Francisco Javier Sáenz de Oíza was perhaps the greatest architect of the twentieth century in Spain, and his masterpiece was the Banco de Bilbao Headquarters, located in the Paseo de la Castellana in Madrid (Spain).
Sáenz de Oíza was a university professor in the Installations and Energy Areas at the University of Madrid. For him, concepts such as indoor comfort, lighting, heat production, cooling, and solar houses were an indispensable part of the student's learning and, of course, he included them in his building design. He applied these concepts in the Banco de Bilbao Headquarters.
Therefore, the purpose of this paper will be to examine the Banco de Bilbao technical knowledge in depth—including its passive architectonical design—and to present the main good decisions (and mistakes) of this first low-energy Spanish building after almost 30 years of intensive use, to extend its lessons for future buildings.