Attic Ventilation Problems

By: Scott L. Muka, S.E., P.E.

Attic ventilation problems can cause elevated relative humidity in an unheated, residential attic. Elevated relative humidity in the attic can promote mold growth and increase the moisture content in the wood framing to a level that promotes fungal growth and rotting. A sustained relative humidity, which is higher than 60 percent (60%), is one of the essential elements for creating an environment that promotes mold growth. High levels of humidity in an attic can raise the moisture content of the wood framing. A sustained moisture content of 20 percent or higher in wood can promote fungal growth that can grow deep into the wood surfaces and produce wood decay.

Elevated relative humidity and moisture from condensation and roof leaks can cause damage to other construction materials or items that are stored in the attic. An understanding of the building envelope, relative humidity, and the dew point are helpful in identifying attic ventilation problems.

The building envelope is a physical barrier that separates the interior environment from the exterior environment. The roof, attic, and ceiling are the building envelope for the top of the house. The roof separates the exterior environmental elements of wind, rain, snow, hail, other loads, and ultraviolet rays from the interior environment of the house. The attic space, attic insulation, and the ceiling separate the temperature and humidity of the exterior environment from the interior environment. Another way to describe these environmental barriers is to refer to the roof as the building envelope and the insulated attic floor as the thermal envelope.

Relative humidity is the ratio of how much water vapor is contained in the air at a given temperature relative to the amount of water vapor required to saturate the air at that temperature. Relative humidity in the attic is commonly measured with a humidity meter. The amount of water vapor that can be contained in air varies with the air temperature, which is why the humidity is referred to as “relative.” Water vapor condenses at a relative humidity of 100%. The dew point temperature in air is the temperature that water vapor condenses.

Warm, moist air, which leaks into the attic from the living area below, can raise the relative humidity in the attic to a level that promotes mold growth. If the warm, moist air in the attic comes in contact with attic surfaces, which have been cooled to the dew point temperature by colder outside temperatures, condensation will form on the attic surfaces. The condensation can damage the surfaces on which it forms and on surfaces on which it drips. If the warm, moist air in the attic comes in contact with attic surfaces which are at or below the freezing temperature of 32 degrees Fahrenheit (32° F), frost will form on the surfaces. When the temperature of the attic surfaces rise above 32° F, the frost will melt, and can damage items that are wetted by the dripping moisture.

Damaging levels of relative humidity, condensation, and frost can be avoided in the attic by eliminating roof leaks, reducing the amount of warm, moist air that migrates into the attic from the interior of the house, and by ventilating the attic to exhaust the warm, moist air to the outside. Numerous moisture sources are present inside the house that can leak into the attic. A list of moisture sources and their estimated moisture output are listed in an article, which is located at this Oregon State University website.

Proper attic ventilation is achieved by the use of static air vents, or by the use of the proper combination of static vents and power ventilators. A common static ventilation system has soffit vents and roof vents. The roof vents are located on, or near the ridge of the roof. A power ventilating system has soffit vents and one or more power ventilators (fans), which are located near the ridge of the roof. Both types of ventilation systems operate on the principle of drawing outside air into the attic from the low vents, and exhausting the air through the high vents. Static roof vents are not used high on the roof in combination with power ventilators because the power ventilators will draw air from the high roof vents rather than the soffit vents, which reduces the air flow from the soffit vents to the roof ridge vents. This ventilation problem is known as “short circuited” ventilation.

Ten common attic ventilation problems are shown in the following figures. Recognizing these problems is helpful in diagnosing the cause of an attic ventilation problem.

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Figure 1: A common ventilation problem is the improper retrofitting of new soffit vents over the original soffit vents. Improper retrofitting of soffit vents can reduce the net ventilation area to less than the required amount. Figure 2: The net ventilation area of roof soffit vents in older houses may be partially obstructed by debris and multiple coats of house paint.
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Figure 3: Roof soffit vents can be obstructed if they are covered with attic insulation. Rafter baffles are commonly used to hold back the insulation from the edge of the roof eave to maintain an unobstructed airway for the soffit vent. Figure 4: An exhaust duct for a bathroom exhaust fan should be vented to the outside. If the exhaust duct is terminated in the attic, warm, moist air is vented into the attic, which can increase the relative humidity in the attic to damaging levels.
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Figure 5: Debris on this static roof ventilator, as viewed from inside the attic, restricts the proper air flow through the ventilator. Figure 6: This static roof ventilator, as viewed from inside the attic, was installed off-center of the opening in the roof sheathing, which reduces the net ventilation area of the ventilator.
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Figure 7: Placing a power ventilator in the center of four static roof ventilators “short circuits” the attic ventilation, which can render the soffit vents inoperative and cause ventilation problems in the attic. Figure 8: A power ventilator should have a dual control: a thermostat that turns on the fan when warm temperatures are detected in the attic, and a humidistat that turns on the fan when high levels of humidity are detected in the attic. A properly set thermostat will ensure that the power ventilator operates during the warm summer months, when the fan is needed to exhaust warm air from the attic. A properly set humidistat will ensure that the power ventilator operates during the cold winter months, to exhaust warm, humid air from the attic.
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Figure 9: A vapor barrier is not installed between the ceiling drywall and the blown-in insulation. The required amount of net ventilation area in the attic is doubled if an approved vapor barrier is not installed between the drywall and the insulation. Figure 10: The gap in the ceiling around this furnace flue pipe is a common location for warm, moist air from the furnace room to leak into the attic, which can increase the relative humidity in the attic to damaging levels. The discoloration in the insulation is caused by the flow of warm, moist air through the opening. All unnecessary gaps that are located around ceiling penetrations should be sealed when possible.