The main goal of IEA Annex 27 "Evaluation of ventilation systems" is to develop tools toevaluate ventilation systems in an objective way in terms of indoor air quality, energy,comfort, noise, life cycle costs, reliability and other building related parameters.To check the developed tools some measurements in real dwellings are necessary. Thedevelopment of the tools is in its final stage. During the AIVC conference some of these toolswill be presented. The indoor air quality tool is not yet ready. The results reported in thispaper are investigations carried out in three groups of 10 dwellings with practically the samefloor plan. Each group had a different ventilation system.The ventilation systems are:-Natural supply and passive stacks (natural)-Natural supply with mechanical exhaust (mech. exhaust)-Mechanical supply and exhaust (balanced)This paper is the first analysis of the results. In a later stage the measurement results will becompared with the results of the tools developed within IEA Annex 27.