The following study consists of the thermal evaluation of the enclosure of a reinforced concrete biodigester, through the theoretical analysis of the influence of each component of the materials in the whole mixture, depending on the distinct values of thermal conductivity of the concrete. To determine this variable in each specific case, a review of the theoretical models developed over the years by experts in the field has been made to find the most suitable model for the case under study. The effect on the thermal conductivity of the variation in type and proportion of the basic elements making up the concrete is studied: the aggregate, the cement, the water/cement ratio, the porosity and the humidity among others, with the aim of providing a tool for estimating the thermal conductivity of a concrete created with the desired proportions. This tool is designed in the form of a nomogram for each specific case, in which conventional concrete, concrete with recycled aggregate from construction and demolition waste (COW) and lightweight structural concrete are differentiated. (C) 2012 Elsevier B.V. All rights reserved.