Thermal conductivity is one of the most important hygrothermal properties of building materials which depends on temperature and moisture content. The one-variable linear function is usually used to describe the influence of either temperature or moisture content on the thermal conductivity when the other one is fixed. However, for real situations temperature and moisture content have simultaneous influence. The thermal conductivities of five typical building materials-expanded polystyrene (EPS), mineral wool, concrete, adhesive mortar and rendering plaster-were measured with the guarded-hot-plate apparatus at different temperature and moisture content. Results show that the two-variable linear function is sufficient to describe the coupled influence of temperature and moisture content on all studied materials. The interaction between temperature and moisture content is negligible. For weakly hygroscopic and capillary materials such as EPS and mineral wool, the two-variable linear function can be simplified to the one-variable linear function of temperature.