Abstract: The air to be supplied to a room is brought to a desired temperature and a desired humidity through the following process steps: making outside air flow as a first air stream through a first line (20) and making a first portion of exhaust air coming from the room flow as a second air stream through a second line (21), wherein moisture is exchanged between the first air stream and the second air stream by means of a device for exchanging humidity, condensing, by means of a dehumidifying and cooling device (19), moisture in the form of water from the first air stream and/or from the second air stream and/or inside the device for exchanging humidity, and making a second portion of the exhaust air flow as a third air stream through another line (32) comprising another dehumidifying and cooling device (33) and returning said second portion back to the room, wherein in the other dehumidifying and cooling device (33) moisture is condensed from the third air stream in the form of water.