layer 9 and the additional layer 10 are directly adjacent to one another the fabric layer 9 which jointly with the additional layer 10 separates the exhaust gas stream 7 from the fresh air stream 6 can be made of a natural fiber material for example cotton with the absorption of moisture the fabric layer 9 begins to swell thus achieving tightness between the gas streams 6 and 7 at the same time the fabric layer 9 absorbs moisture from the moist exhaust gas stream 7 which is released to the dry fresh air stream 6 the additional layer 10 is located on the side of the fabric layer 9 which is facing the moist exhaust gas stream 7 advantageously the additional layer 10 consists of plastic fibers for example viscose or a mixture of fibers made of different plastic materials expediently the additional layer 10 is designed so as to be thinner than the fabric layer 9 and preferably has a basis weight between 30 g m 2 and 70 g m 2 the additional layer 10 has highly hydrophilic properties and is capable of absorbing moisture from the exhaust gas stream 7 in a short period of time this causes a relatively high water concentration on the side of the.