Winter-time air exchange is a challenge in calf barns. The calf barn I was in on February 5th was a good example of this. 
 
The barn is tucked in among several other large barns so there is no direct access to the prevailing southwest winds common in that location. The day of my visit was one of many since the first of the year with night-time lows in the single digits and day-time highs in the mid-teens - persistent cold, freezing weather.
 
The barn's curtained sides had the curtain full up on the west side and down maybe a foot on the east side. One 18" exhaust fan was laboring away in the north peak of the building. The occupants were about 30 preweaned calves and another 30 transition-age heifers.
 
I measured both temperature and relative humidity outdoors and indoors. As you might suspect the readings were higher inside compared to outside.
 
                                           Temperature (F)           RH (%)
Outdoors                              15                                39
Indoors                                 27                               51
 
The owner reported having to treat "too many" preweaned calves for pneumonia - a persistent problem during the winter months with many fewer treated calves when they were able to keep both the doors and curtains open. 
 
I reinforced her decision to seek assistance to install positive-pressure ventilation equipment. In my experience when both the temperature and relative humidity in a calf barn shows clear increases like those shown above there is inadequate air exchange. And, almost without exception, many of the calves in these barns require treatment for clinical pneumonia symptoms.
 
As a point of reference on this matter of air exchange rates you may be interested in this resource developed by Curt Gooch at Cornell; click HERE for the table of recommended air exchange rates for different seasons of the year for four different age groups of heifers.