Historic structures are a part of our cultural heritage and nowadays, in the polluted environment, the need of their preservation is more intense than ever. One of the anticipated problems includes new materials that have to be compatible with those existing in older structures. In the case of mortars, traditional binders such as lime, natural pozzolanas, brick dust, and white cement have been combined successfully. In the present article a series of mixtures combining lime, two types of natural pozzolanas, brick dust, and different types of cement have been produced in order to measure their thermal conductivity for the first time. The parameters tested are: the binder type, the proportion of the binders, and the water/binder ratio. For the measurement of the thermal conductivity of the samples, a commercial instrument was used. To test its operability and extend its range, a transient hot-wire instrument was employed.