A rectangular piece of alumina fiber insulation was designed to prevent heat loss. The heat source was generated by a susceptor material, which was used as a microwave (MW) absorber (heater). The thicknesses of different sides (frontal, lateral, and longitudinal) of the insulation piece were varied from side to side to study the penetration level of MW electromagnetic field coming from both horizontal and vertical incidences. The experience has shown an increase in temperature when top and longitudinal thicknesses were decreased. It has also been found that specific orientation of the insulation box leads to a maximum MW heating efficiency.