We describe a mathematical model applied to analysis of hygrothermal behavior of buildings. A lumped approach to model the room air temperature and humidity is used and a multi-layer model in finite differences for the building envelope is employed. The capacitance model allows studying the transient air humidity and temperature of a room when it is submitted to the weather of the city of Curitiba-PR, Brazil. To evaluate the building performance with thermal parameters, we have written a code which includes air infiltration, conduction loads, internal gains of people, lights and equipment and short and long wave radiation. In the results section, we show the influences of simulation time step on internal temperature and humidity and temperature profiles within the building envelope.