Indirect method versus direct method for measuring ventilation rates in naturally ventilated dairy houses

• CO2-balance overestimated AER; H2O-balance underestimated AER.
• Integration time is important for reliable AER in indirect methods.
• Differences between indirect and direct methods varied with wind velocity.
• AER unreliable when inside-outside ΔT, ΔAH, and ΔConcentration were low.

Indirect methods are widely used for determining air exchange rates (AER) in naturally ventilated barns because they are relatively easier and cheaper than direct methods, which measure actual airflow in and out of the barns. The main goal of this study was to evaluate a common indirect method (CO2 mass balance) against a direct method, and identify factors influencing this indirect method. The mean AER based on 24-h averaging, irrespective of method, ranged from 13 to 39 h−1 during the study-periods. The CO2-balance method tended to overestimate barn AER. The cows' CO2 production rate, in the current study, was estimated at 0.178 m3 h−1 hpu−1 based on 24-h averaging. The CO2-balance method with 24-h data averaging yielded more reliable barn AER than with shorter averaging times (i.e., 1, 2, and 12 h). The 1-h averaging, however, was chosen to analyze the effects of other pertinent factors on the CO2-balance method to capture diurnal variations of AER. Both wind speed and wind direction had significant effects on barn AER as well as the difference between the CO2-balance method and direct method. Barn AER, in general, increased with wind speed. The CO2-balance method was unreliable during milking times, and when indoor–outdoor CO2 concentration and temperature differences were less than 70 ppm and 0 °C, respectively.