Natural ventilation has obvious advantages in that natural forces are used to ventilate buildings, reducing costs for both fans and electrical power. Natural forces include wind moving through, against, and over buildings, as well as thermal buoyancy of warmed air rising inside of the building. Wind roses that summarize wind conditions are available for most parts of the United States, and can be accessed through the US Department of Agriculture at the following Web site: http://www.wcc.nrcs.usda. gov/ftpref/downloads/climate/windrose/. When the wind is still, naturally ventilated barns depend on thermal buoyancy and the stack effect for ventilation (Fig. 1). Because calves do not generate sufficient heat to effectively create thermal buoyancy, ventilation becomes insufficient.
Further limitations of natural ventilation occur when outside air is warmer than the air inside of the barn, a situation that occurs for a period of several hours almost every day as the sun warms the air outside of the barn more quickly than inside. During these periods of time, air entering the cooler interior of the barn through eaves will rise and leave the barn without good mixing near the floor.
Because of these occasional limitations with natural ventilation, we have advocated the use of positive-pressure tube ventilation systems to supplement naturally ventilated calf barns to provide the minimum ventilation rate of 4 air changes per hour. Our preference is based on the widespread success in improving calf health.
Fig. 1. On still, winter days, cold air enters a naturally ventilated barn through the eaves and exits out of the ridge as it heats up. Positive-pressure tube ventilation (PPTV) systems ensure the appropriate winter ventilation rate and direct fresh air to calves year-round, especially on still days. When wind is present, wind enters through the sidewalls and mixes with the air inside of the barn before exiting through the ridge. The sidewalls should be open as much as possible to capture prevailing winds during warm weather.
Traditional recommendations for naturally ventilated barns focus on ridge openings, eave openings, and interior roof slope for winter conditions, along with a method to open the sidewalls and having sufficient distance from obstructions to wind for warm and hot weather.
The recommended ridge opening provides a net opening of 2 in per 10 ft of building width (5 cm/3 m),15 and it is critical for ventilation by wind and thermal buoyancy to work. Because calves are commonly housed directly beneath the ridge, the ridge openings are commonly covered. Our experience suggests that the elevated ridge cap with upstands on each side is the preferred covering to limit the amount of moisture coming into the barn. A series of cupolas on the ridge can serve the same purpose; however, they need to provide an equivalent cumulative open area to the traditional ridge opening.
The eave openings running above the long walls on both sides of the barn match the net area of the ridge opening, each providing a 1-in opening per 10 ft of building width (2.5 cm/3 m).15 Sometimes eave openings are constructed into the building structure above the walls, whereas other times there may be a space above or between retractable curtains. If so, the curtains should be fitted with blocks to keep them open to provide minimal opening.The interior slope of the roof should rise toward the ridge at a minimum ratio of 1 height to 4 width. 15 With sufficient slope, warmed air will rise toward the ridge opening much like water moves downward toward a drain.
As the weather warms, it is important to open sidewalls and allow winds to move directly into curtains. If so, the curtains should be fitted with blocks to keep them open to provide minimal opening. The interior slope of the roof should rise toward the ridge at a minimum ratio of 1 height to 4 width.15 With sufficient slope, warmed air will rise toward the ridge opening much like water moves downward toward a drain.
As the weather warms, it is important to open sidewalls and allow winds to move directly into and through the naturally ventilated barn. To allow this, the sidewalls should be able to be opened a minimum of 50% of the sidewall area. In modern barns, opening 80% of the sidewall area is preferable. The recommended total side-wall height is relative to the building width. In general, 12 ft (3.7 m) should be viewed as the minimal sidewall height for buildings less than 40 ft (12.2 m) in width, and reaching ft (4.3 m) in buildings of 60 ft (18.3 m) in width or greater. The permanent found at the wall or footings beneath the curtains should be very short, limited to approximately (61 cm) above the floor of the barn. During warm weather when the sidewall curtain is fully open, the low sidewall allows wind to move directly into the calf pens a maximize ventilation. The sidewall should be fitted with retractable curtains, prefer a split so that all air does not have to enter above the top of a single curtain.
The naturally ventilated building must be spaced so that winds can enter the o sidewalls in warm weather. Barriers that block winds are called wind shadows and include adjacent buildings, earthen banks, woodlots, and even seasonal cornfield approaching full height. (See Mario R. Mondaca's article, "Ventilation Systems Adult Dairy Cattle," in this issue.)
The overall rule is the narrower the barn, the better. Narrow barns are easier to ventilate by wind forces in warm weather. To maximize wind forces across the width of the barn, our preference is to limit the overall barn width to approximately 40 ft (12.2 m less. For barns with single or paired calf pens, optimal barns have a single row of pens Very good performance can be achieved with 2 rows of pens. When barns get wither than 40 ft (12.2 m), natural ventilation is frequently perceived to be insufficient in warm weather.
In addition, it is easier to limit the spread of disease from calf to calf in a long, narrow barn. In single-row calf barns, new calves are placed in freshly cleaned pens, there is usually a space between them and the oldest calves in the barn. In a barn with 2 rows of pens, both rows are filled simultaneously and parallel, similar to a single-row calf barn, leaving the new arrivals in freshly cleaned pens with a few empties between them and the oldest calves about to be weaned. In barns with or more rows, the situation is almost always present in which vulnerable young calves are directly across a service alley from older calves that are potentially shedding pathogens.