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.