1. Ventilation controls the indoor environment and is critical for poultry production and welfare. Ventilation is also crucial for assessing aerial pollutant emissions from the poultry industry. Published ventilation data for commercial layer houses have been limited, and are mostly based on short-term studies, mainly because monitoring airflow from large numbers of fans is technically challenging.2. A two-year continuous ventilation monitoring trial was conducted at two commercial manure belt houses (A and B), each with 250 000 layers and 88 130-cm exhaust fans. All the fans were individually monitored with fan rotational speed sensors or vibration sensors. Differential static pressures across the house walls were also measured. Three fan performance assessment methods were applied periodically to determine fan degradations. Fan models were developed to calculate house ventilations.3. A total of 693 and 678 complete data days, each containing >16 h of valid ventilation data, were obtained in houses A and B, respectively. The two-year mean ventilation rates of houses A and B were 2·08 and 2·10 m 3 h -1 hen -1, corresponding to static pressures of -36·5 and -48·9 Pa, respectively. For monthly mean ventilation, the maximum rates were 4·87 and 5·01 m 3 h -1 hen -1 in July 2008, and the minimum were 0·59 and 0·81 m 3 h -1 hen -1 in February 2008, for houses A and B, respectively.4. The two-year mean ventilation rates were similar to those from a survey in Germany and a 6-month study in Indiana, USA, but were much lower than the 8·4 and 6·2 m 3 h -1 hen -1 from a study in Italy. The minimum monthly mean ventilation rates were similar to the data obtained in winter in Canada, but were lower than the minimum ventilation suggested in the literature. The lower static pressure in house B required more ventilation energy input. The two houses, although identical, demonstrated differences in indoor environment controls that represented potential to increase ventilation energy efficiency, and reduce carbon footprints and operational costs.