Ohio State University Fact Sheet

Food, Agricultural and Biological Engineering

590 Woody Hayes Dr., Columbus, OH 43210

Chimneys: A Natural Ventilation Alternative for Two-Story Barns

AEX-115-98

Animal health and diseases are influenced by air quality, which is directly related to ventilation. The natural flow of air through properly sized and managed ventilation openings continuously replaces contaminated air with fresh outside air. This air exchange helps control relative humidity and reduces the concentration of pathogens, ammonia, carbon dioxide, and other air pollutants.

Many livestock barns in the Midwest are two-story facilities which pose significant ventilation challenges (see Figure 1). While these facilities may be 50 to 75 years old or older, farmers are often reluctant to stop usage and are likely unwilling to invest much money in redesigning them. Frequently, providing a continuous open ridge for natural ventilation (see MWPS-33, Natural Ventilating Systems for Livestock Housing) is not seen as a workable option in these buildings. Chimneys can often be utilized in such barns to conveniently provide effective ventilation.

How a Chimney Works

Chimney exhausts can be designed for remodeling purposes in two-story barns or modified environment buildings (Table 1). Keep in mind that heat is generated from animals (primary source), equipment, solar gain, and supplemental sources inside a building. When inside air temperature is higher than outside air temperature, a pressure difference is created that causes the warmer, less dense air to rise. The natural process behind this air movement is commonly referred to as thermal buoyancy or stack effect.

Natural ventilation by thermal buoyancy is affected by inlet and outlet area, height difference between the inlet and outlet, and difference between inside and outside temperatures. Utilization of the stack effect for ventilation (e.g., chimney) can provide the primary air exchange needed during cold winter periods and supplemental ventilation during warmer weather.

Figure 2 represents a two-story barn with a loft or machinery storage above the animal living space. The goal of chimney ventilation is to draw warm, moist air from the lower story of the barn, through the upper story, and out of the barn. Air that otherwise would become stale is exhausted from the barn via openings in the lower story ceiling and the barn roof. Inlet openings must exist around the perimeter of the animal living space to allow cooler outside air to enter the building. This fresh air replaces air that is exhausted from the lower story of the barn.

Typical Arrangements

The ideal chimney ventilation design keeps the exhaust air separate from the air in the upper story. In many instances, good air exchange can be achieved by letting the upper story serve as the airflow duct, provided that air is allowed to continuously rise (no obstructions or tiered roof lines) toward the air exhaust. A continuous, fully constructed chimney, on the other hand, is easier to insulate and maintains more dependable airflow patterns than the upper story of a barn, so there is less likelihood that moisture in the exhaust air will condense within the system.

In many barns, construction of more than one chimney (air exhaust) provides better ventilation of the animal living space. This is especially recommended in long rectangular-shaped barns, L-barns, and barns with attached building wings. To ensure more uniform removal of contaminated air in a long rectangular building, locate chimneys as shown in Figure 3. For barn floor plans having more complex shapes, divide the floor area into a convenient number of nearly square sections and construct a chimney centrally within each area.

Barns that are 60 feet wide or more and have flat ceilings dividing the stories also require multiple chimneys (or multiple openings in the barn ceiling) to ensure that the entire animal living space is ventilated. Divide large barn areas into smaller sections and locate a chimney (ceiling opening) centrally within each section as illustrated in Example 1.

In many two-story barns, an open driveway or machinery storage area is centered between two mows. In these cases, removing perimeter floor boards running perpendicular to mow floor joists, as shown in Figure 4, allows air to move upward from the animal living space into the mow space. The same procedure can work in a barn without a driveway if center floor boards are removed. Remember that, in either situation, an upper story air outlet must still be provided to remove moisture-laden air from the mow space.

Figure 4. Alternative means of exhausting air from the animal living
space in a two-story barn (top view of mow floor).

Openings in the upper portion of two-story barn endwalls (Figure 5) can serve as outlets for exhaust air. Removal of contaminated air has been noted up to 25 feet from endwall openings. Thus, any barn over 50 feet long needs at least one chimney (roof opening) in addition to endwall air exhaust openings.

Figure 5. Air exhausts in a two-story barn (gambrel roof shown).

Sizing Chimney Openings

A proper chimney design requires 1 square foot of opening for every 100 square feet of barn floor area to be ventilated. To calculate the chimney size necessary, refer to Example 2.

Remember that all chimney applications need inlet openings that provide as much or more total open area as is provided by the chimneys (18 sq. ft. in the previous example). The inlets could be existing (eaves, doors, leakage, etc.) or newly constructed openings. The walls of some older two-story barns are not very airtight. Once a chimney is constructed in these barns, natural infiltration of air usually supplies adequate fresh air for good wintertime ventilation. If windows and doors are distributed around at least one side of the building, cracking them open can provide additional inlet area. Inlet openings may need to be constructed in barns that have walls around the animal living space that are more solid and airtight. Refer to MWPS-33 for guidance in providing and locating proper air inlets in these situations.

Depending on the application, chimney location and design (size, insulation, use of damper, etc.) may vary. In all cases, check your plans before starting construction to ensure that the required chimney open area is maintained throughout the design (especially at critical points such as the outlet and where the chimney extends through the roof). If not careful, ventilation can easily be compromised by restricting airflow through the chimney.

Observed Benefits

More than twenty two-story livestock barns in west-central Ohio were retrofitted with chimney exhausts following recommendations provided by Ohio State University Extension. The barns were visited on one or more occasions before and after making modifications to document, both firsthand and as reported by the producers, the performance of chimney ventilation systems (Table 2). The lead author and other visitors observed that, overall, air within the barns was less humid and had less of an odor after chimney ventilation systems were in place. Over the few years since modifications were made, livestock managers have reported that drier barn environments resulted from improved ventilation. Bedding needs were reduced, in some cases substantially, in these barns. Moisture condensation under roof lines was eliminated in some barns and dramatically reduced in others. In addition, working conditions for management and employees were improved, according to several farmers.

Swine managers have utilized two-story barns for large groups of finishing hogs. After making recommended ventilation improvements, air transfer from the living quarters to the second story was easily observed by watching cobweb and dust particle movement in newly created openings. Fewer respiratory problems have been observed in these facilities since the barns were retrofitted. Consequently, reduced medication usage and improved growth rates were reported. Pigs did not pile as frequently after barns were retrofitted, which producers credited to proper airflow patterns and reduced dampness. Location of chimneys over desired manure collection areas also improved pig dunging habits.

Dairy managers have commented that two-story barns utilized for maternity pens seemed more comfortable for cows and calves after ventilation improvements were made. Where stale air was removed by chimneys over freestalls, those stalls were used more often. Dairy managers credited the improved air exchange that was achieved by implementing recommended building modifications with reducing somatic cell counts. Managers have also associated fewer incidents of mastitis in their herds with ventilation improvements they made.

Beef and sheep producers generally utilize two-story barns for calving and lambing, respectively, during inclement weather. Building modifications that provide drier environments contribute to fewer incidents of pneumonia and scours, and improved vitality of youngstock, according to cooperators.

Some limitations to naturally ventilating barns using chimneys also exist, including lack of precise control over the barn environment, occasional difficulties in placement of chimneys, and construction problems associated with retrofitting a barn. These limitations may be especially apparent with an older barn. However, a chimney or chimney-like design often is the only viable alternative for ventilating barns where construction of an open ridge and installation of a mechanical ventilation system have been ruled out as workable options. Improvements in air quality and benefits associated with providing proper air exchange have been demonstrated in barns where chimney design concepts have been applied. The chimney merits consideration as a natural ventilation alternative for two-story barns.

This fact sheet was reviewed by:

S. Foster
D. Frobose
D. Gerber
W. Joslin
J. Papritan
S. Ruhl
B. Strobel
M. Veenhuizen

Reference

MWPS. 1989. MWPS-33, Natural Ventilating Systems for Livestock Housing. Iowa State University, Ames, IA.