Abstract: A continuous building basement wall and floor cavity, formed by the foundation wall and floor enclosure on the outside, and an attached inner air and vapor barrier structure, insulated as appropriate, is drained and ventilated to inhibit and prevent soil moisture and gases, such as radon and methane, from entering the basement living space. The air exhausted from the cavity can be drawn from the basement living space, as well as from the surrounding soil and the outdoors. This ventilation of the cavity serves to thermally condition the basement enclosure structure and, coincidentally, to beneficially augment the ventilation rate of the living space. Cavity exhaust air can be used for combustion air supply and heat recovery pruposes. Such a cavity can be economically constructed, using novel modifications to current construction practices, in both new and existing building structures.