This type of ducting can be used to connect to an extractor fan in your bathroom or shower room when the duct passes through your loft or attic, In this situation warm air being extracted by the fan can condense due to the cold air above and result in water droplets coming back down into the extractor fan and possibly damaging it, Using this type of ducting normally solves the problem as the insulation on the duct means that the air passes through without coming into contact with the cold temperature above, Used to minimise the heat gain or loss resulting from the temperature differential between the air flow and the surrounding ambient air,