Transient behavior of an active thermal insulation system consisting of a semi-transparent porous medium with gas injection was considered. Numerical analyses were carried out on the semi-infinite layer of a porous medium, and hydrogen was chosen as the injected gas. During the early stages of heating, thermal penetration depth, surface temperature and surface heat flux do not depend on the gas injection velocity. As the heating time increases, the thermal penetration depth of the active thermal insulation system reaches a constant value for each gas injection velocity, and the surface heat flux vanishes keeping the back face at a low temperature. However, the thermal penetration depth of a conventional insulation without gas injection never reaches a constant value. Transient solid temperature distributions in the layer for two different surface boundary conditions are almost identical except in the vicinity of the surface.