With regard to room comfort and potential moisture damage hygrothermal conditions inside a building are of high interest, especially in highly humid or dry climatic regions. In order to achieve satisfying climatic conditions not only thermal storage, but also moisture storage of walls needs to be taken into consideration. Dynamic effects of walls and the impact on thermal room climate require a detailed thermal wall simulation. In the simulation program NANDRAD a Finite Volume approach is used to discretized each building construction. The balance equation system is now extended with coupled moisture mass balance, both for zone air volumes and all construction elements. This leads to a duplication of system unknowns with highly nonlinear interactions between solution variables, resulting in strong increase of simulation time. In order to improve performance for larger simulation models, a mixed grid approach is evaluated. Hereby, a detailed grid is used for energy balances and coarser grid for moisture balances. In the article we discuss the procedure, evaluate the impact on accuracy of solution results, and discuss numerical problems. We show that the procedure indeed helps reducing the overall number of unknowns, which, however, appears to pay off only for large building models.