Mass transfer due to pressure-driven air flow is one of the most important processes for determining both environmental quality and energy requirements in buildings. Heat, moisture, and contaminants are all transported by air movement between indoors and outdoors as well as between different zones within a building. Measurement of these air flows are critical to understanding the performance of buildings. Virtually all measurements of ventilation are made using the dilution of a tracer gas. The vast ma,jority of such measurements have been made in a single zone, using a single tracer gas.

Tracer-gas techniques have become widely used to measure the ventilation rates in buildings. A tracer gas is an idealized substance used to tag volumes of air so as to be able to infer their bulk movement; the properties of a perfect tracer are discussed. The basic principle involved is that of canservation of mass (of both air and tracer gas) as expressed in the continuity equation; by monitoring the injection and concentration of the tracer, one can infer the exchange of air. This report will summarize the techniques in use.

Increasing air-tightness of houses in Canada in recent years, and related concern over indoor air quality and excessive humidity in winter, has led to provisions in the National Building code of Canada for mechanical ventilation systems in all new selfcontained