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. In buildings without air recirculation, the ventilation rate can also be determined with reasonable accuracy by measuring 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