Straw
from vernacular
to prefabrication The barn with straw bales is slightly dark and the air is heavy of the dusty scent of straw. In secret, a hut is built inside the pile of strawbales. The narrow entrance of the hut is camouflaged, however, once inside there is enough room for three school boys to sit. One of them, the most daring one, lights a firelighter. The boys watch indulged in awe and bewilderment of doing something naughty and dangerous. No one has to know... that straw bale walls in buildings are not actually fire prone. The loose straw in the straw bale pile is, but a well-built wall with compressed straw will only smolder slowly on its surface. This natural material is therefore not as dangerous as we may think, it is actually a versatile material which is highly applicable in the construction sector. Text: Thimo Hillenius Benefits of the material Straw as building material has been used in vernacular building since the beginning of humanity. Only quite recently the material was rediscovered in western building industry. So why don’t we see straw buildings more often? Straw is a bio-based building material, and as such from a regrowable resource. It is a residual product of the cereal industry, so it is available in huge quantities as long as people eat bread or drink beer. Straw as building material is CO2-negative, so instead of emitting CO2, it stores CO2 within the building. Research to make cement CO2-neutral is still ongoing, but with straw we are already there.
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Availability and ecology are not the only benefits of this building material. Buildings built with this material also have very good properties. Walls made with a straw layer of 400mm have an R-value of 8.25 m2.K/W which is almost two times the value which is necessary according to Dutch norms. With straw bale walls a building can be built damp-open, which creates a healthier environment inside than damp-tight buildings. It is also the case that with traditional building, for example with ‘PUR’, ‘PVC’ and several insulation materials, materials are slightly poisonous during production or during the usage phase. Straw is slightly dusty to work with, but that is not the case after the wall is finished. Straw has excellent insulating properties, which gives the building a high energy efficiency.
If TU/e was a straw built university, you would possibly finish your studies faster The material also contributes to pleasant indoor acoustics. A healthy building has benefits for the atmosphere inside, therefore a straw bale building makes people more productive.
Material characteristics It is worthwhile to note some of the characteristics of the material straw. Straw has other insulation qualities in the longitudinal direction than in the transverse direction. So when a straw wall is made, care should be taken with the orientation of the straw. A wall insulates better with a panel with straw in the transverse direction (20%). However, the straw in the wall would not be stable that way. Straws can easily picked loose on both sides. It is found that using straw in a crisscross direction is an optimum between insulation and strength. In pressure related to insulation there is also an optimum. Straw needs to be compressed +/100 kg/m3. With low pressure the straw turns into a loose gathering of strands, with high pressure the air in the voids within the straw reeds diminishes. The material also copes with wrong presumptions about its properties. Like that the wall will decompose. This will, just as various other building materials, occur only when it gets wet. Water leaking on top of the straw bale walls has to be avoided at all costs. Another prejudice is that unwanted animals start to inhabit the walls. In fact, in 100kg/ m3 compressed straw there is no room and no food for such animals. Also, the walls have a finishing layer that shield the straw. Still, some people also have doubts because of their hay fever. However, you are not going to sneeze yourself out of your house, since hay fever results from hay, and not from straw.