The solar radiation balance in buildings has a significant impact on their energy needs, as well as on their potential BIPV energy production. It also influences the potentials of daylight, its healthiness and sustainability. Solar radiation models for urban environments require the characterization of the obstruction degree to which each point is subjected due to other buildings, topography, vegetation, etc. This characterization is carried out with the parameter known as sky view factor (SVF). In this paper, we deepen and extend the study of SVF on vertical surfaces that have a high level of obstruction. This paper aims to present a general mathematical method to obtain projection equations in vertical planes, which allow the study of SVF as a surface ratio. It is also proposed an adequate projection for vertical planes under the hypothesis of angular distribution of diffuse radiance based on Moon-Spencer's model. (C) 2014 Elsevier Ltd. All rights reserved.