In the case of three-layered house chimneys which are formed from shaped casing pieces in which an inner pipe is inserted, problems may be expected if the inner pipe is not sufficiently insulated from the shaped casing pieces. Up until now, insulating mats have been pushed in in order to provide an insulation between the inner pipe and the shaped casing pieces. The insulating mats can no longer fulfil their purpose, however, if they become moist, which is always the case, in particular at low chimney temperatures, and are destroyed by the resulting sulphuric or sulphurous acid. To remedy this, the invention proposes to arrange the insulating mats at a distance from the outer wall of the inner pipe, so that ducts running along the inner pipe are produced, and to bring these ducts into connection with the outside air, by way of openings in the lower chimney region and an upper-side opening, with the result that, when the chimney is operating, a constant air movement which is guided upwards and which reliably keeps moisture away from the insulating mats is achieved. IMAGE
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