Thermal Conductivity of Straw Bales: Full Size Measurements Considering the Direction of the Heat Flow

Abstract

Abstract:The thermal conductivity of straw bales is an intensively discussed topic in the international straw bale community. Straw bales are, by nature, highly heterogeneous and porous. They can have a relatively large range of density and the baling process can influence the way the fibres are organised within the bale. In addition, straw bales have a larger thickness than most of the insulating materials that can be found in the building industry. Measurement apparatus is usually not designed for such thicknesses, and most of the thermal conductivity values that can be found in the literature are defined based on samples in which the straw bales are resized. During this operation, the orientation of the fibres and the density may not be preserved. This paper starts with a literature review of straw bale thermal conductivity measurements and presents a measuring campaign performed with a specific Guarded Hot Plate, designed to measure samples up to 50 cm thick. The influence of the density is discussed thoroughly. Representative values are proposed for a large range of straw bales to support straw-bale development in the building industry.

Physical Properties of Straw Bales as a Construction Material: A Review

“…To this end, the guidelines of ASTM and ISO standards were followed to the extent they can be applied. The results obtained were found to be comparable with Costes et al [42] results. Finally, Seitz and MacDougall [45] described the design and construction of a small-scale, affordable hot box apparatus for testing of straw bale and other nonconventional building panels.…”

“…Consequently, the measurement apparatus for thermal conductivity is usually not designed for such thickness. Costes et al [42] presented a measuring campaign performed with a specific Guarded Hot Plate according to Dubois and Lebeau [43], designed to measure samples up to 50 cm thick, in order to guarantee reliable measurements. In particular, the influence of density and thickness on the thermal conductivity of the samples were investigated.…”

“…Since thermal resistance is affected by density, several authors analyzed the thermal conductivity with varying straw density (Figure 4) [42].…”

Hygrothermal Behaviour of the Timber-Framed Sauna with Straw-Bale Walls

“…The predicted calculation was done using λ = 0.08 W/(m·K) [10]. According to the regulations of the Government of the Republic of Estonia, Minimum energy performance requirements for the thermal transmittance of the external wall could be 0.12 -0.22 W/(m²·K), so the tested straw-bale wall conforms to the set requirement.…”

Static and Dynamic Thermal Characterization of Timber Frame/Wheat (Triticum Aestivum) Chaff Thermal Insulation Panel for Sustainable Building Construction

“…Due to their anatomical structure, the heat capacities and thermal conductivities of these materials depend strongly on the sample orientation towards the heat flow from the source. Unlike conventional thermal insulation materials, they are anisotropic; for example straw bales show the thermal conductivities of 0.045-0.056 W m −1 K −1 and 0.056-0.08 W m −1 K −1 in perpendicular and parallel direction, respectively [17].…”