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Increases in air moisture concentration, presented as the change in humidity ratio from outside to inside air, were compared to ammonia accumulation within an experimental room housing 120 White Leghorn layers. Threshold ammonia concentrations (25 ppm) were not reached with a Humidity Ratio Difference (HRD) from outside to inside air of less than 0.008 kg H2O/ kg dry air. A more appropriate HRD of 0.006 kg H2O/ kg dry air was related to ammonia concentrations of 10 ppm. Ventilation rates (observed during the data collection periods) that were able to reduce ammonia concentrations within the room were applied to a steady state mass balance and the corresponding humidity ratio difference between inside and outside air. The calculated HRD difference ranged from 0.005 to 0.007 kg H2O/ kg dry air. Based on the two mentioned techniques a Critical HRD for the experimental room was determined to be 0.006 kg H2O/ kg dry air. Results from the experimental room were compared to data obtained in commercial hen houses. Data from three high-rise hen houses located in York County, PA, each housing approximately 125,000 birds were evaluated based on humidity ratio difference. Results were similar to the experimental room findings but demonstrated that the Critical HRD is affected by management techniques that change the ammonia production rate of manure. The advantage of cold weather environment control based on the inside and outside humidity ratio difference is that variations in building management as well as differences in moisture production rates can be accounted for in the HRD term.