The straw bale
Straw bales are stalks pressed into cuboids from threshed and then field-dried cereal plants, not to be confused with dried grass, which we call hay. Hay is much more prone to decomposition in the presence of moisture, probably also because it contains much less air and therefore has poorer insulating properties. Straw, on the other hand, is very similar to wood in terms of its chemical composition, which is why this combination works very well in construction. The so-called equilibrium moisture content of both materials is also between 13-15% by mass. Straw consists mainly of cellulose, lignin and silica. It is covered with a waxy layer which, in combination with the high silicate content, results in a high resistance to moisture. A rotting process therefore only begins after weeks of increased moisture. Even then, a straw bale retains its volume and releases the moisture under its own power, provided the structure allows this. At first, the straws themselves do not absorb moisture at all or hardly at all; it is stored in the air spaces and can also escape again due to the capillary effect. Many insulating materials (mineral wool, etc.) start to clump as soon as a certain moisture content is reached and then release the absorbed water again only with difficulty.
Material properties
Just as settlers in the U.S. at the end of the 19th century were amazed that their buildings constructed with straw bales were still standing years later and used for more than just temporary housing, we are still surprised today by the potential, or rather the multitude of properties in a single building material.
- very good insulating properties, U-value of superstructures for small bales between 0.1 and 0.14 W/m2K, for big bales mostly below 0.05 W/m2K
- very low energy consumption in production and thus real heating cost savings ( conventional passive house has already consumed more energy in production than it can save in heating)
- ecological material
- renewable raw material from the region
- fully compostable building material
- Cost-effective
- Use of an agricultural waste product
- very good sound insulation and acoustics
- good indoor climate (fully breathable wall structure), non-allergenic
- excellent space energy (despite super insulation and soundproofing energetic openness to the environment).
- absorbs radiation like e- or cell phone smog (Attention: no reception with small windows)
- Fire resistance plastered mostly tested with 90min
- very high ductility, which enables earthquake-resistant construction (confirmed by various international tests)
- Participation of the builders ( and -women) very well possible
- most different building styles possible, favors creative design
- very durable with proper construction
Straw has been used in construction for many thousands of years:
- as an aggregate for clay plaster or rammed earth
- in the form of straw clay as a static element, but also to improve thermal insulation
- for roofing; the straw of specially grown rye (e.g. in Hungary) can withstand up to 50 years of weathering, or alternatively as reed straw
Straw in bale form has been used in construction only since the invention of the baler about 120 years ago. Straw bales can be used in house construction both in the external walls (insulation, plaster base, static element), as roof, ceiling insulation, but also in the floor slab, provided that we can ensure adequate protection against moisture and condensation.