The thermal behaviour of Medium height building, built in cross laminated timber (CLT) was studied and validated for Chile at the present paper using Builder Design software. Simulation was done for the hottest and coldest day in Santiago, Chile and it was verified that temperature's distribution produced in the building ensures comfort levels in the different areas, and each floor, closed to the ideal comfort temperatures. This was checked even with no use of isolation installations or air conditioning. Furthermore, Energyplus module from the mentioned software was used to obtain the values of internal gain and sun losses in the different floors of the building. It is also shown that there is no great variation in thermal behavior between the ground floor and higher ones.