A wall structure for a building, comprising a cavity (10) for allowing air to flow through the structure between the building's interior and exterior, between the wall structure's inner (2) and outer layers (4). An electricity generating member (20) for generating electricity using heat transfer is located for transferring heat from the building's exterior to interior when exposed to relatively warm external conditions (THOT) and from the building's interior to exterior when exposed to relatively cooler external conditions (TCOLD). The electricity generating member (20) being located for promoting an energy transfer with air within the cavity (10).