Why is it so wet in here?
Put yourself in a calf barn with 100 preweaned calves between birth and weaning. It is summer. Curtains are wide open and doors on both ends are open, too.
Not much air moving today. Humidity in the calf barn seems pretty high. Floors are not drying out. Why is it so wet in here? Well, "Hello, Don't be surprised!" Calves generate waste water and lots of it.
Calves release about 0.2 pounds (91gm) of moisture per 100 pounds (45kg) body weight per hour into their environment via urine, feces and respiration. For example, the 100 calves in this barn averaging around 150 pounds (68kg) release between 80 and 90 gallons (303-341L) of water daily.
Only by providing adequate fresh airflow can airborne moisture be removed and the humidity brought down to a level at which pathogens cannot survive. Reducing noxious gases depends on airflow rates, as well.
When the ratio of calves to area open for natural ventilation is low we can get away without mechanical ventilation most of the time. The barn I had in mind originally had four rows of calves the length of the barn separated by two work alleys. With only natural ventilation most days, even with the curtains open, the ratio of calves to opening for ventilation was too high to exhaust the excessively humid air.
This calf barn was improved by adding tunnel ventilation (a row of large fans all across one end) so that even on a still day I could feel a draft from the open end toward the exhaust fans.
What's the take home message? Calves generate a lot of waste water. Provide enough air exchange to get rid of it.