This paper compares apartments in two residential blocks in Vienna; one passive and the other one low-energy. These blocks were constructed simultaneously in the same location and with comparable building construction features and floor plans. The main difference between the two blocks (other than the higher thermal insulation level in the passive building) lies in the ventilation system: passive buildings deploy controlled ventilation, whereas the low-energy buildings rely mostly on user-operated natural (window) ventilation. We measured indoor environmental conditions (indoor air temperature, relative humidity, and CO2 concentration) in two units of each block over a period of five months. Additionally, the buildings were compared in view of operation and embodied energy use, CO2 emissions, and construction costs. (C) 2010 Elsevier B.V. All rights reserved.