Ventilation of Barn Attic Space
Provisions should be made for continuous ventilation of the attic space in order to minimize the build-up of moisture caused by changes in climatic conditions. Warm, humid air in contact with the cold roof will result in condensation on the roof deck, as the cooling air is unable to hold as much water as warm air. This condensation may result in rotting of wood, rusting of metal, or molding of stored hay.
Prevention of humid air rising from livestock quarters or arenas is the first step to keeping the roof and hay dry. A moisture barrier between the livestock and the attic space will prevent humidity from animal respiration or wastes. The original barn cupolas were connected by an insulated chute passing down through the attic spaceto the lower level of livestock housing, allowing heat and moisture from the animals to escape through the hay mow, using the chimney effect. The air inlets were along the top of the side walls, equal to the cupola openings. Today, fans can be placed on the wall of the livestock area to draw hot, moist air away from the animals.
Attic ventilation is best accomplished by continuous eave vents along both sides of the roof overhangs and a continuous ridge vent at the peak. Provide one square inch of vent area for each square foot of ceiling area. The vent area should be evenly divided between the eaves and the ridge for best results. Provide 2 inches of ridge slot for every ten feet of barn width. In typical installations with screened soffits, the air enters the soffit, flows over the plate, and as it warms – rises along the roof to exit through a ridge vent. Be sure to keep hay bales (and beams) away from the roof deck so as not to obstruct upward airflow along the underside of the roof. Use of cupolas, instead of a continuous ridge vent, are a more expensive option and require horizontal movement of air along the ridge before escaping. Louvered openings on the gabled endwalls can be used for short buildings less than sixty feet long. The airflow from end to end should not be obstructed at the height of the louvered openings, unless an exhaust fan is used on one endwall to pull the air through the attic.
Example:
Attic floor 37’ X 98’ = 3626 square feet
Total vent area = 3626 square inches (minimum)
1)Ridge outlet area 4700 square inches (98 feet X 12 inches X 4” minimum ridge opening)
Eaves inlet area 4700 square inches (98 feet X 12 inches X 2 sides X 2 inch eave opening)
Total = 9400 square inches
Or
2)Cupola outlet area 2880 square inches (24” X 30” cupola louvers X 4 sides)
Eaves inlet area 4700 square inches (98’ X 12” X 2 sides X 2” eave opening)
Total = 7580 square inches
Prepared by J. E. Meader (January 2006)
Reference: Ventilation of Dairy and Other Livestock Buildings
by Brugger and Brooks