In the context of low energy buildings we study the impact of the incoming radiation through a window (sun patch) on the heating and cooling demand. Existing studies have shown that not considering the sun patch and fast climatic variations (Figure 4, Global radiation) can lead to important differences in energy power evaluation [1, 2, 3]. In this paper we present a 3D envelope model taking into account the minute-wise sun patch evolution. Simulation results are analysed for a low energy cell. A numerical model has been developed in order to simulate the transient thermal behaviour, with a refined spatial (3D) and temporal (down to one minute time step) discretization of the single room. For each node of the grid, the energy conservation equations are developed. They traduce balance between short-wave and long-wave irradiations, convection, air enthalpy and three-dimensional heat conduction. The particularities of the program are that it projects the sun patch on the inner walls, the conduction is treated in three dimensions and climatic minute-wise variations are taken into account. As main results, the surface temperature evolution, the air temperature evolution and the heating or cooling power necessary to maintain an inner air set-point temperature are calculated at each time-step. Heating or cooling power is compared to the power calculated with no sun patch incorporation (solar loads only on the floor). Conclusions are made on the importance of the integration of the sun patch and its impact on the observed results.