HOUSING PIH-120
PURDUE UNIVERSITY. COOPERATIVE EXTENSION SERVICE.
WEST LAFAYETTE, INDIANA
Non-mechanical Ventilation of MOF Swine Buildings
Authors:
Gerald R. Bodman, University of Nebraska
Don D. Jones, Purdue University
Reviewers:
L. Bynum Driggers, North Carolina State University
Larry Jacobson, University of Minnesota
Russ and Mary Jeckel, Delavan, Illinois
Ventilation is a key element in the successful operation of
any swine production facility. A well-designed, manageable venti-
lation system enables the producer to ensure environmental condi-
tions within the pig zone conducive to good performance.
A ventilation system removes low-quality air and replaces it
with fresh, high-quality air. Excess heat, moisture, dust, odors,
pathogenic organisms and irritating, noxious or toxic gases
decrease air quality. Ventilation systems are judged, all too
often, by air quality in the people zone--4 to 5 ft. above floor
level. Accurate assessment of ventilation system performance
requires evaluation of air quality in the animal zone--0 to 2 ft.
above floor level. Good ventilation of the animal zone is depen-
dent upon system design, management skill and inputs, building
layout, farmstead arrangement, construction techniques, judicious
use of insulation, and careful material selection.
Our goal is to provide healthful conditions for animals and
personnel and to control deterioration of structural components
and equipment. Given the widely varying climatic conditions
encountered in most locations, there is no perfect system. Conse-
quently, producers need to select the system which best fits
their management abilities and goals with the most acceptable set
of compromises. The selection process should include careful
evaluation of all viable options or alternatives.
There are two basic types of ventilation systems. Both
require good design and management. A mechanical ventilation sys-
tem relies upon fans or other mechanical air-moving devices to
achieve airflow through the building. A non-mechanical ventila-
tion system (also sometimes referred to as ``natural'' or ``grav-
ity'' ventilation) relies upon wind forces and the thermal buoy-
ancy of air to achieve airflow through the animal zone. Buoyancy
is the tendency of warmer, lighter air to rise and cooler,
heavier air to fall or descend. This phenomenon is also referred
to as natural convection.
Building Design
Dissatisfaction with the operating cost of mechanically ven-
tilated buildings and the performance of pigs reared in open
front buildings and shelters during winter conditions resulted in
producers installing doors or panels to allow partial closure of
the open front during extreme weather--and the modified-open-
front building or MOF was born. Research and field experience
have shown reduced incidence of pneumonia and improved feed effi-
ciency in a well-designed, well-managed MOF compared to open-
front housing.
Buildings with gable or ``A'' roofs and those with mono-
slope, shed or single slope roofs can be designed and managed as
an MOF. Gable-roof MOF buildings are used primarily for growing-
finishing pigs though some are used as breeding-gestation facili-
ties. Monoslope roof buildings with a southward facing high wall
are often referred to as the ``Nebraska MOF.'' Originally the
monoslope roof MOF was used for growing/finishing pigs. More
recently, the design has been adapted for farrowing, nursery, and
breeding-gestation units. Most MOF buildings are non-mechanically
ventilated year-round. Some use mechanical ventilation (often
manure storage ventilation) to provide cold and a portion of the
mild weather ventilation needs.
The performance and manageability of both gable and mono-
slope roof buildings are enhanced by adherence to sound design
principles. The construction practices and techniques unique to
each building style are discussed later.
Basic Principles
Siting--Locate feed bins at the ends of the building or on
the downwind wall relative to prevailing summer breezes. Provide
at least 75 ft. separation from other buildings, windbreaks, sig-
nificant geographic features that protrude above grade (e.g.,
hills and lagoons), and tall crops such as corn. A separation of
100 ft. is preferred. The minimum separation between non-
mechanically ventilated buildings or other obstructions to air-
flow can be calculated from the equation
_
S=0.4H\|L
(S = minimum separation, in feet;
H = height of upwind building [building causing interference with
access of wind to MOF building], in feet; and
L = length of upwind building, in feet). Examples of minimum
separation distances based on this equation are given in Table 1.
Closer spacings also increase the risk of major loss in case of
fire.
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Table 1. Absolute minimum feet between nonmechanically ventilated
buildings and other buildings or obstructions to airflow (based
on
_
S=0.4H\|L).
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Windward
building or
obstruction Windward building or obstruction length
_____________________________________________________
height 50 75 100 150 200 250
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feet feet
8 75* 75* 75* 75* 75* 75*
12 75* 75* 75* 75* 75* 76
16 75* 75* 75* 78 91 101
20 75* 75* 80 98 113 126
24 75* 83 96 118 136 152
28 79 97 112 137 158 177
30 85 104 120 147 170 190
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* Separation distances less than 75 feet are not recommended.
Closer spacings impede ventilation and risk fire. Preferred
minimum separation distance is 100 feet.
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Building Orientation--Orient buildings perpendicular to pre-
vailing summer breezes. For most of the U.S., this means an
east-west orientation. This enhances across-the-building airflow
through individual pens, permits interception of more wind, and
reduces solar heat load on the building roof. The chimney effect
associated with wind pressures is also enhanced when wind strikes
the side of a building as contrasted to the end.
Roof Slope--Best year-round ventilation of a monoslope MOF
is achieved with a roof slope of 2:12 (smooth ceiling) or 21/2:12
(exposed roof purlins) for growing-finishing and breeding-
gestation buildings. Farrowing and nursery units built to date
have 3:12 roof slopes. (Note: In a monoslope MOF, the ceiling and
roofline are the same.) Gable roof buildings work best with a
4:12 roof slope and without a ceiling. The steeper roof slopes
enhance movement of warm, moist air through air outlets at the
high part of the roof, i.e., chimney effect.
Insulation--Insulate walls to at least R12, rooflines to at
least R19, and building perimeter to at least R10 (to a depth of
2 ft. below grade or beneath the outer 2 ft. of the floor).
Uninsulated concrete walls and masonry walls with insulation in
the block cores are not adequate, except in the lower tier of
southern states. Earth berming moderates extreme temperatures
but does not eliminate the need for insulation. Reducing heat
loss through the floor, walls, and roof simplifies management of
the building environment since more heat is available for removal
through the ventilation system. This allows higher ventilation
rates and better control of moisture and other air contaminants
without excessive lowering of building temperatures. No insula-
tion material is totally water-vapor, fire, rodent or bird proof.
Use a polyethylene (plastic) vapor barrier and interior protec-
tive liner in all cases. In colder climates, insulated curtains
or sidewall panels are often used to reduce heat losses. Insu-
lated curtains are more susceptible to rodent damage than are
other vent closures.
Air Inlets/Outlets--All effective ventilation systems
require air inlets and air outlets. The proper quantity of well-
distributed, good quality air is necessary for effective ventila-
tion. Provide air inlets over the desired dunging area and
outlets at the high part of the roof. Outlets must allow a con-
tinuously upward flow of warm, moist air. Ventilation during warm
weather requires openings on both sides of the building for
across-the-building airflow. There is no physical phenomenon to
make warm air move downward. Locate openings so that air enters
the pig zone. Inlets should be within 32 in. of the floor and
direct air down toward the floor. A single opening, such as a
curtain, cannot function as both an inlet and an outlet. All
inlet and outlet openings must be easily controllable. PVC pipes,
concrete blocks on edge and similar wall openings provide insuf-
ficient air inlet area except during severe winter weather.
Sidewall Closures--Install sidewall panels (pivot or top-
hinged doors) or curtains to allow independent operation of vari-
ous sections of a building. This allows temperatures in different
parts of the building to be varied to meet animal requirements
and aids in controlling longitudinal (end-to-end) airflow. Do not
try to keep one part of the building warm by allowing or
encouraging airflow from one part of the building to another.
Such practices accelerate the transfer of disease. All pigs
require fresh air. Limit the length of curtain or panels operated
by one control device and thermostat to 100 ft. Curtains and wall
panel vents must be sized and installed so that they do not pro-
trude into the ventilation opening when in the ``full open''
position. Open curtains should be ``stored'' beyond the opening.
Size panels to assure the required opening is provided, consider-
ing any reduction in opening size because of the panel itself.
Panels that direct air down into the pig zone are best.
Partitions--Provide full height room partitions at 75 to 100
ft. intervals in long buildings and between the grower-finisher
parts of all buildings. Gable-style roof, partly-slatted hog
buildings should use pen partitions that are mostly solid between
pens (in the solid floor area) to improve performance and dunging
patterns. Some airflow is desirable in all areas of the pen, but
air velocity in the resting areas should be kept lower than in
the desired dunging area. In gable buildings with a work-service
aisle on the north side, install a walk door at each room parti-
tion location. The partitions aid in maintaining appropriate room
temperatures and controlling longitudinal airflow. Match the
location of curtain or panel junctions and solid room partitions.
In large operations, consider separate buildings, instead of
separate ``rooms'' for improved disease control. Multiple,
smaller buildings enhance ``all in all out'' management and con-
trol of disease.
Ceiling--A smooth roofline or ceiling improves airflow,
especially during calm winds. Avoid exposed purlins more than 4
in. high.
Group Size--Limit pen or group size to 35 animals (25-30
preferred). Smaller groups allow easier size matching of the
pigs, feeder spaces, and waterers; reduce social stress; and
facilitate the use of hovers over the sleeping area during
extreme weather.
Pen Arrangement--Arrange pens to allow ventilation air to
enter over the dunging area. The low north wall of the monoslope
building provides a natural ``hover effect'' without adversely
affecting cold weather ventilation. Additional hovers may be
required over the sleeping area for small pigs and during extreme
winter weather, especially in gable buildings.
Pen Size--Design pens with a width:length ratio of 1:2 to
1:4 for better definition of the sleeping, eating, and dunging
areas. A combination of three pen widths (for example, 6, 8, and
10 ft.) is best in most situations to optimize animal density
without overcrowding or excessive numbers of pigs per pen. This
will maximize animal heat production and reduce the need for sup-
plemental heat. Use partially slatted floors or open gutters in
MOFs to provide animals a draft-free, solid floor resting area.
Gates and Walls--Use ``open'' vertical rod gates around the
intended dunging area for increased airflow and socializing. Use
solid or nearly solid walls along or around the intended sleeping
area for draft control. Limit solid wall height to 32 in. Let
reinforcing bars extend above the solid pen partition and weld
horizontal bars to them (6 in. on center) where a greater pen
partition height is required.
Electrical System Controls--All electrical equipment such as
lights, augers, and power washers as well as devices to control
curtains, panels, etc. must comply with Article 547 of the
National Electrical Code (NEC) (in many instances, state law).
The Code requires equipment that is corrosion-resistant and
water- and dust-tight. Many thermostats on the market are not UL
listed and have unknown performance and safety characteristics.
Failure to install equipment in accordance with the NEC could
lead to higher insurance premiums or disqualification for
insurance. (See Pork Industry Handbook fact sheet PIH-110 for
more information.)
Airflow Patterns--Airflow patterns cannot be precisely
predicted in any building. Variations will occur because of rate
of heat production in the building, size and location of ventila-
tion openings, inside and outside temperatures, wind speed and
direction, number and location of partitions, etc. Generalized
airflow patterns to be expected under different conditions are
shown in Figure 1.
In addition to the cross-building airflow patterns illus-
trated in Figure 1, wind can also cause longitudinal or end-to-
end airflow in non-mechanically ventilated buildings. The
expected airflow patterns are illustrated in Figure 2. Shelter-
belts, nearby buildings, grain bins, feed rooms, ventilation
openings, etc. can alter these generalized patterns.
Monoslope (MOF) Design
Building Width--Limit building width to 28 ft. for growing-
finishing and breeding-gestation units and 24 ft. for nursery and
farrowing facilities. See Figure 3 for a typical building cross-
section.
South Wall--Plan these openings for easy management and
adjustment. They are the primary openings for winter ventilation
and significantly influence summer ventilation. Provide a 3-in.
(minimum) continuous full length air outlet near the top of the
wall. Equip the opening with an adjustable baffle or other clo-
sure device. Two construction techniques are shown in Figure 4.
These designs allow easy adjustment of the opening from floor
level, have minimum components susceptible to corrosion, and use
readily available materials. The opening should be ``full open''
except during extremely cold or windy weather. Daily adjustment
is neither necessary nor recommended. A curtain that opens from
the top down can serve as the air outlet if the opening is within
12 in. of the roof. Flashings, fascias, etc. must be installed so
that they do not interfere with the continuous upward flow of
air.
Provide an air inlet near the bottom of the wall. Masonry
blocks turned on edge or PVC pipes placed in the wall are inade-
quate openings except during extreme winter weather, are diffi-
cult to manage, and are nearly impossible to rodent-proof. The
opening should be within 32 in. of the interior floor. Excess
height allows cold winter air to move farther into the pen before
reaching the floor level. A curtain that is fastened at the top
and that opens from the bottom can be used as an air inlet.
Large sidewall openings are needed for warm and hot weather
ventilation. A 4-ft. high continuous opening is minimum. Larger
openings are desirable. A flexible curtain can function as either
an inlet or outlet--but not both! If the curtain opens from the
bottom up, it can also serve as the winter air inlet. Keep the
bottom of the curtain opening within 32 in. of the inside floor.
The curtain must be complemented with an air outlet near the top
of the wall (Figures 3 and 4).
If the curtain is fastened at the bottom and opens from the
top down and if the opening is within 1 ft. of the roof, it can
serve as the outlet for ventilation air. An inlet must be pro-
vided near the bottom of the wall.
Install all curtains so that they do not protrude into the
sidewall openings when in the ``full open'' position. This
requires a curtain to be at least 1 ft. wider (measured verti-
cally) than the opening being controlled and attached beyond the
opening. Hemming generally reduces the actual curtain size by at
least 4 in., e.g., a 6-ft. curtain is about 5' 8'' wide. Also
plan for shrinkage. Lumite-saranO curtains typically shrink about
6-10%. Lumite 50O curtains shrink about 1-2%. Non-porous curtains
are preferred as they provide more positive control of airflow.
An alternative being used by several producers is a set of
two curtains. Both curtains are attached about two-thirds of the
distance from the front wall to the roof along the south wall.
The bottom curtain is thermostatically controlled, opens up from
the bottom, and functions as an air inlet in cold weather. The
top curtain is either manually or thermostatically controlled,
opens down from the top, and functions as an air outlet. This
arrangement allows the south wall to be about 80% open for summer
ventilation.
North Wall--Make these openings easy to adjust. They must be
reasonably airtight for winter operation.
Provide a 3-in. wide continuous baffled inlet along the top
of the wall (Figure 3). The inlet is used primarily during the
widely varying weather of late fall and early spring when it is
too warm to have the building closed tightly and too cool (espe-
cially overnight) to open the large vent doors. Without this
inlet, twice-a-day adjustment of the north wall vent doors might
be necessary. The inlet must be reasonably airtight during
extreme winter weather.
Large doors or panels are needed for warm weather ventila-
tion. They should provide a continuous unobstructed opening at
least 2 ft. high. The bottom edge of the opening should be within
32 in. of the interior floor. Construction limitations, panel
framing, and vent opening characteristics (e.g., butterfly doors
or other designs that reduce the effective size of the actual
framed opening) generally mandate use of panels at least 30 in.
high. Install welded hog panels across the openings to control
pig access to the framing materials. A non-insulated curtain can
be used in some locations, but insulated curtains or panels are
required for winter operation in the central and northerly parts
of the U.S.
Gable Roof MOF Design
Building Width--Limit building width to 50 ft. A width of 32
ft. or less is preferable. See Figure 5 for a typical cross-
section. In buildings with two rows of pens, make sure the dung-
ing areas are along the walls. Buildings with center dunging
areas are very difficult to ventilate and manage.
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Table 2. Recommended minimum openings for gable-roof buildings,
plus recommended raised ridge cap dimensions.
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Sidewall
opening Ridge cap
_________________________________
Building Ridge height Vertical Width (inches)
____________________
width opening minimum clearance Min. Max.
_________________________________________________________________
feet inches feet inches
20 4 2 2 6 8
30 6 3 3 9 12
40 8 4 4 12 16
50 10 5 5 15 20
60 12 6 6 18 24
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Make the air outlet along the ridge continuous the full
length of the building. The opening should be at least 2 in.
wide for each 10 ft. of building width (Table 2). Equip the open-
ing with a device to allow partial closure during adverse
weather. One option is shown in Figure 6. The ridge opening
should never be closed completely. If a ridge cap is installed,
it must be set high enough to prevent interference with the
upward flow of warm moist air (Table 2). Use extreme caution in
selecting commercially fabricated ridge ventilators. Most cause
significant restrictions to airflow. If rain blow-in is a prob-
lem, consider an ``interior raingutter'' below the ridge opening
(Figure 7), rather than a ridge cap.
Provide continuous openings the full length of both sides.
The openings should provide a minimum clear unobstructed open
height of at least 2 ft. Increase sidewall opening height by 1
ft. for each 10 ft. of building width over 20 ft. Thus, a 40-ft.
building should have continuous unobstructed sidewall openings at
least 4 ft. high. Equip the north wall with insulated curtains or
panels. If located in northern or north central U.S., install
insulated or double glazed panels, double curtains, or an insu-
lated section on the south wall also. Openings should be within
32 in. of the floor. Use a bottom opening curtain or provide a
baffle inside the sidewall posts to direct incoming air down into
the dunging air during cold weather.
Ventilation System Management
Proper management of the ventilation system by adjustment of
inlets and outlets is essential so that outside air temperature
variations do not cause extreme, abrupt inside air temperature
changes. The goal in ventilation system design and management is
to maintain animal zone conditions in the range of optimum feed
intake and utilization and to maximize animal performance and
comfort. The conditions provided must not pre-dispose the animal
to stress, excessive risks to health, or secondary infections and
illnesses. This can be achieved by distributing the air to
prevent ``dead air'' spaces without creating drafts. Moving air
reduces the effective temperature (wind chill effect). Such air
currents are a ``draft'' anytime they produce an undesirable
side-effect or reaction in the animal. (Note: The same air velo-
city might be considered a desirable ``cooling breeze'' during
warm weather!) Hovers are an effective way to allow animals with
different metabolic rates and reactions to air currents and tem-
peratures to seek out conditions where they are most comfortable.
Healthy, comfortable animals perform well, are less susceptible
to infections from opportunistic pathogens, and lead to maximum
profits. Allowing animals access to a range of environmental
conditions permits them to select conditions where they are most
comfortable and thus serves as a guage in managing the ventila-
tion system.
Cold weather ventilation requires that heat produced within
the building be conserved through good construction and proper
use of insulation. The conserved heat is needed to maintain room
temperature and to heat cold, incoming ventilation air, helping
to remove moisture. Inadequate insulation usually leads to under
ventilation. The result is uncomfortable conditions because of
elevated relative humidity and low temperatures. A decrease in
animal health and productivity frequently follows because of
chilling or respiratory ailments. The recommended range of rela-
tive humidity in livestock buildings is 50-65%.
Non-mechanically ventilated buildings should be near capa-
city during cold weather. Lightly loaded buildings or mostly
small animals may require supplemental heat to maintain proper
ventilation rates and temperature.
The distribution of ventilation air is determined by the
design and location of inlets and outlets. Ventilation rates are
varied by manual or automatic adjustment of inlets and outlets.
Allow cold air to enter through openings near the floor to cool
the desired dunging area and encourage proper animal behavior. As
the cool air mixes with warm interior air, it picks up moisture
and other contaminants and rises as it becomes ``used.'' The
warm, moist air follows the underside of the roof and exits
through the outlets at the high point of the building.
The outlets should be the last opening closed or restricted
and the first opening to be re-opened. Restrict the outlets only
when outside conditions are so severe as to prevent maintenance
of the desired interior conditions by adjustment of inlets only.
Observe the pigs and check conditions in the animal zone before
restricting outlets.
Mild weather conditions require increased airflow to limit
inside temperature rises. Airflow through the building can be
increased by adjusting sidewall inlets. The outlets should be
full open.
During hot weather, ventilation airflow rates must be high
enough to prevent overheating. Increased airflow through the
animal zone is achieved by opening the large panels or curtains
on both the south and north walls. The air must move through the
animal zone to be effective in cooling the animals.
Summary
Proper design will allow energy efficient non-mechanical
ventilation of buildings for any phase of swine production.
Management of non-mechanically ventilated buildings is fairly
easy if good construction practices are followed and appropriate
openings are incorporated. Non-mechanical ventilation systems
require a thorough understanding of the principles of ventilation
and ``natural'' airflow. Management inputs to properly operate an
MOF are slightly greater than with mechanically ventilated
designs. Usually, however, this simply means taking time to
observe the building and animals and making appropriate changes
during morning and evening chores.
_________________________________________________________________
Reference to products in this publication is not intended to
be an endorsement to the exclusion of others that may be similar.
Persons using such products assume responsibility for their use
in accordance with current directions of the manufacturer.
_________________________________________________________________
NEW 5/89 (5M)
Figure 1a. Expected airflow patterns in monoslope-roof building
for (A) extremely cold weather, (B) mild weather and (C) hot
weather.
Figure 1b. Expected airflow patterns in a gable-roof building.
Figure 1. Generalized airflow patterns in monoslope- and gable-
roof buildings under selected conditions.
Figure 2. Expected longitudinal airflow patterns in MOF buildings
due to wind."
Figure 3. Typical cross-section (schematic) of monoslope MOF.
Figure 4a. An air outlet over the plate and between the rafters
requires insulation along the roof and a narrow fascia.
Figure 4b. An air outlet using a double plate to form a thru-
the-wall opening.
Figure 4. Air outlets for the top of the south wall on monoslope
roof MOF buildings.
Figure 5. Schematic of a typical gabic roof" "MOF building"
Figure 6. Ridge openings for a gable roof building. A plastic
pipe in a rope sling can be used to adjust the size of a ridge
opening. Install flashings along ridge and over each truss to
prevent precipitation and condensation from wetting insulation
and structural members.
Figure 7. An interior trough or rain gutter can be added to an
open ridge to intercept precipitation and prevent wet floors.
Figure 8a. A building with a single row of pens requires a winter
inlet on one side only (above dunging area).
Figure 8b. Buildings with two rows of pens and dunging areas
along the walls require inlets along both sides.
Figure 8. The location of inlets in a gable roof MOF is dependent
upon the number of rows of pens and location of the dunging
area(s).
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Cooperative Extension Work in Agriculture and Home Economics,
State of Indiana, Purdue University and U.S. Department of Agri-
culture Cooperating. H.A. Wadsworth, Director, West Lafayette,
IN. Issued in furtherance of the Acts of May 8 and June 30, 1914.
It is the policy of the Cooperative Extension Service of Purdue
University that all persons shall have equal opportunity and
access to our programs and facilities.
.