The thermal diffusivity of hemplime, and a method of direct measurement

Hemplime concrete is a material that has a different combination of properties to other, more traditional, building materials, and as such it exhibits potential advantages. Its thermal mass characteristics are often viewed as favourable to reducing the energy used to heat or cool indoor buildings. As a dynamic property, the effects of thermal mass on thermal behaviour depend not only on the conductivity but also on the heat capacity of the materials; the combination of conductivity and heat capacity being the diffusivity. The thermal diffusivity of hemplime concrete is comparatively under-researched, particularly given that most measurement techniques are designed for small samples of homogeneous material and may not give accurate results for inhomogeneous, anisotropic materials such as hemplime. This paper uses a technique recently developed for testing the diffusivity of whole wall panels, to measure the diffusivity of a section of hemplime wall, establishing the diffusivity as 1.56 × 10 - 7 ± 3.8 × 10 - 8  m2 s−1. This method may offer more accurate results. This paper compares the results of diffusivity measurements using this new technique, to diffusivity values calculated through measurements of the thermal conductivity and heat capacity via separate tests. The new technique is found to give results in agreement with previous techniques. The property space filled by hemplime is contrasted with timber, hard aggregate concretes, and insulating materials.