A building panel may be installed below a slab in the construction of buildings. The building panel supports the slab and also provides a ventilation layer that may be depressurized to eliminate or reduce infiltration of radon gas into the building. The ventilation layer may comprise channels which provide a two-dimensionally interconnected void. Ventilation panels which include collars for connecting to ventilation systems may be provided. The panels may be installed directly on compacted soil. The building panels may additionally provide sub-slab insulation and/or a capillary break for water drainage. In some embodiments the building panels are formed substantially entirely of thermal insulating material such as rigid polystyrene foam. In an example embodiment the panels are approximately 4 inches thick and have a grid of intersecting channels formed on an underside of the panels.
