Future global warming scenarios will have a significant influence on existing building, especially in those most vulnerable due to major percentages of glazed area. Increasing temperatures will result in an increase in energy consumption due to cooling, especially in the Mediterranean climate with high solar radiation levels. Sun protection systems may be one of the first strategies employed to resolve this issue. Among these, bidirectional shading devices display an excellent behavior although its numerical simulation is very difficult. This research aims to develop an open source numerical method to calculate the short wave solar transmittance in an egg-crate shading device for use in the rehabilitation of tertiary buildings in Mediterranean climate. For this purpose a dynamic three-dimensional hybrid model has been developed combining raytracing and radiosity methods. The numerical method has been validated with monitoring data in southern Spain. The results obtained confirmed the accuracy of the model and the suitability of this type of shading devices in reducing the solar gains in summer with reduced blocking of solar radiation in winter.