This paper deals with the real behavior of a humidity controlled air inlet, regarding to relative humidity and temperature. It has been often heard, that relative humidity is not the best indicator for detecting a need of ventilation in main rooms (living room and bedrooms) because it is season dependant (the absolute humidity outside varies in a wide range from winter to summer, which should lead the air inlet to be closed in winter and open in summer). Designing a humidity controlled air inlet is much more complex than designing a humidity controlled extract grille because this effect of the wide variation of the outside absolute humidity must be fought to ensure a proper functioning throughout the year. The sensor is sensitive to relative humidity, in its own environment. The local relative humidity depends on the absolute humidity and the temperature of the sensors environment, which is linked to inside and outside temperature. The know-how of designers allows to have a controlled drift of the response curve (aperture Vs internal relative humidity) that makes it possible to keep a good answer to occupancy, without any manual adjustment during the year.