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Natural Ventilation for Freestall Barns
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.
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: (1)
Continuous, low level air exchange, which is the
minimum required all the time, even during subfreezing
conditions to remove moisture continuously produced by
animals. (2) Temperature control air exchange, which is
necessary during cool and mild conditions to remove
excess body heat from the barn, and (3) 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
Requirements
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.
1. 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.)
2. 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.)
3. 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.)
4. 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
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Natural Ventilation for Freestall Barns
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.
Construction
For best performance, consider using the following
construction tips for naturally ventilated freestall barns with
light insulation:
1. 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.
2. 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.
3. 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).
4. 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.
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.
6. 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.
7. 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.
8. 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.
9. 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.
5. 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
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Natural Ventilation for Freestall Barns
Insulated Freestall Barns with
Automatic Temperature Control
Requirements
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 6. Insulated barn with spaced ridge outlets and attic
ventilation.
Figure 7. Insulated barn with sidewall only ventilation.
Figure 4. Insulated barn with ceiling stacks (chimneys) and
sidewall 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.
Figure 5. Insulated barn with ridge and sideall ventilation.
The following guidelines are suggested for ventilation
openings in different seasons and climates.
1. 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
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Natural Ventilation for Freestall Barns
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.)
2. 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.
3. Summer: 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 tongueand- 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
funnelling 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.
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Natural Ventilation for Freestall Barns
Summary
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
Contact Information
Robert Graves
Prof Emer Ag Engineering
[email protected]
814-865-7155
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Code: G-75
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Natural Ventilation for Freestall Barns