A simplified approach to thermal building behaviour using natural cooling techniques has been proposed within the frame of the Pascool/Joule II European project. This provides useful information relating to the feasibility of integration of natural cooling techniques into a given building, taking into account only the climatic potential of the site, the indoor comfort criteria and the building cooling requirements. This approach is useful for architects and building designers, offering them helpful tools for the selection stage of the cooling system types. However, it is insufficient for the building design stage, where more accurate results, dealing with thermal behaviour and energy consumption of the building, are required. In order to solve this, we propose a thermal evaluation approach to building using direct and indirect evaporative cooling systems. This work has been realized using the integration of the passive cooling system models developed by our team in Passport+, a thermal building software, developed in the European projects Pascool/Joulell and Altener/Sink. This approach has been applied to a typical office building in La Rochelle (France). This procedure offers useful informations relating to the building behaviour (thermal and comfort behaviour), defines the cooling installation by sizing the passive and/or conventional cooling systems and shows the building energy saving by both the direct and indirect evaporative cooling system.