The theory of ventilation drying applied to New Zealand cavity walls (May 2005)

Presented at the 2005 Institute of Refrigeration, Heating & Air Conditioning Engineers (IRHACE) Conference, Nelson, New Zealand, 20-22 May 2005

Authors

MR Bassett and S McNeil

Abstract

Drained and ventilated cavity designs are standard practice in masonry veneer construction and open rainscreen stucco walls have been around for much of New Zealand’s construction history. Both of these wall designs have a satisfactory record of managing moisture, but there is no calculation basis for linking the size of ventilation openings or cavity dimensions with the need for moisture control. This paper offers a preliminary analysis of the cavity ventilation processes and shows how ventilation rates depend on wind and buoyancy pressures in the New Zealand climate. Ventilation rates are estimated for drained and ventilated cavities having well defined vent sizes, but it turns out to be more difficult to do the same for open rainscreen walls. Here the ventilation process is likely to be dominated by natural infiltration through the wall. Differences between open rainscreen and drained and ventilated walls are presented in the context of theoretical drying rates in the New Zealand climate.