An Impact of Moisture on Thermal State of Flax and Hemp Shives Thermal Insulations

Abstract

Plant-based materials exhibit different moisture absorption properties than synthetic materials. In the case of synthetic fibrous insulation, the effect of moisture on thermal conductivity can be relatively easily determined based on the mass fraction of moisture in the material’s skeleton. In the case of cellulosic materials with an open capillary structure, determining this effect requires laboratory testing. The authors conducted laboratory tests of the thermal conductivity coefficient of dry and wet plant-based insulation, such as flax and hemp shives. The effect of material densification at various moisture levels was also considered. The article also presents a numerical analysis of the thermal state and moisture content of thermal insulation used in walls operating under moderate climatic conditions. For damp shives, thermal conductivity increases noticeably with increasing densification, while for dry shives, thermal conductivity decreases until a certain level of densification is achieved. The obtained results were compared with values calculated using a linear model of the relationship between thermal conductivity and moisture content in the material. At higher moisture values, around 14–15 wt.%, thermal conductivity results are significantly lower than those obtained from the linear model (12.5–16.3% in the case of flax shives and 8.4–11.3% in the case of hemp shives) This is a favorable characteristic of shives compared to the performance of, for example, mineral wool in elevated humidity conditions. The authors believe that their results will be not only scientific but also practical, facilitating the assessment of heat loss in buildings.