A simplified simulation module has been developed to calculate the thermal performances of multizone buildings, using modal analysis. This technique reduces the computation time and therefore allows architects to perform accurate simulations using a low cost micro computer (type AT for instance). The thermal calculations are based upon a data structure in which the building is decomposed into elements (thermal zones, walls, windows, materials, ...). Each building element has been modeled as a computer object. Components of higher complexity are linked to simple components via pointers. Thus, if a component is modified, the modification is automatically transmitted through the whole structure. This environment is particularly adapted to connect the thermal calculation module (COMFIE) to an expert interface. At the moment, no expert system or inference engine is implemented, but simple algorithms which work in the following way. The user inputs a project and several characteristics (heat losses, solar gains, ...) are studied by the expert tool, which then proposes alternative designs. The user may test these modifications by repeating the simulation. This tandem simulation/expert interface takes into account the specificity of the project and its climate when applying the expert knowledge. A few expert rules are proposed, in order to enhance the energy savings and the thermal comfort. New expert rules may also be generated and/or validated. This design tool not only provides the user with fast and accurate simulations, but also offers him an opportunity to include the thermal aspects in his design approach with the help of expert rules, and this constitutes a real computer aided design.