Green building envelopes in urban environment are well-known for their environmental benefits. They enhance building energy efficiency and improve the urban microclimate and urban living comfort. However, in dense built environments, the evaluation of performance of green building envelopes is more difficult as shading can affect the solar radiation, while different surface temperatures of the green and built surfaces have a strong influence on the incoming longwave radiation. Although many studies have investigated the cooling and energy-saving benefits of vertical green building envelopes, research on proper evaluation of incoming longwave radiation (LWR) from different surrounding surfaces remains limited. This knowledge gap hinders the proper evaluation of thermal and hydrological response of green building envelope surfaces that include living walls. Therefore, this study proposes the improved model for LWR on vertical surface in built environment. The model is based on meteorological data, such as solar radiation, air temperature, and sky LWR which can be gathered from databases. View factors of sky and vegetated and built surfaces are needed in the LWR model and have to be determined separately as they are site specific data. The novelty of the proposed LWR model is that it takes into account heat accumulation in non-insulated built surfaces. The proposed model was experimentally validated and compared with existing models presented in relevant literature. A presented case study of hydrological response of living wall located in a street canyon clearly shows the influence of incoming longwave radiation, determined with different models, on living wall water balance and needs of irrigation water. The results show up to 20% higher irrigation water needs for living wall in heat-stressed urban environment. Improved LWR model integrated into software tools can contribute to sustainable irrigation water management in cities.