The microwave drying process intensification of new inorganic thermal insulation board, containing expanded perlite and inorganic binder, was investigated in a multimode microwave applicator, with a 1.8 kW, 2.45 GHz microwave source. Ability to generate heat inside moistened material and to avoid the problem of heat conduction to the core of the material is especially important when dealing with efficient thermal insulators. A 3D unsteady mathematical model was developed, considering Lambert's law of exponential decay of microwave energy and nonuniform power distribution inside the microwave cavity, for temperature and drying rate predictions. The predicted temperature profiles at critical locations of the exposed board and cumulative drying rate were compared and verified with experimental data.