Measured ventilation rates in water managed wall cavities

A tracer method has been used to measure ventilation rates in the water managed cavities of eight different walls in a test building at BRANZ. Four were drained and ventilated (vents top and bottom), three were open rainscreen designs and one a drainage plane. Carbon dioxide was found to work well as a tracer once the absorption characteristics of the gas in building materials were understood. The tracer was released continuously into the cavity to a concentration of around 1 % and the CO₂ emission rate and concentration interpreted in terms of an average cavity ventilation rate. Ventilation rates were found to be similar to theoretical estimates in drained and ventilated walls, but higher than expected in open rainscreen walls. The reason for this was traced to natural infiltration paths allowing similar ventilation processes, but on a smaller scale, to those in drained and ventilated walls.