A minimal ventilation rate is required in animal housing in cold weather, regardless of outside temperature. This minimal rate is necessary whether the barn is designed to be a warm barn or a cold barn in winter. In addition, the minimal ventilation should be continuous to maintain concentrations of contaminants in the air at minimal levels. The minimal rate depends on outside weather, design conditions, animal factors (number, type, age, and size), and whether the barn is intended to be cold or warm.
Barn environments can be categorized according to temperatures maintained in the barn in winter. The particular environment, based on desired indoor temperature, must be established before ventilation system design can begin. In cold barns, indoor temperatures are allowed to fluctuate with outdoor temperatures. Ventilation is sufficient to maintain indoor temperatures within 3°–6°C of outdoor temperatures. Ventilation is largely unregulated, except to adjust for seasonal changes. In general, a cold barn with natural ventilation has no insulation, an open ridge and eaves, and sidewalls and endwalls that open. Providing an open ridge along with open eaves has long been recognized as a way to use a stack effect to cause air exchange, especially to control moisture in winter. Indoor temperatures are expected to be a few degrees warmer than outdoor air temperature because of the heat given off by the animals being housed. Current recommendations call for providing a ridge opening of 5 cm per 3 m of barn width and equivalent open area divided between the two eaves. Raised ridge caps are to be avoided. Their performance is unpredictable because of local wind patterns, which often channel winds into the structure and increase the entry of snow and rain. The combination of the open ridge and eaves should be viewed as the sole source of ventilation only during the most severe winter weather, ie, during periods when temperatures reach the lowest levels or times when conditions are especially windy or stormy. During all other times in winter, additional ventilation should be provided.
Warm barns are well insulated and, by necessity, have well-controlled ventilation systems. These barns are designed to provide a relatively uniform environment throughout the winter. Tie stall barns for dairy cows (indoor temperatures at least above freezing) and farrowing and nursery barns for swine (indoor temperatures 25°–30°C) are examples of this type of housing. Keys to success include ventilating fans and controls chosen to match the needs of the animals being housed, a well-insulated building, supplemental heating (if required), and a ventilation system that is well managed and regulated to compensate for changing outside climatic conditions.
Some barns do not fit into either the warm or cold category. In-between or modified environment barns usually have indoor temperatures in winter above freezing. These buildings have some insulation, perhaps only under the roof, and are naturally ventilated (usually with an open ridge, eaves, and sidewalls). Unfortunately, even though a minimal ventilation rate is always necessary, ventilation openings may be closed or blocked during extreme weather to keep manure from freezing and for other reasons. This practice can result in inside temperatures rising 10°–20°C above the outside temperature, significantly higher than the 3°–6°C temperature difference limit considered acceptable for cold barns. This alone can create problems because of excess moisture buildup and a high relative humidity. Even more seriously, openings remain closed or blocked after severe weather conditions have passed. As a consequence, blocked ventilation openings restrict airflow during less severe conditions, resulting in underventilation and poor environmental conditions. A properly designed and managed in-between barn is more like a warm barn, in terms of both design and operation, than a cold barn. Thus, to avoid problems, the design and management of in-between barns should follow the guidelines for a warm barn.
Underventilation in winter is one of the most serious threats to the environment of animals. Both improper design and improper management of the ventilation may compromise animal health. In colder climates, problems are most likely during winter, spring, and fall, especially during rainy weather and warmer days coupled with cold nights.
Adjusting ventilation for severe winter weather and not readjusting to allow increased ventilation for milder winter weather appears to be a major reason for air quality problems in barns in winter. This is especially true for cold barns with manually controlled natural ventilation. For example, ventilation openings are closed in anticipation of a windy, cold, blustery night, but are not opened the next day when, although the temperature may still be cold, the wind subsides and the sun shines. The lack of wind reduces ventilation and thus air exchange and the positive effects of dilution.
Sometimes, good ventilation system management does call for openings to be reduced. For example, a slot inlet in a mechanically ventilated tie stall barn may be adjusted, or fabric may be placed over an open sidewall to match the lower ventilation rate in winter. All openings should never be covered, however, because this leaves no means of air exchange for moisture control.
In addition to adversely affecting the animal environment, the design and operation of naturally ventilated barns also influence moisture-related deterioration in wood members and metal fasteners. In naturally ventilated dairy free-stall barns in Michigan, where air exchange in winter was defeated by blocking ventilation openings, average wood moisture contents >30% dry basis (capable of supporting wood decay and corrosion in metal fasteners) were found after 2–3 mo of cold weather operation. Moreover, restricted air movement in these barns inhibited drying and allowed wood moisture content to remain high even during warm weather. Warm, moist conditions favor growth of mold, bacteria, and decay fungi and accelerate metal corrosion. The presence of insulation under the roofing in these barns fostered the situation. In barns in which design details and management efforts allowed optimal air exchange rates, increases in moisture were minimal. Even if free water from precipitation and condensation caused slightly increased moisture contents, adequate air exchange, especially during warm weather, promoted drying of wood truss components so that deterioration was not a problem.