The modal method is a very powerful way of modeling thermal systems such as buildings. The modal expression offers a pertinent view of the thermal inertia through specific analysis tools. Modal-based simulations are easy, accurate and fast. The modal dominance of some of the eigenelements is the basis of the modal reduction. This reduction does not affect the static characteristics of the model, and introduces, if used correctly, small deviations which are generally acceptable, and always compensated by an increase in speed of the dynamic simulations. The modal facilities have been implemented with the software package m2(m), a freely available set of modules which perform the different calculations needed to get the results explained in this paper.