In the developed countries the air-conditioning of buildings involves a great electric energy demand: for example in the United States around one-sixth of all the electricity generated is used by HVAC units. The study of innovative building envelope can greatly reduce the heating and cooling loads increasing also the quality of life inside the buildings. The horizontal roof gives a strong contribution to the summer loads of buildings due to the insulation. The utilization of a green roof was considered to reduce the correspondent cooling load, having into account both the effect of the thermal capacity of a mass of soil and the evaporative cooling due to vegetative greenery. This analysis was applied to a factory for the production of solar cells, to be built up in the next months, with a novel design to reduce the HVAC loads. The analysis aims to study the effects of different thickness of soil, separating the influence of thermal capacity and of the evaporation.