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 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. In particular, in a porization of crystalline silicon, starting from an edge area of the region, the crystalline orientation of the silicon relative to the edge area is selected such that a thermal conductivity comes about along a direction perpendicular to the edge area that differs from, in particular is lower than, the thermal conductivity, that comes about in this direction in an otherwise analogous porization of crystalline silicon having a orientation or an equivalent orientation relative to this edge area.