The total ventilation rate, because it includes infiltration through the building envelope as well as outdoor air flow through the ventilation system, can usually be measured only by tracer gas methods (Sandberg and Sjoberg, 1983 or ANSI/ASHRAE, 1997). In buildings without air recirculation, the mechanically-supplied ventilation rate can also be determined with reasonable accuracy by measuring total supply or exhaust air flow rates. The choice between supply and exhaust flow rate measurements should be based on the indoor air pressure relative to that outdoors. Typically, to reduce moisture problems, the design intent is to underpressurize buildings in cold climates and to overpressurize buildings in warm climates. In underpressurized buildings, the measurement of exhaust flow gives a good estimate for the total ventilation rate, and in over-pressurized buildings the measurement of supply flow rate indicates the total ventilation rate. If return air is mixed with outdoor air and recirculated back to the rooms, the supply flow rate must be multiplied by the proportion of outdoor air in the supply air stream which is measured by a tracer gas procedure or estimated from CO₂ measurements in the return air, outdoor air, and mixed air. Another measurement method used by air balance professionals is a traverse of air velocities at the outside air intake louver, often with a rotating vane anemometer. Wind or low air speeds often make it very difficult to accurately determine ventilation rates using this method. Recently a few vendors have started to market measurement systems for outside airflow that are installed just upstream or downstream of the outside air intake louvers.

• Blower door
• Tracer gas

Tracer gases can be used to measure the ventilation rate in buildings. The gases used are odorless, nontoxic and typically used at concentrations in the range of parts per billion (ppb) to part per million (ppm). Some typical tracers that are used include:

Sulfur Hexafluoride (SF₆) , Perfluorocarbons and Carbon Dioxide (CO₂). The tracer gases can be injected continuously or in a single burst in the supply air duct or in the ventilated occupied space.

• Others such as flow hoods, and VAV box measurements

* With the ventilation system providing the minimum rate of supply of outside air, a normal condition during cold weather and also when outside temperatures exceed the indoor temperature.