HOUSING                                           PIH-120


          Non-mechanical Ventilation of MOF Swine Buildings

Gerald R. Bodman, University of Nebraska
Don D. Jones, Purdue University

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 = 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

Table 1. Absolute minimum feet between nonmechanically ventilated
buildings  and  other buildings or obstructions to airflow (based
building or
obstruction        Windward building or obstruction length
  height       50       75       100      150      200      250
   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
* Separation distances less than 75  feet  are  not  recommended.
Closer  spacings  impede  ventilation  and  risk  fire. Preferred
minimum separation distance is 100 feet.

     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-

     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

     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

     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.

Table 2. Recommended minimum openings for  gable-roof  buildings,
plus recommended raised ridge cap dimensions.
                      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

     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.


     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

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


Cooperative Extension Work in  Agriculture  and  Home  Economics,
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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
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