The influence of two crop by-products on the hygrothermal properties of earth plasters

The incorporation of natural fibres in earth construction elements is a common practice that is found both in traditional and in contemporary building systems. The positive effect that such materials have on the mechanical properties of the clay mixtures has been established in previous research. However, their effect on the hydric and thermal properties is less well understood and these properties are important for thermal mass and passive humidity control in buildings, aspects linked to occupant health and reductions in energy use. The present paper includes the first in-depth study of the thermal conductivity and diffusivity, as well as the water vapour permeability and moisture buffering of compressed earth blocks and plasters incorporating natural fibres. Two different vegetable materials, barley straw and corn pith, were mixed to the clay materials in two different percentages (1% and 2%). The results show that the vegetable materials have a great impact on the thermal properties and the apparent density of the mixtures, but a limited effect on the hydric properties. The greatest improvement of the moisture behaviour was shown by the specimens incorporating 2% of corn pith. This improvement is greater for short time exposure than long time exposure. After 3 h, these mixtures adsorbed 15.5 g of moisture more than the plain samples, but after 8 h the difference was reduced to 8.0 g. This indicates that such mixtures might be more appropriate in environments with short and intense moisture loads, such as bathrooms. Previous research has demonstrated that earth provides the highest moisture buffering capacity of common building materials, and this research demonstrates how these properties can be enhanced for specific applications.