Double-skin facades (DSFs) are promising and attractive building envelopes. DSFs can reduce the undesired heat gain during cooling season. However, due to complicated air flow and heat transfer in a DSF, economical and practical methods suitable for evaluating the thermal performance of a naturally ventilated DSF for the duration of a cooling season are still lacking. A large number of computational fluid dynamics (CFD) simulations are required to reach an assessment for a naturally ventilated DSF. This paper proposes a new fast calculation method. The proposed method significantly reduces the CFD simulation workload by establishing a correlation between indoor heat gain and solar radiation intensity. The novel calculation method demonstrated good computational accuracy. The results were verified by CFD models and were then applied to obtain the total indoor heat gain for a cooling season. This method could be used in optimization of DSFs. (C) 2015 Elsevier Ltd. All rights reserved.