherine Oermann Trinity College Gawler.
It contains information derived from literature on the topic and some unique research on the insulation qualities of straw bale walls.
Please use the links below to jump to specific information:
- History of strawbale building
- Some benefits of straw bale construction
- Cool in summer, warm in winter
- Insulation against the movement of heat
- Our test of insulation against heat movement
- Fire resistance
- Australian Bushfire Test Results
- Fire retardants
- Sound insulation
- Our test of insulation against sound
- Appendix Construction Site
Our project was essentially undertaken to compare the insulation properties of strawbale walls with other types of walls used for house construction in Australia however the project grew as we researched the topic. This webpage summarises some trials we conducted on test walls and a review of books, internet research and information received from people who build with strawbales. Most of the information comes from North America where there are more than 10,000 strawbale houses, some of them more than 100 years old.
Strawbale construction seems to offer homes that are:
- easy to keep cool in summer (reducing air conditioning costs and energy consumption)
- easy to keep warm in winter (reducing heating costs and energy consumption)
- relatively cheap
- able to be partly built by people with no trade skills
- kind to the environment in that less energy is used to make them than other types of house
We researched some of the most important of those claims and found from our trials and research that strawbale buildings use less energy to construct and insulate against heat and sound better than commonly used building materials. From our reading and conversations with people who have built using strawbales it seems that the system is easy to learn for people that have minimal building skills and is financially more economical.
In other words more people should be building with strawbales.
A large amount of information in this project was derived from the following books:
- 'Build it with Bales' - Matts Myhrman and SO MacDonald
- 'Straw Bale Building' - Chris Magwood and Peter Mack
Why we chose this topic for our research
Because almost everyone lives in a house and can do something about the way it is built or altered. In South Australia we suffered power cuts last summer because badly built houses had to be cooled by hundreds of thousands of air conditioners, each one increasing the greenhouse problem. We wanted to show how ordinary people can do something for themselves and the environment by building with strawbales.
History of strawbale building
As long as human beings have been creating shelter, straw and grasses have been used in conjunction with a variety of building methods to provide safe, dependable, and comfortable housing in many climates and environments.
Walls made from tied bundles of long lengths of straw, stacked in mud mortar, have been constructed for centuries throughout Asia and Europe. Another ancient method, also employed in Asia and Europe, used compacted loose straw coated with a clay slip for walls. Those methods and materials remain in use today, their use declining only where modern construction methods, materials, and codes have become common. In the United States, a new era of building with straw and grasses began in the late 1800s with the development of stationary horse-powered and the steam-powered balers, which made it possible to compress hay and straw into string or wire-tied rectangular units called bales. It took only a slight stretch of the imagination for early homesteaders in the timber-poor region of the Great Plains of North America to think of using bales as oversized bricks. It was in the sand hills of Nebraska, a land that produced magnificent stands of meadow hay, from which the first bale buildings were constructed.
The reasons vary as to why many pioneers chose to build with baled straw or hay. Some seem to have simply been intrigued by the method, while others found building with bales easier than building with sod. Some families decided to replace their original small sod houses with larger bale houses. It was also noted that using baled meadow grasses seemed much more efficient than stripping a large section of productive meadow land for sod.
In some cases, families were in immediate need of housing, and bales were looked upon as the quickest way of getting a roof overhead. Many of those structures were first viewed as temporary, but when it was discovered that they were both durable and comfortable in the extremes of the Nebraska winter and summer, they were soon plastered and adopted as permanent housing.
Whilst strawbale building became unfashionable with the easy availability of ordinary building materials as transport systems improved, it has had a huge comeback as humans have become more environmentally aware in the last ten years. In Australia the last 5 years have seen the pioneering of cheap and efficient strawbale building systems which people with no trade skills can easily master. In our testing of the wall modules for insulation value we actually used a commercial strawbale cold room to provide the constant temperature needed.
Some benefits of straw bale construction
Straw bale construction can provide benefits in regions where straw has been an unwanted waste product. The slow rate at which straw rots makes its disposal a problem for farmers because unlike nitrogen-rich hay, straw is not used for animal fodder, and the stems are too long to be thoroughly tilled into the soil. In California alone almost a million tons of rice straw burned each autumn. This produces more carbon monoxide (a toxic gas) and fine burned particles in the air than all of the electric power generating plants in the state combined. This air pollution has prompted the state's Air Resources Board to initiate the process of banning this burning.
|Annual carbon monoxide production from power plants and straw burning
||Tons of carbon monoxide
|California Agricultural Magazine, Vol 45, (1991)|
The Oregon Department of Environmental Quality has stated that the smoke is "carcinogenic ... containing particles that irritate the lungs" Strawbale construction could also be useful in the effort to control global warming and atmospheric deterioration. A large reduction in the amount of straw burned would cut back the production of carbon monoxide, carbon dioxide and nitrous oxides by many thousands of tons per year.
There could be a significant decline in the devastation of timber sources if homes were built from straw bales instead of the wood-hungry construction methods so common today. In many developing countries, the cutting of firewood for heating is as devastating to forests as wood cutting for the buildings themselves. Energy-efficient bale buildings could lessen this impact. According to the American strawbale expert Matts Myhrman; "If all the straw left in the United States after the harvest of major grains was baled instead of burned, five million 2,000-square-foot ('20-square') houses could be built every year."
In contrast to the timber used for building houses, straw can be grown in less than a year in a sustainable production system. Straw can also be grown on saline or low quality land. Tall Wheat Grass (Aelongatum), for example, is long-stemmed and durable, and productive in soils with high water tables, high salinity, and alkalinity. Straw bypasses much of the energy and waste needed to produce conventional building materials. For example, the production of one ton of straw requires 112,500 BTUs (British Thermal Units) in comparison to 5,800,000 BTUs for concrete.
According to calculations performed by Richard Hoffmeister at the Lloyd Wright school of architecture in Scottsdale, Arizona, straw bales are at least thirty times less energy intensive than a wood frame or equivalent fibreglass insulation.
Cool in summer, warm in winter
If you can stop the heat of summer days getting into your house by closing up your home early in the morning while it is still cool, you can keep the inside of the house cool, as long as the heat cant get in through your wall, roof windows etc. That is where a super-insulating strawbale wall comes in handy - it wont let the heat through.
It is just the opposite in winter when you warm your house with sun allowed in through the windows and trapped in the house or with a slow combustion heater, air heaters, radiators etc. You dont want the heat to escape - again the strawbale wall will reduce the movement of that heat.
A strawbale wall is made of hundreds of thousands of stalks of straw, each one containing and trapping air. Air is a great insulator so if you can keep it still in the wall, almost no heat can move through it. Even the solid part of the straw itself is quite good insulator but it is interesting to think that a very well compacted bale will give worse insulation than a lighter bale, because the insulation is mainly provided by the air.
The layer of cement render on the inside of the wall stores warmth or coolness so it evens out changes in the air temperature, so if you opened the door letting lots of hot air into the house for a few minutes the cool wall would absorb the heat from the hot air and leave the house much the same temperature as it was before you opened the door.
Insulation against the movement of heat
Strawbale wall insulation is two to three times better than the wall system of most well-insulated homes, and often five to ten times better than older houses. Additionally, the mass gained from the plaster of the bale wall can help increase the thermal performance of the wall system because it stores heat.
Straw bale walls call provide greatly improved comfort and big energy savings compared to more expensive conventional building systems, as they allow smaller heating or cooling systems to be installed than in conventional homes because of the increased insulation. Bale building is of special value in severe environments where energy is expensive.
To get the most benefit from the highly efficient walls of a bale building, the building should include a well-insulated roof (straw can be used in many cases), foundation insulation, insulated windows and doors, proper sealing to minimise drafts, and good ventilation achieved either by plastering and covoring the walls with a breathable finish. The high insulation and mass of bale walls will make it possible to keep the windows open much of the year, providing cleaner air inside.
Strawbale homes should be designed according to Passive Solar principles, orientated with the long axis running east-west and most of the windows on the north side, an ideal position for solar heating and natural cooling.
The R-value of a material is its ability to resist heat flow. An R-10 material will allow one tenth of a British Thermal Unit (BTU) through a square foot of the material in one hour if there is one degree Fahrenheit difference between the temperature on the opposite sides of the material. An R-30 wall would allow one thirtieth of a BTU through. Obviously the huge benefits come say from R-5 to R-30; above that, so little heat would be moving through a wall that it becomes insignificant compared with heat leakage through doors, windows, roof, floor etc.
The R-value of wood is about 1 per inch, brick is 0.2 and per inch fibreglass batts are 3.0. The higher the R-value the better the insulation. Research by Joe McCabe at the University of Arizona found that the R-value for both wheat and rice bales was about R-2.4 per inch with the grain, and R-3 per inch across the grain, which would give two-string bales laid flat (18 inches wide), the R-value of 42.8, and for a two-string bale on edge (14 inches wide), it would be R- 32.1. It has also been shown that thermal resistance is affected by moisture and density of the bale. (Joseph McCabe, January 1993).
In Australia we use the metric system (heat movement in watts per square metre of wall per second) which produces values which are about one sixth of the Imperial ones eg R-20 = Rmetric 3.5, R-40 = Rmetric 7, R-50 = Rmetric 8.8. So a standard 2.5 batt used in construction in Australia would translate to about R-15 in the imperial measurement system used in America and quoted in most of the literature.
It has been suggested that Australia adopt minimum standards of Rmetric 1.5 for walls and Rmetric 2.5 for roofs. A cavity brick wall usually only manages Rmetric 0.5
A layer of plasterboard (Gyprock) on wooden battens adds insulation value mainly because of the trapped air space involved.
From the information from the American researcher McCabe and Australian Building Standards tables (see appendix) you can deduce that a strawbale wall (with bales laid flat) has an Rmetric value of about 7 and with bales laid on edge an Rmetric of about 5.
Australian information suggests that a brick veneer wall with good (Rmetric-2) insulation will achieve a total of Rmetric-2.4, whilst a cavity brick wall will be around Rmetric-0.5
Because brick veneer is the most common type of construction in Australia we decided to actually test the insulation power of a strawbale wall with bales laid on edge against a brick veneer wall with Rmetric-2.5 fibreglass insulation in the space between the plasterboard and the brick skin. As far as we know this is the Worlds first test of these two walls against each other.
A strawbale wall has a long lag time as heat moves through it; in fact it can take up to 12 hours for the temperature inside to normalise with the outside temperature. This length of time means that day and night temperatures may well play off against each other leaving the inside temperature almost constant in conditions where days are warm and nights cold. So the test would need to continue over quite some time to be give us all the information we needed.
Our test of insulation against heat movement
We made two small 'house models' with the same inside floor area of 0.25 sq metre and identical floors and roofs (5cm thick styrene) but one was built with strawbale (on edge) walls and one with brick veneer (with plasterboard lining and Rmetric-2.5 fibreglass insulation inside the bricks). Both had walls 0.4 metre high. See photos below.
Constructing Brick Veneer model
Measuring starting temperature
Trial of insulation effectiveness -
The models were built on pallets so that they could be moved quickly into the testing chamber, a well insulated cold room which was kept at between 0 and 2 degrees Centigrade for the test.
We placed identical buckets containing 10 litres of water at 56 degrees Centigrade into each model and sealed them in the cold room. The models were wrapped in a single layer of plastic to prevent any air movement into the models from the outside environment as considerable draft would be experienced because of the fans in the cold room. The temperature of the water in the buckets was taken regularly to see how much heat was escaping through the walls.
The results are shown below.
From the results above it can be seen that the temperature of the water dropped about 10 degrees in the first hour. The temperature in the strawbale model fell slightly faster because the render inside it was absorbing heat. However during the second hour there was more heat loss from the brick veneer model.
From that point the brick veneer model continued lose significantly more heat than the strawbale model, showing the superior insulation quality of straw bale construction. It is worth pointing out that the model was built with bales on edge; had they been on the flat the insulation would have been even better.
From these test results it is clear that strawbale construction is ideal for maximising comfort and minimising energy use.
We would have liked to do our own test to compare strawbale with brick veneer but we were not able to get suitable equipment, so we researched tests done by other people.
Straw bale buildings are extremely hard to burn. This is because the bales hold enough air for good insulation but because they are compacted tightly they dont hold enough air to permit combustion.
*The National Research Council of Canada tested plastered straw bales for fire safety and found them to perform better than conventional building materials. In fact, the plaster surface withstood temperatures of about 1,850° F for two hours before any cracks developed. According to the Canada Mortgage and Housing Corporation, "The straw-bales/mortar structure wall has proven to be exceptionally resistant to fire. The straw bales hold enough air to provide good insulation value, but because they are compacted firmly, they don't hold enough air to permit combustion."
*In 1993 the New Mexico Straw Bale Construction Association commissioned SHB Agra to test strawbale walls for fire resistance. The tests showed that such walls are fire tolerant to the point where they were included in the New Mexico building Code. A video of the tests is titled Building with Straw Vol 3 : Straw Bale Code Testing - Black Range Films Box 119 Kingston, New Mexico.
Australian Bushfire Test Results
SHB Agra's Report on Fire Testing
In 1993, as part of the testing commissioned by the New Mexico-based Straw Bale Construction Association which eventually led to the inclusion of straw bale in the New Mexico building code, fire testing was undertaken on a straw bale wall panel by SHB Agra, Inc.
Transmission of heat through the unreinforced [unplastered] straw bale during its test was not sufficient to raise the average temperature at the exterior face of this wall to 250F above the initial temperature (the governing criteria for ASTM E-119). The highest average temperature recorded on the unexposed face of the unreinforced straw was 52.8F at thirty minutes. Transmission of heat through the wall did not exceed the allowable limit for any single thermocouple. Additionally, there was no penetration of flames or hot gases through the unre-inforced straw bale wall during the thirty minute test.
The burning characteristics of the unreinforced straw bales were observed through observation ports during the test. The test panel was also examined after it was removed from the combustion chamber. The straw was observed to burn slowly and the charred material tended to remain in place. The residual charred material appeared to protect the underlying straw from heat and ventilation, thereby delaying combustion.
The maximum temperature recorded inside the furnace was 1,691F at thirty minutes. Upon removal, the bales did. not burst into flames, but slowly smouldered. The fire was easily extinguished with a small quantity of water.
After the unplastered bales passed the 30 minute fire test, plastered bales were tested more closely simulate real-life burning characteristics on finished walls, with the following results:
- The highest temperature recorded on the exterior face of the stuccoed straw bales after 120 minutes of exposure was 63.1F, less than a 10 degree rise in temperature. The highest average furnace temperature recorded during this period was 1,942F, however at least one thermocouple recorded temperatures exceeding 2,000 F. There was no penetration of flames or hot gases through the stuccoed straw bale wall.
The burning characteristics of the stuccoed straw bales was also observed. The reaction consisted of initial cracking of the stucco surface as the heat was applied, with little other evidence of distress."
Where exceptional fire risks exist (eg bushfire areas) some very conservative Australian officials insist that since no Australian standard exists proving the fire resistance of straw bale construction, extra precautions need to be taken. One can use fire resistant foil which satisfies such regulators. This is placed on the outside of the wall before the wire netting and plaster layers. Possibly also a fire retardant mixture could be used. Solomit ceilings which have been used in Australian buildings since the 1940s incorporate such a mixture.
One mixture is 2 parts by volume Borax to 1 part of granular Boric Acid. Mix with warm water till no more will dissolve. Soak straw in the solution and dry in the sun. This also prevents the development of any fungi. A built wall can also be sprayed with a fairly high pressure jet of the solution before rendering with plaster.
Knowing how quickly brick veneer houses burn and the need for smoke alarms in modern houses suggests to us that a strawbale house would be a much safer place to live. In SA there has been one case of vandalism where a pile of dry hardwood planks was stacked against the wall of a strawbale building in Whyalla and lit up. The fire was in an isolated place with no one there to raise the alarm. The fire went out by itself having burned for some hours, leaving minor damage to the wall. A brick veneer home would have almost certainly burned to the ground.
In houses today there are often televisions, stereos, cooking, arguments and musical instruments all making noise. As well there may be traffic or aircraft noise coming in from outside. For people to feel comfortable there needs to be good sound insulation from the outside and between rooms.
|Loudness of sounds|
|Source||Intensity in decibels|
|Threshold of pain||130|
Stopping sound is quite complicated because it comes in many frequencies and different wall materials are better at stopping some frequencies than others. For building materials scientists use STC (Sound Transmission Class) to rate sound barriers according to the Australian Standard 1276-1979. It measures the airborne transmission losses for all one-third octave bands from 100Hz to 5000Hz.
Our test of insulation against sound
We were unable to find any test information on testing strawbale walls for sound insulation so we did the following test to compare the two types of wall.
- We obtained the materials for the sound testing from school.
- Placed the speaker into the brick-veneer model.
- Put plastic sheet and polystyrene lid on the top of the model to keep the sound in (acting as the roof).
- We had the volume of the stereo on a constant setting and used the same piece of music for each test
- We did the same for the straw bale model and recorded the results.
- We then tested the sound levels with the speaker in the open
Catherine Oermann testing sound levels in a comparison of straw-bale and brick-veneer walls
Graph showing the transmission of sound through the two types of wall compared with sound levels in the open.
Our sound test was at the level that the music was fairly loud; we had to almost shout to have a conversation and it could certainly have annoyed someone nearby who didnt like Jimi Hendrix. From another angle, the level was roughly that of two people having a heated argument.
On the other side of a brick veneer wall the sound that got through (50 decibels) was equivalent to two people in conversation. However the other side of a strawbale wall (40 decibels) the sound was down to the level found in a very quiet house.
So people in strawbale houses would not be worried by noise from outside the house or (if they use internal strawbale walls), what is happening in the next room, whilst people in brick veneer homes will be much more aware of noise from outside the house and (if they use plasterboard walls which is usual) they will hear virtually everything that goes on in next door rooms.
Appendix Construction Site
Thermal Resistance of standard building materials (metric R Value)
Source - 'Building Your Own Home' Wilkie and Arden 1997
|Construction||Summer heat flow||Winter heat flow||With appropriate Insulation added|
|Pitched roof, tiled with plasterboard ceiling||0.76|| 0.29
||With reflective foil: Summer 1.81, Winter 0.55
With R2 bulk insulation: Summer 2.76, Winter 2.29
|Metal deck roofing with raked ceiling||0.48||0.38||With reflective foil and R2 insulation:
Summer 3.13, Winter 3.067
|Timber frame with weatherboards||0.46||0.46||With foil 1.685
With R2 bulk insulator 2.305
|Brick veneer||0.45||0.45||With foil 1.517
With R2 bulk insulator 2.457
|Hebel Brick 100mm: 0.71|
|Hebel Brick 150mm: 1.07|
|Hebel Brick 200mm: 1.42|
|Concrete block (200mm)||0.35||0.35|
|Single glazed window
|Double glazed window
|Timber floor||0.39||0.43||With foil: Summer 0.617, Winter 1.28|
|Suspended Concrete Floor||0.43||0.54|
|Steel partition wall||0.52||0.52|
|Mud brick (300 mm)||0.4||0.4|
The US Department of Energy commissioned Jim Hanford of Lawrence Berkeley Laboratory (LBL) to analyse the thermal characteristics of the various wall materials. He produced the table below.
|Wall Section Thermal Characteristics
including strawbale construction (Imperial R values)
|2x4 studs w/R11batts||10.2||0.098||9.2||2.2|
|2x6 studs w/R19 batts||15.4
|Compressed Straw Panel|
|uninsulated 4.8" panel||10.1
|insulated 4.8" panel
|Fibrous Concrete Panel|
|insulated 3 inch panel||16.7 0
|insulated 4" panel
|23" bale @ R-1.8/inch (-25%)||42.7
|23" bale @ R-2.4/inch
|23" bale @ R-3.0/inch (+25%)
|6" form w/ concrete/adobe fill
|8" form w/ concrete/adobe fill
|exterior insulated 24"
Note: All walls have stucco exterior and drywall interior, except adobe and straw walls have plaster.
- Wood frame walls have 25 percent (R-11) and 20 percent (R-19) stud areas. The R-19 batt compresses to R-18.
- Compressed straw panel, insulated case, has 2 inches polystyrene on exterior.
- Fibrous Concrete panel have 1 inch polystyrene inside and out.
- Straw bale wall R-value is calculated for 3 unit R-values for straw to cover potential variability.
Average material thickness across foam block wall sections are as follows:
- 6 inch foam has 2.9 inches polystryene each side and 3.4 inches of fill.
- 8 inch foam has 3.1 inches polystryene each side and 4.8 inches of fill.
- Wall properties are based on 75 percent adobe and 23 percent concrete fill.
- Adobe walls , insulated case, have 2 inches of polystyrene on exterior.
- 24 inch wall is two 10 inch layers with 4 inch air gap.