Ammonia (NH3) and carbon dioxide (CO2) concentrations, air temperature, and building static pressure were monitored during winter in a large-scale (425,000 bird) commercial laying hen barn with manure belts by using 4 Intelligent Portable Monitoring Units (iPMUs). The sampling points of the 4 iPMUs were inside and between cages, and at the top of two minimum ventilation fans. Recorded NH3 and CO2 concentrations varied with the sampling location inside the building and the ambient temperature. High NH3 concentration (up to 40 ppm) was found at the sampling point on top of the minimum ventilation fan between the barn and the manure drying room. Air leaked through the non-operating fan louvers from the manure drying room back into the barn, and was caused by the back pressure between manure drying and bird cage areas resulting in high NH3 concentration near the sidewalls between the barn and the manure drying room.