Longwave radiative heat exchange plays an important role in heat transfer of building facades with exterior louver blinds, which is a well-known solution for constructing energy-efficient buildings and the good indoor thermal-visual environment. However, errors may arise,in the calculations for this process, due to the simplification of surroundings by the Blind Enclosure Model (BEM), which is the most common method to determine the view factors for a facade with louver blinds. In this study, we used field measurements to determine boundary conditions, and estimated longwave radiative heat exchange results of BEM by numerical calculations. It indicates the net radiative heat gain of the building fa ade will be underestimated by the BEM, for its view factors calculation is inaccurate, and thus, the BEM does not provide a conservative prediction for load and energy consumption of HVA&C system. Then, we established a combined method of street canyon geometry and crossed string algorithm (named SCCS method), in order to improve the view factor determination. The difference between this method and numerical simulation is less than 5% in the majority of cases, representing an improvement compared to the BEM. (C) 2017 Elsevier B.V. All rights reserved.