Modeling the behavior of an enclosure for use by a control system of an HVAC system is described. A model for the enclosure that describes the enclosure's behavior for use by the control system is updated based on weather forecast data. The weather forecast data can include predictions more than 24 hours in the future, and can include predictions on temperature, humidity and/or dew point, solar output, precipitation. The model for the enclosure can also be updated based on additional information and data. The model for the enclosure can be updated based also on an enclosure model stored in a database, and/or enclosure information from a user. The model can be updated based on active testing of the enclosure which can be performed automatically or in response to user input. The testing can include heating and/or cooling the enclosure at times when the enclosure is not likely occupied.