The sky temperature is an important parameter for simulation codes in building studies. A preliminary campaign of validation of a simulation software CODYRUN has demonstrated the misinterpretation of the radiative exchanges of long waves between the building and its environment. A bibliographical research has then been led to the use of models using dry air temperature to estimate sky temperature. However, these models has not been completely satisfactory as far as night clear sky are concerned. In this case, sky temperature remains overestimated. A research of a non linear models has been undertaken, leading to use of neural networks with satisfactory results. Sky temperature is then calculated and reinjected in the simulation code CODYRUN. Comparison between simulated temperature ad measures has turned to be acceptable.