The software bSol addresses to the professionals (engineers and architects) eager to optimize the parameters of a building project according to the local environment. It allows, hour per hour over a whole year, to determine the thermal response of a building located under specific climatic conditions (including local horizon): the solar gains are taken into account in a very efficient way. Moreover, this software offers a sensitivity study by varying eight constructive parameters, (thermal protection, size of the glazed openings, choice of the glazings, attenuation of the blinds, ventilation with/without recovery of heat, thermal inertia). In addition, bSol gives a very good idea of the comfort obtained inside the building.