Abstract: A structural element having a region of porous silicon or porous silicon oxide, which was obtained from a porization, starting from an edge area of the region, in at least largely crystalline silicon. Relative to the edge area, the crystalline silicon has a crystal orientation that has an orientation that differs from a <100> orientation or from an orientation that is equivalent for reasons of symmetry. This structural element is suited for use in a mass-flow sensor, in a component for the thermal decoupling of sensor and/or actuator structures, or a gas sensor. Furthermore, methods for setting the thermal conductivity of a region of porous silicon or porous silicon oxide of a structural element are described.