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Design Recommendations for Natural Ventilation Systems
 
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ISU Publication #: none - electronic only
Authors: Jay D. Harmon, PhD, PE and Steven J. Hoff, PhD
Department of Agricultural and Biosystems Engineering 
Iowa State University
Date: 11/94                                   
 
     Naturally ventilated livestock facilities are designed with one of two goals. They can be designed to function as a cold environment or as a modified environment.  Cold environment buildings generally are thought only to provide shelter from winter winds and rain and generally are within a few degrees of the outdoor temperature. These are generally used in dairy, beef, sheep and adult swine housing. Modified environment buildings maintain higher temperatures than cold buildings and require more attention to design and operation. The following factors pertain to the design of modified environment, naturally ventilated livestock buildings.
 
Building Siting
 
     Naturally ventilated buildings should be sited so that the ridge of the building is perpendicular to the summer winds.  In Iowa,  the predominant winds are from the south so buildings should be sited running east-west. This will not only help to better ventilate the building, but will reduce the solar heating during summer months by facing the roof surface to the south, instead of toward the west. 
 
     Buildings should be located in an area where summer wind obstructions are minimal.  Avoid locating buildings in a low point where natural air flow patterns are absent.  Hill tops receive much more wind. All other buildings should be at least 50 feet from the naturally ventilated building.   Buildings in the prevailing windward direction should be separated based on the following equation:
 
DSD = 0.4 x HGHT x sqrt(LGTH)
 
 
where:
     DSD = separation distance (ft) 
     HGHT = total height of the obstruction (ft)
     sqrt(LGTH) = the square root of the length of the
                 obstruction (ft)
 
Example: A swine breeding building is being built to the north of a gestation building.  The gestation building has a total height of 18 feet and a length of 144 feet. What is the necessary separation distance?
 
Solution:
DSD  =   0.4 x 18 ft x sqrt (144 ft)
     =   0.4 x 18 x 12
     =   86 feet
 
 
Insulation
 
     Walls and ceilings should be insulated in order to reduce the possibility of condensation forming on building surfaces.  Buildings using supplemental heating should be insulated to an R value of 20 in walls and 33 in the ceiling.  Buildings that do not use supplemental heating should be insulated to an R value of 12 in walls and 25 in the ceiling. Vapor barriers should be used on the inside surface of walls and the ceiling to prevent the passage of moisture into the insulation.  Do not leave insulation exposed unless it meets requirements for fire rating.
 
Ceiling Slope
 
     The ceiling slope depends upon the configuration of the building.  If no ceiling is used, the lower side of the roof should be sloped between 4/12 and 6/12.  This range allows air to move up toward the ridge at  an adequate rate, not too fast and not too slow.  If purlins are 2 by 6 or larger and installed on edge, air flow may be slowed due to the deep recesses.  A ceiling along the purlins may be needed.  
 
     Some buildings have a partial chimney.  These buildings typically have a flat ceiling at the outer edges of the ceiling and slope to the ridge in the middle.  Generally, these buildings are constructed by creating a ceiling along the truss middle web members. Slope seems relatively unimportant if the ridge opening is adequate.
 
Ventilation Openings
 
     Ventilation openings are critical to the success of naturally ventilated buildings.  All openings should be made bird proof by using some type of bird screening. Commercial bird wire is available, however, if hardware cloth is used, be sure it is 3/4 inch or larger. Smaller openings may become plugged. Bird proofing is essential not only to reduce the chance of disease transmission but also to protect materials from bird damage.
 
     The ridge opening will occasionally have rain or snow blow into it.  Do not locate feeders directly below the ridge. Exposed truss chords should be covered with flashing to provide protection from water damage.
 
     The size of the ventilation openings is critical to assure proper ventilation rates. Use the illustration below to find the proper size of the ridge and sidewall openings in Table 1.
     The ventilation curtains should be installed to move freely without excessive friction on structural members.  Use cotton or nylon cord to create an X pattern on either side of the curtain. This prevents the curtain from flapping during windy conditions. The top of the curtain should go up past the top of the opening in order to create a seal when needed.  Use an insulated curtain on the north and an uninsulated one on the south in buildings where temperature control is critical and there is exposure to north west winds. In less critical situations, both may be uninsulated. 
 
Table 1. Natural Ventilation Openings
 
Building             Ridge             Curtain
Width              Opening            Opening
BW (ft)              RO(in)            CO (in)
 
to 20                4 to 6              30
22 to 24             5 to 6              36
26 to 30             6                   42
32 to 34             7                   48
36 to 40             8                   60
42                   9                   60
44                   9                   60
 
Adapted from MWPS-33
 

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