Cascade ventilation systems are based on minimal ductwork. This is being achieved through the use of active transfer vents, that connect air supply rooms, e.g. bedrooms without ducting to the MVHR system. Such transfer vents are fans that pull air out of the room into e.g. an hallway, creating an influx of fresher air through the door undercut or other transfer opening from other parts of the dwelling. They measure CO2 and humidity in the respective room and speed up accordingly, in order to maintain good air quality levels.
The reasoning behind this set-up is that living and sleeping spaces are typically used at different times. Fresh air brought into the dwelling through the MVHR unit is either going to be used in the living room or the bedrooms. This allows to reduce the ventilation rate and therefore the running costs without compromising the air quality. Although it contains a fan, the unit is extremely silent and normally inaudible at with 21dB(A) at 40 m3/h.
Please see below how the cascade ventilation works in dwellings throughout the day and the night: