This International Standard specifies a method to determine the thermal transmissivity of a moist material (λ*) under steady-state moisture conditions, i.e. not affected by moisture movement. It is measured using standardized guarded hot plate and heat flow meter methods, at temperatures above 0 °C. This material property is a function of the moisture content and does not represent the thermal performance of a material under service conditions. However, it can be used, together with knowledge of the moisture conditions in the material, to predict the practical thermal performance. The use of λ*, the distribution of moisture under service conditions and consequently the prediction of thermal performance under service conditions are outside the scope of this International Standard. However, the moisture distribution under service conditions should, where possible, be considered when λ* is determined. Furthermore, transient methods of measurement are not included due to the difficulty involved in analysing and interpreting the results of these methods.