Sky radiation models play an important role in the longwave radiation exchange between the opaque surfaces of buildings and their surroundings. The article is focused on an accuracy analysis of sky models implemented in the MZELWE simulation module of the ESP-r program. Dynamic outdoor tests and spectral laboratory methods were used for the validation of thermodynamic procedures in the given simulation module to demonstrate its applicability to calculating the accuracy level of models simulating longwave radiation phenomena. Based on high and low-emissivity properties, the presented issue focuses on vertical opaque walls in relation to highly insolated and weakly insulated building components for which longwave radiation exchange has been observed to be of most significance. Clear sky climatic conditions were predominantly applied for the purpose of validation at a Central European location. Among all the analyzed longwave sky radiation models implemented in the ESP-r program of the MZELWE module, only Clarke's model provides an applicable result with regard to the overall heat flow measured. The effects of the sky models were compensated by the greater thermal performance of the building components, for which the influence of the longwave radiation exchange on the resulting heat flows was significantly reduced. (C) 2015 Elsevier B.V. All rights reserved.