Highly porous silicon has been successfully used to create thermally isolated local areas on bulk crystalline silicon, for the fabrication of silicon integrated sensors. Heat distribution and heat transfer have been simulated by using finite element analysis with the ANSYS software. Three cases were compared: the case of bulk crystalline silicon, the case of porous silicon and that of free standing polycrystalline silicon membranes. A flow sensor of the thermal type using porous silicon isolation has been designed and fabricated, showing high sensitivity and rapid response. Its operation principle and main characteristics will be presented.