There are approx. 5.5 million residential buildings in Poland including 5 million single-family houses, 90% of which were built before 2002. Most of the existing single-family houses are not energy-efficient and a large amount of energy is wasted. It is therefore essential to develop, experimentally validate and then disseminate strategies for thermal modernization of the existing single-family houses, including strategies for ventilation systems renovation. In two-storey single-family houses with sloping roofs it is possible to locate a ventilation heat recovery (VHR) unit and ventilation ducts in the unheated roof space. The paper presents the results of tests of combined ventilation systems: mechanical ventilation with heat recovery in rooms on the second floor and stack ventilation in the rooms on the first floor. On-site measurement of the efficiency of ventilation heat recovery was performed, the airtightness of the house was measured twice before and after its improvement. The results of the experimental tests of the hybrid system were used to estimate the ventilation airflow in the house and the energy demand for ventilation. The ventilation airflow was calculated using the software CONTAM. The estimated heat demand for the tested ventilation system was 37.4 kWh/m(2)/season; though it seems possible to reduce it below 20 kWh/m(2)/season. (C) 2017 Published by Elsevier B.V.