Winter months brings the inclination to close up the rabbitry to preserve warmth from escaping. While this may comfort the caregiver, one must remember that rabbits prefer a cold climate verses a hot building. Even in the extremes of winter it is important to provide proper ventilation to prevent respiratory problems. If you walk into the rabbitry and notice an ammonia smell or your eyes water, then you need to consider better ventilation.
The number of air changes needed is going to be dependent upon several considerations, including, the number of animal housed in the square footage, proximity of ammonia build up to animals when stacking cages are used, the square footage of cages versus the number of rabbits housed.
The greater the density of rabbits, the more important the need for ventilation. A rule of thumb—if you walk in and the atmosphere is noticeable to your perceptions—you need to ventilate.