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Why build with straw?
Why build with straw? Why do straw bales make such a fantastic building material?
Why are straw bale buildings so special?
Straw is a locally available, plentiful and environmentally friendly building material. It is very strong, durable and super-insulating. It can be used to create beautiful yet practical buildings of all shapes and sizes.
Building with straw bales is not a new concept. Some of the oldest known straw bale buildings still in use today include houses built in France in 1921 and Nebraska, USA in 1925. There are an increasing number of straw bale buildings being constructed in every corner of the world.
The oldest straw bale buildings in the UK were built in the mid 1990s.
The benefits of building with straw are many and varied:
Click on each straw bale benefit to read more
- very high level of thermal insulation
- cost effective
- renewable
- carbon store
- low embodied energy
- low carbon emissions
- locally available
- plentiful
- reduced material requirement
- vapour permeable
- excellent acoustic insulation
- non toxic
- no waste
- adaptable and creative
- user friendly
Using straw in combination with lime or clay plasters adds further benefits:
Click on each straw bale benefit to read more
- excellent results in worldwide testing for fire resistance, compression, racking and wind load
- improved thermal performance
- healthy indoor environment
>> Click here to read our frequently asked questions about straw bale building
Very high level of thermal insulation
Super-insulated straw bale walls help to keep a building warm in winter and cool in summer – saving energy and keeping fuel bills low.
The thermal conductivity of wheat straw is in the region of 0.060 W/mK. The table below shows how this information is used to calculate a u-value of 0.123 W/m2K for the Straw Bale Cabin’s walls where the bales were 475mm thick.
Wall assembly |
Thickness |
Conductivity (λ) |
Resistance |
Internal surface resistance (1) |
- |
- |
0.130 |
Earth plaster (2) |
0.025 |
0.800 |
0.031 |
Straw bale |
0.475 |
0.060 |
7.917 |
Lime render (2) |
0.025 |
0.870 |
0.029 |
External surface resistance (1) |
- |
- |
0.040 |
|
|
|
|
Total thickness of wall |
0.525 |
|
|
Total resistance of wall |
|
|
8.147 |
|
|
|
|
U-value (u=1/Rt) |
0.123 |
W/m2K |
|
To put this result into perspective, UK building regulations require a u-value of 0.3 W/m2K for extensions, the AECB silver standard requires a u-value of 0.25 W/m2K and gold standard requires 0.15 W/m2K. The Cabin walls are easily good enough to meet even the best of these standards. The bales used for the walls of the Straw Bale Cottage are even wider at 500mm giving a u-value of 0.117 W/m2K Insulation, however, is only one of several factors affecting the thermal performance of a wall system. The way building standards currently assess only heat loss by conduction is seriously flawed. We should also look at heat loss by convection within the wall itself, thickness and heat storage for example. To include all other parameters would be even more advantageous to straw and other natural building methods. For more information about this, see Carol’s thesis.
Straw, being an agricultural by product, is a very cheap source of insulation but straw bale builders should be prepared to pay a little extra to ensure good quality building bales.
However, the cost of the walls is only a small part of the total build cost and ideally, natural walls should be complemented with the use of other natural materials such as sheep’s wool or wood fibre. These materials are currently more expensive then conventional, mass-produced building materials – but the cost to the environment and human health is much less!
The biggest saving is the significant reduction in heating and cooling costs over the entire life of the building.
The use of non-renewable resources for construction in the UK is three times the sustainable rate. In other words we need three planets the size of the Earth to continue to provide the construction materials we are currently using.
More renewable materials must be brought into the equation – especially materials such as straw or hemp that can be grown every year.
Bricks and blocks could never be classified as renewable although they could potentially have a very long life and be “recyclable” if they were stuck together with lime mortar rather than cement. This very rarely happens, however, in modern day construction.
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Cereal crops photosynthesise as they grow, taking in carbon dioxide from the atmosphere. Every 10kg of straw absorbs nearly 14kg of carbon dioxide, sequestering it in the walls for the lifetime of the building. A much better carbon sink than piping it under the sea!
The Straw Bale Cabin has stored over 3,700 kg of CO2 in its walls (96 full bales plus 45 half bales weighing an average 23 kg per bale). After deducting the emissions generated to build the complete wall, the net balance of CO2 locked up in the walls is approximately 2,650 kg (over 80kg/m3).
Straw is a by-product. It is grown for the cereal crop rather than the straw. It is either removed from the field by baling or it is chopped for incorporation into the soil. Little extra energy is required to turn it into a construction material. Compare this to bricks whose manufacture requires huge amounts of energy, mainly due to the high temperature production process involved (1,462 KWh/m3).
Currently, 10% of total energy used in the UK is embodied in construction materials. More widespread use of straw bales could significantly reduce this figure.
The low energy requirement for straw bale production as described above, means that there are very low carbon emissions at the construction stage. Compared to the construction of the average UK home where approximately 50 tonnes of carbon dioxide is emitted, the construction of a house with local and/or renewable and recycled materials would emit only a fraction of that.
The exceedingly good thermal performance described above ensures very low carbon emissions from heating or cooling over the life of the building (which can be a very long time).
A cereal crop, and therefore straw, can probably be grown within one hour of even the most inaccessible places in the UK. Even where building materials are made in the UK rather than being imported, the manufacturing sites are unlikely to be on the doorstep.
As with any material, the closer your source of bales the less energy required to transport them.
In 2006, nearly 3 million hectares of cereal crops (mainly wheat, barley or oats) were grown in the UK. Each hectare can yield approximately four to six tonnes of straw. As an average home could be built with less than 10 tonnes of straw, that is a vast quantity of potential building material grown every year!
On the other hand, trees can take 50 years to grow to construction grade and as the UK has only 10% tree cover (much less than many other European countries), most of the timber used in construction here has to be imported.
Currently, most straw is used for animal bedding or as a soil conditioner. However, more and more green material is being diverted from landfill to improve agricultural soil and there are other materials such as waste paper or wood chips that can be used for animal bedding. In the future, there could be increased demand for straw to be used as a source of renewable, biomass heating fuel but good quality straw must surely be of more environmental benefit when providing super-insulated homes.
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A straw bale is the building block, the insulation and the surface for plaster. In addition, a load bearing design reduces the timber requirement. Currently, 50% of all raw materials are used in construction.
A straw bale wall breathes – it allows moisture to evaporate.
Straw bale buildings are unbelievably quiet. The thick walls deaden sound.
Straw bales are non-toxic. There is no off gassing from nasty chemicals. It is best to use clean and dry straw to minimise dust and mould exposure during the build. Once encapsulated in the wall there is no problem.
There is no waste in the construction phase as all loose straw or spare part bales can be either composted or used for animal bedding. All it takes is a little discipline on site to tidy the straw up into spare sacks at regular intervals and to ensure that scraps of baler twine are separately stored.
A straw bale or other building made with natural materials can last a very long time because they are carefully detailed to protect them from their main enemy – rain! But when a natural building has served its useful purpose, it can be exposed to the elements, allowing it to biodegrade. No need for landfill!
Straw bales are in effect large building blocks but they can easily be cut and shaped to form a desired shape. Their natural undulations can be accentuated or they can be used to create very straight walls depending on preference.
Standard straw bales are light enough to carry, easy to shape and quick to build. It is possible to build with larger bales but lifting machinery would then be needed.
The main reason behind this is to reduce the energy “embodied” in the building and the associated carbon emissions. Constructing the average UK home emits 50 tonnes of carbon dioxide but constructing a house with local and/or renewable and recycled materials emits only a fraction of that. In addition, a straw house needs far less energy for heating once built and when using all natural materials, the healthy, toxin free environment is an extra bonus.
Using straw in combination with lime or clay plasters adds further benefits;
Excellent results in worldwide testing for fire resistance, compression, racking and wind load
Bruce King’s book Design of Straw Bale Buildings, The State of the Art is the best reference for these tests. See our recommended reading list.
Improved thermal performance
When straw bales are plastered both sides, their thermal performance improves. The plaster makes them airtight and provides thermal mass. Read Carol’s thesis for a more in depth explanation.
Healthy indoor environment
A straw bale wall with a clay plastered interior is great for regulating heat and moisture to provide a very stable indoor environment. Again, there is more on this subject in Carol’s thesis.
The following graphs show the inside and outside temperatures at the Straw Bale Cabin during five unoccupied and unheated days in 2007. The flatter blue curves show how the straw bale walls moderate the internal temperature compared to the external diurnal temperature swings.
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What is the fire risk with a straw bale building?
A bale of straw is often compared to a telephone directory. Loose straw or a single page of the directory will readily burn. However, it is very difficult to set fire to the dense bale or the whole directory as there is very little oxygen in there. Add to that 25 mm of clay or lime render and you have a wall assembly that will easily withstand more than two hours in a fire test. There have been many tests carried out around the world to confirm this.
Are rats and mice a problem in a straw bale building?
Some say that rodents will not live in straw because there is no food supply but this is not true. As any farmer will tell you, a straw stack is a very popular home for rats and mice. Luckily though, there is a big difference between a farmer’s straw stack and a straw bale building; in a well designed building the straw is completely sealed on all sides to prevent rodent entry and the bales are dense and carefully stacked together with no gaps whatsoever.
There is a further advantage if the bales are under compression as in a load bearing building – the bales are so tightly pressed together that the rats and mice would be unlikely to live there.
Our building bales for the cottage were riddled with mice in the over winter store. Out of 700 bales we struggled to find 300 that had not been chewed through. However, it was very reassuring to find that one part of the stack where some large, heavy big round bales where placed on the top there were no mice taking up residence!
Can rain damage a straw bale building?
Water is the main enemy of all natural building materials as prolonged exposure to water would result in rot. However, with appropriate design, attention to detail in construction and timely repair in use straw bales, timber and hemp can last a very long time.
Design techniques include a large roof overhang or porch, a breathable render or a ventilated rain screen and raised foundations or wall base. These simple measures were widely used in the past.
How long should a straw bale building last?
Straw bale buildings are relatively new in the UK but we have been using straw in cob buildings and in plaster for centuries. As described above, if it is kept dry it can last forever.
The straw bale house in France built in 1921 is the oldest European example but the are several houses in the USA which are over one hundred years old.
Why renewable building materials?
The following startling statistics highlight how modern construction techniques place heavy demands on resources:
- 50% of all raw materials are used in construction
- 10% of total energy used in the UK is embodied in construction materials
- 8% of global pollution is attributable to the cement industry.
Recent analysis has shown that the use of non-renewable resources for construction in the UK is three times the sustainable rate. In other words we need three planets the size of the Earth to continue to provide the construction materials we are currently using.
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Why renewable energy?
The majority of scientists now agree that rising global temperatures are attributable to human induced greenhouse gases, mainly carbon dioxide and mainly produced by the combustion of fossil fuels for energy use.
Urgent action is needed to reduce the emission of greenhouse gases to limit significant changes in the world’s climate system and their serious consequences for humanity.
However, households are consuming ever increasing amounts of energy:
- 29% of UK delivered energy is consumed in the domestic sector
- Domestic energy consumption has increased by nearly 10 million tonnes of oil equivalent in the last 20 years.
Therefore, a greater understanding by the public of energy use, energy conservation and the contribution renewables can make to energy generation at a local level is essential to reverse the current trend.
A recent report by the Sustainable Consumption Roundtable highlights the profound effect that personal experience of micro generation can have. It is “convinced that significant and sustained progress will not be made towards the UK’s carbon reduction targets without actively harnessing consumer concern and converting it into action. The challenge is to raise people’s use of energy in the home from the subconscious to the conscious and enable them to feel part of the solution”.
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Please contact Home Grown Home if you have found this information helpful or if you feel there is anything we can add.
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