Basic Requirements for Natural Ventilation
Cows continuously produce heat and moisture. When cows are confined in freestall barns, loafing sheds or under shade structures, a ventilation system is necessary to continuously exchange warm, humid inside air for drier, cooler outside air. This exchange must occur regardless of outside temperature or weather conditions. Even when its snowing on a cold windy night, fresh outside air is required to keep cows healthy and to reduce moisture levels inside the barn. This ventilation process also removes odors and gases. Barn design, construction, and operation must consider the year round needs of the cow for optimum ventilation.
Typically, Holstein cows can maintain high levels of productivity between 20° and 76°F as long as relative humidity is not allowed to go too high. Cows are much more tolerant of temperatures below this optimum range than above. Kept dry and out of the wind cows will do very well at temperatures far below 20°F. Barns that are foggy, that are wet and smelly, or have condensation on interior building parts are too humid and under ventilated. Ventilation openings need to be increased until these problems go away. Wintertime productivity problems are the result of animals being shut inside poorly ventilated, damp smelly barns. Too often, barns are run for the convenience and perceived comfort of the operator disregarding the welfare of the cows.
Even the most basic ventilation systems should provide for the following:
Air exchange. Ventilation systems can have either a mechanical driving force (fans) or natural driving force (winds and buoyancy). Sufficient air exchange can be accomplished by using either one or a combination of the two.
Control or the ability to modify ventilation rates based on inside or outside conditions. Ventilation rates are changed by turning on and off fans or opening and closing curtains, dampers, windows or ventilation doors. Automatic control provides the most uniform conditions and response to changes in weather.
Flexibility. Ventilation systems should be flexible so they can provide healthy conditions during various seasons of the year. There are three distinct operating conditions:
- Continuous, low level air exchange, which is the minimum required all the time, even during subfreezing conditions to remove moisture continuously produced by animals.
- Temperature control air exchange, which is necessary during cool and mild conditions to remove excess body heat from the barn, and
- Air velocity and high rates of air exchange, which are required during hot weather to help the cow remove large amounts of heat from her body and the immediate space around her.
Barn construction is also important to the performance of the ventilation system. When close temperature control is desired, the barn must be well-insulated to control heat loss and constructed to minimize unplanned air exchange. Barns relying on natural air exchange must have sufficient and properly located openings to take advantage of breezes and thermal buoyancy (the "chimney effect"). Fabric curtains provide a convenient and economic means of closing ventilation openings. Curtains that are left loosely folded or rolled up near the ground may attract nesting rodents and accumulate blowing dirt or bedding.
Natural Ventilation for Freestall Barns
The primary purpose of the freestall barn is to protect the cows, freestalls, and feed areas from winter winds, rain, snow, and hot summer sun. It is not necessary to maintain freestall barns continuously above freezing temperatures because exposed milk and water lines are not required and cows are not washed or milked in the freestall barn. Natural ventilation systems work well for freestall barns and are classified based on the degree of temperature control and the amount of insulation used. Fans may be required to increase airflow over cows in extremely hot weather.
Most freestall barns are built with little or no insulation. In areas with extended periods of extremely cold weather, barns may be built with moderate levels of insulation. This will allow the barns to operate at moderately higher inside temperatures during cold weather. Insulated barns are more costly to build and severe problems for both cows and the building can result if they are improperly built or operated. Too often, in an effort to maintain high temperatures, ventilation is restricted causing the relative humidity inside the barn to become dangerously high. This is unhealthy for cows and accelerates deterioration of the building.
Freestall Barns with No or Light Insulation
These barns, sometimes referred to as "cold barns," follow outside temperatures and serve only as a shelter from blowing winds, rain, and snow during cool and cold weather and sunshades during hot weather conditions. Roof slope and a smooth underside surface allows hot moist air to quickly flow to the open ridge. Adequate sidewall opening height and eave height help to capture breezes and keeps the warm layer of air beneath the roof farther away from the cows' living zone. Low levels of insulation under the roof reduce the radiant heat load which contributes to summer heat stress. If the building is closed too tightly during cold weather, condensation and unhealthy conditions for cows will occur. During cold weather there should be no more than 10°F temperature difference between the inside of the barn and outside. Decreasing ventilation to maintain a higher inside temperature will result in condensation of warmer humid air on cold building parts such as roof sheets, siding and rafters.
Figure 1. Uninsulated or lightly insulated barn with sidewall and ridge openings.
The following guidelines are suggested for ventilation openings in different seasons and climates.
Year-round: Continuous ridge openings. Provide a two-inch ridge opening width for every ten feet of building width. (Example: a 90' wide building requires an 18" unobstructed open ridge.)
Very cold winter weather: Continuous sidewall openings at the top of both sidewalls equal to one half of ridge opening. (Example: a 90' wide building requires a 9" clear opening at the top of each sidewall.)
Mild and cold winter, spring or fall weather: Continuous adjustable four foot high sidewall openings above cow level and adjustable end-wall openings above cow level or traffic doors. (Adjust openings to prevent high humidity, minimize drafts and maintain temperatures within 10 degrees of outside temperatures.)
Summer: Additional sidewall openings and end-wall openings at animal level. Complete opening of all four walls from the ground to the roof will provide maximum opportunity for cooling breezes during hot summer weather. Maximum exposure to wind and breezes is necessary to minimize heat stress on cows when temperatures are above 80°F. Because the barn basically serves as a sunshade and rain umbrella in the summer, total side and end-wall removal is optimal.
Side and end-wall closures may be curtains, removable or hinged panels, sliding doors, or hinged windows. Selection should be based on ability to achieve maximum openings, cost, convenience and longevity. Openings that require regular adjustment should allow convenient centralized operation. Areas that are opened or closed based on the season can be attached by nailing strips or other less convenient but more cost effective methods.
For best performance, consider using the following construction tips for naturally ventilated freestall barns with light insulation:
Use a 4/12 - 6/12 roof pitch. Lower roof pitches may result in slow movement of air along the underside of the roof and make it easier for pockets of warm moist air to become trapped against cold roof surfaces. Steeper pitches increase costs and may result in too high an air flow along the underside of the roof. This can cause poor fresh air distribution at cow level during cold weather.
Use continuous open ridges. Protect exposed structural members or trusses at ridge openings with three coats of exterior paint or clear penetrating urethane sealer or flashing. Apply paint or sealer from ridge to at least the first purlin. Paint or sealer must be regularly maintained. Consider pressure-treated material for the portion of the truss under the open ridge. Use double hot-dipped galvanized truss plates. Trusses with vertical king posts should not be used with open ridges. Short sections of flashing installed over trusses can prevent precipitation from falling directly on the truss parts. Do not wrap trusses with metal. This will trap moisture and interfere with drying.
Ridge caps are not usually recommended. Minimal snow and rain will enter a properly-sized open ridge when the building is fully populated. Properly designed ridge caps will provide some reduction of entering rain and snow but are expensive. Improperly designed or installed ridge caps will hinder ventilation air flow and may increase snow or rain entry. (See Figure 2 for further explanation.)
Figure 2. Performance of various ridge openings.
A completely protected ridge cap will exclude most if not all precipitation. It is very expensive to build and must be sized to assure that adequate clearances are maintained to prevent restriction of exhausting air.
A rain gutter under the open ridge can intercept precipitation without interfering with air flow (Figure 2).
Adequate sidewall height, 12 - 14 feet. When open, higher sidewalls will capture more wind for summer cooling. Sidewall heights and bottom truss cord heights must allow for machinery access. Higher sidewall heights are recommended for barns that are sheltered by other buildings, growing crops or other obstructions. Under normal conditions heights above 16 feet will not result in noticeable improvements in air quality.
Use positive-type bird protection. Roosting birds will foul building parts, feed, and animals. Birds can also rapidly destroy insulation by pecking and burrowing. Bird elimination is difficult. The following tips may help:
- Minimize locations where birds can perch or roost, especially over feed area
- Use plastic bird netting to exclude birds from truss areas. If bird netting is placed over open ridge, increase opening by 20% to account for blockage of air flow by the netting.
- Use steel or wooden beams or arches instead of open trusses. This may cost slightly more than traditional wood trusses.
- Protect exposed insulation surfaces, ends and joints with solid barriers.
Sidewall construction must allow for changing winter conditions and complete opening in hot weather. Easily adjusted curtains or gang ventilation doors above cow level are necessary for ventilation adjustment during cold and cool weather conditions. Completely removable sidewalls are required at cow level for hot weather conditions.
Endwall construction must allow for changing winter conditions and complete opening in hot weather. Roll up doors or curtains will allow for more endwall openings for hot weather ventilation. Large adjustable ventilation openings in gables will aid in exhausting hot air from under the roof.
Minimum insulation (R=2) under roof sheets will reduce inside surface temperatures on hot, sunny days. Use water-resistant, plastic-type insulation and protect from birds. A 5/8" to 3/4" wooden roof deck with asphalt shingles is an alternative.
Allow a minimum 3' roof or eave overhang to minimize precipitation and sun problems when sidewalls are open. A general rule is to extend the roof 1/3 the sidewall height. Eave overhang also protects rolled up curtains and moves snow slides farther from building.
Figure 3. Completely protected ridge cap. Upstands and cap must be positioned to allow uninterupted upward flow of exhaust air.
Insulated Freestall Barns with Automatic Temperature Control
Freestall barns with moderate levels of insulation (R=10-12) in walls and ceilings or roofs can be operated at higher inside temperatures than uninsulated barns. The extra cost may be justified in areas where temperatures remain below 10°F for extended periods of time. These barns are classified by location of ventilation openings, sidewall ventilation (Figure 7) or sidewall and ridge, stack or chimney ventilation (Figure 4, 5 and 6). The required insulation may be placed immediately under the roof, below roof purlins or on the lower chords of the roof trusses. The unheated attic space between the insulation and the cold roof can become a particular problem with these type of buildings. Refer to the later construction section.
Figure 4. Insulated barn with ceiling stacks (chimneys) and sidewall ventilation.
Figure 5. Insulated barn with ridge and sideall ventilation.
Figure 6. Insulated barn with spaced ridge outlets and attic ventilation.
Figure 7. Insulated barn with sidewall only ventilation.
Insulated curtains will further reduce heat loss. The barn will be dark when curtains are closed. Uninsulated curtains allow more light into the barn but condensation will form on the curtains during very cold outside conditions. Regardless of curtain type or installation methods, maximum sidewall opening for summer conditions must be maintained for adequate summer ventilation.
During extreme cold conditions, these barns can usually be maintained 30-40°F higher than the outside temperature and still provide a good environment for cows. As outside temperatures fall below 0°F, the barn will approach and eventually go below freezing. Closing ventilation openings to keep the barn above freezing will compromise conditions for the cows. During cold and mild conditions, this type of barn can be maintained about 40°F.
For good cow comfort, openings should be automatically controlled based on temperature. Set automatic controllers to assure curtains can be opened at least four feet if barn temperatures start to rise above 40°F. The openings should never completely close to assure a continuous minimum air exchange. Moisture levels and air quality, not comfortable temperatures for the operator, must be the basis for control.
The following guidelines are suggested for ventilation openings in different seasons and climates.
Very cold winter weather:
Continuous sidewall openings at top of both sidewalls about 1" for each 10' of building width. (Example: a 90' wide building requires a 9" clear opening at the top of each sidewall). An equivalent combination of sidewall and ridge or stack openings may also be used. (Size stack or ridge openings to provide one square foot of outlet for each 100 square feet of floor area.)
Mild and cold winter, spring or fall weather:
Open ridge and stack or high endwall openings. Adjust sidewall openings and endwall openings or doors to maintain fresh inside conditions and minimize drafts at cow level. These barns are often not sufficiently opened on winter days when temperatures get above 40°F.
Additional sidewall and endwall openings at animal level are needed. Barn should function as wide open sunshade.
Continuous ridge openings can remove moist air from the entire length of the barn. However, they are more difficult to control when low ventilation rates are required. Also, wind may result in air flowing in portions of the ridge and out other locations. Proper attic ventilation is more difficult with continuous ridge openings because the ridge opening interferes with attic exhaust openings. A system that places adjustable ridge outlets from the stable along one-third to one-half the roof length and attic vents between them accomplishes both requirements (Figure 6). Chimneys or stacks with adjustable dampers are easier to control when small exhaust openings are desired. Stack or chimney sidewalls must be insulated all the way to the exhaust opening to prevent condensation of warm moist air exhausting from the barn. Chimney caps or ridge covers must be high enough to allow free air flow beneath them. Insulate the undersides of chimney cover or ridge caps to prevent condensation and dripping back into the barn.
Construction of Insulated Barns
A moderately insulated freestall barn requires careful design and construction. The most critical area is the enclosed attic space between the insulated ceiling and the roof (Figure 8). Serious damage and shortened building life will occur if moisture is allowed to accumulate in this area. There are two primary paths that allow direct access to the attic space by moisture from the animal space. Any holes or cracks in or around rigid insulation, or the air/vapor barrier, can allow large amounts of rising warm moist air direct access to the attic space. Also warm moist air being exhausted out sidewall vents can be trapped and pulled into the attic space through eave openings. This is especially critical in barns that only ventilate through sidewalls.
Figure 8. Air leakage into attic space.
Following are some construction guidelines that can help control moisture problems in attic spaces:
- Carefully install a water-resistant insulation and vapor barrier. Fiberglass blankets or other materials that can absorb moisture are not recommended. If insulation is also used as ceiling material, it should have a fire-rated covering.
- Excellent air/vapor barrier between animal space and attic. If insulation is also used as the air/vapor barrier, careful cutting, fitting, and caulking of tongue-and-groove insulation joints and all edges and penetrations of the insulation is required. Carefully installing a separate 6-mil polyethylene air/vapor barrier is also recommended.
- Construct and enclose eave overhangs to prevent funneling moist air from the sidewalls into the attic. Use tightly closed horizontal soffit. Provide air inlets in the vertical fascia.
- Attic space ventilation should be one square inch for each one square foot of ceiling area. A 90 foot wide barn requires 90 square inches of total attic opening per foot of length. The following are recommended:
- large industrial-type ridge vents
- maximum endwall louvers
- two- to four-inch slots in vertical fascia
- Carefully control relative humidity in animal spaces. Leave a minimum six-inch opening at the top of the curtain when it is closed. Automatic control of both sidewall curtains is also desirable.
- Inspect attic space twice a year. If moisture is accumulating in spite of the above guidelines, ventilate the attic with pressure fans during cold weather to create a positive pressure in the attic.
Hot Weather Considerations
In the summer, the barn should act as a sunshade and rain umbrella. Select a method of barn construction that allows easy complete removal of side and end walls to facilitate good summer breezes. Circulation fans and water sprinklers may also be required during extremely hot humid conditions with no wind.
Properly designed, constructed, and managed naturally ventilated freestall barns can provide an excellent environment for dairy cattle. The primary purpose of the freestall barn is to protect cows, freestalls and feed areas from cold winter winds, rain and snow and to provide shade from hot summer sun. There must always be some sidewall (also, ridge or stacks when present) open to allow air exchange to remove the large quantities of moisture exhaled by the cows. If barns are closed up too tight, they will become humid and smelly, resulting in an unhealthy environment for the cows and wet damaging building conditions.
Management of barns to maintain certain temperatures for the convenience of the operator or equipment will likely cause inferior conditions for the cows. Temperatures inside barns with little or no insulation should be no more than 10 degrees higher than the outside temperature. Barns with moderate levels of insulation and insulated curtains may be operated 30-40 degrees higher than outside temperatures during extreme cold weather. As outside temperatures fall below zero degrees Fahrenheit, these barns will go below freezing if adequate air exchange is provided to remove moisture.
The ultimate question on ventilation is, "What is best for the cow?" If a freestall barn is under ventilated and fills with moist air in the winter or hot stuffy air in the summer the cow and her productivity will suffer and so will farm profitability.
Prepared by Robert E. Graves, Professor emeritus of Agricultural Engineering and Michael Brugger, Associate Professor of Agricultural Engineering, Ohio State University