Impact of fibre orientation on the hygrothermal properties of straw bale insulation

This paper presents findings from a research project that aimed to develop the use of wheat straw bales as non-load bearing building insulation with a new prototype insulation product. Demands for more sustainable construction products, with lower net carbon emissions and improved thermal performance, have grown in recent years. There is an increasing recognition that bio-based insulation materials, including hemp, wood fibre and straw, can provide more sustainable alternatives to current insulation products. Straw bales, produced as an agricultural co-product from cereal production, can be used directly in construction. However, both the bale sizes and orientation of straw fibres in these agricultural bales are sub-optimal for most construction uses, and have become a barrier to wider adoption. This paper reports on the technical development and characterisation of a prototype insulating product produced from wheat straw, in which both the bale size and straw fibre orientation has been produced to optimise insulation performance. Thermal conductivity and hygric performance studies are presented for the prototype bales, characterising improvement in performance from optimising straw fibre orientation. Optimising orientation of the straw fibres in bales has successfully reduced thermal conductivity, compared to conventional agricultural bales, by 38%, allowing similar reductions in insulation thickness for an equivalent performance. Furthermore, by optimising the orientation of the straw fibres a reduction in water vapour permeability, moisture buffering, and moisture infiltration by as much as 76%. This research will support opportunities for greater uptake of a novel bio-based insulation materials into mainstream construction.