The air tightness of ventilation ductwork was measured in two recently built low-energy houses and in two conventionally built houses in the summer of 2013. The ducts and components were metal in three houses and plastic in one house. The air tightness of the ductwork had been checked by an installation survey after construction. The measured leakage airflows corresponded to air tightness class A or lower, therefore did not satisfy the minimum requirement set for class B regarding the air tightness of ventilation ductwork. Leakage airflow of the ductwork per duct surface area was estimated to be 47 times higher at a typical ventilation duct pressure of 100 Pa than leakage airflow in the house envelope per internal surface area at a typical house air pressure of 5 Pa. The results showed that the installation survey was not a sufficient method for ensuring the air tightness of ductwork in these houses. Therefore, it is recommended that the air tightness of ventilation systems that serve a single room or a single dwelling are measured during commissioning. A larger study of the air tightness of ventilation systems in Finnish buildings would be necessary.