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How Natural Ventilation Works
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ISU Extension Publication #: none - electronic format only
Authors: Steven J. Hoff, Ph.D. and Jay D. Harmon, Ph.D., P.E.
Department of Agricultural and Biosystems Engineering, Iowa State University
Date:  11/94

       Natural ventilation, as the name implies, uses natural forces in nature to deliver fresh air into the building.  Two forces; wind and thermal buoyancy, are the predominant forces used to deliver proper fresh air to the building.  Properly operating naturally ventilated buildings use wind and thermal buoyancy in advantageous ways. To determine "proper" operation, we must first discuss how wind and thermal buoyancy act to ventilate buildings.

Wind Generated Ventilation

       The most prevalent force of nature driving building ventilation for naturally ventilated buildings is wind.  During summer months, when outside temperatures are high, advantage is taken of wind forces to supply the building with as much fresh air as possible.  This is done in an attempt to control inside building temperature. Conversely, during cold winter periods, we want to reduce the effect of wind on the building to help maintain inside temperature.  Air exchange is maintained at levels required to remove excess moisture, gases, and airborne dust.

       Wind, as one might expect, forces outside air through any opening on the windward side.  Windward openings will be pressurized implying that fresh air will enter any opening.  Leeward and ridge openings will, in general, be depressurized implying that stale inside air is exhausted from the building at these locations.  The illustration below gives a general feeling for pressure distributions based on wind direction. (no illustration available)

       With wind being a predominant driving force for ventilation, building orientation becomes a critical issue. In general, it is recommended that the building be oriented to take full advantage of predominant summer winds. For Iowa, predominant summer winds originate from south to slightly south-east. A wind rose can be used to decide orientation. The June windrose for Iowa is shown below. The windrose shows the percent time winds are from the indicated directions.

       A building oriented with the roof ridge east-west will be subjected to predominant summer winds.   Unfortunately, an east-west axis will also subject the building to predominant winter winds. Winter winds originate from the north-west in Iowa thus special provisions must be maintained on
the north side of buildings during cold winter periods.  The January windrose for Iowa is shown below.
 (no illustration available)

       Special provisions such as insulated north-side doors or insulated curtains are common provisions incorporated on the buildings north side to combat these winter winds. A northern wind break may also be used.

Thermal Buoyancy Generated Ventilation

       Thermal buoyancy simply implies that air inside the building is being heated.  When air is heated, it becomes lighter and rises. If openings are provide in the ceiling/ridge, the heated air will escape from the building. As stale, inside air escapes, fresh-air must enter as a replacement.  Thus, a ventilation process is developed. The overall effect is identical to a home fireplace/chimney system.  For livestock buildings, the "logs" are replaced with pigs/poultry and the chimney is replaced with the ridge vent system.

       Thermal buoyancy is a force that we try to take advantage of during cold winter periods. As mentioned previously, wind will force fresh air into the building at relatively rapid rates. During cold periods this is not desired so small openings are used.

       If the north and south-side openings are reduced to minimal levels, we can draw fresh air into the building using thermal buoyancy.  For thermal buoyancy to be effective, the ridge must be designed to behave as a chimney and provide as little restriction to airflow as possible.

Chimney/Ridge Performance

       The purpose of the ridge vent system is to generate fresh air ventilation during cold winter periods and cool periods during fall/spring periods. During summer periods, the ridge provides little benefit in ventilation.

       Chimney and ridge must work together to provide a path for stale, moist inside air to exit the building.  Winter winds, when flowing perpendicular to the ridge, will produce a suction at the ridge. This suction, combined with thermal buoyancy will provide the force to exhaust air from the building.  The ridge should be free of obstructions to allow air to freely flow out of the building. Upstands 6 to 12 inches above the ridge will greatly increase suction at the ridge and hence increase the ventilation rate.  A baffle control can then be used to decrease ventilation if the need arises.

       Ridge caps are commonly used on ridges to prevent rain and snow from entering the building. These caps tend to restrict airflow from the building and generally would not be recommended.  Instead, a sloped trough below the ridge along the ridge length will provide for rain/snow removal and simultaneously provide for a clear opening for exhausted air.

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