Air flow in a cavity wall.

The external facade of a nine storey office building has been reclad with a ventilated cavity structure with a length to height ratio greater than forty. As there is little published information regarding the likely air flows within such cavities a research programme has been set-up to investigate the ventilation and energy performance of this structure. This paper will address the cavity air flows through both theoretical and full scale measurements. Theoretical predictions were calculated using the COMIS infiltration simulation programme utilising different pressure profiles from around the building. A detailed wind tunnel study [1] was carried out prior to this in order to obtain accurate wind pressure distribution profiles for the building. The experimental measurements were carried out on the cavity wall structure on the second (west facing) floor of the building. Using a N2O tracer gas detector system, the flow velocity and the air flow volume were deduced from the measurements and compared with the theoretical prediction. The comparison of the two sets of results show good agreement, considering the varying wind and temperature conditions which are difficult to simulate. The maximum difference between the calculated and the predicted airflows, for the tests carried out is in the range of 30 %. Air flow pattern and mixing of the tracer gas were demonstrated using a 2.44 metre long segment of the wall structure under laboratory conditions.