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Ventilating today’s cathedral ceilings is more of a challenge than ventilating regular roofs. Because of the isolated air spaces in rafter cavities, cathedral ceilings are especially prone to moisture damage - posing unique ventilation problems. Special attention to the avoidance of water condensation and the management of water vapor and its flow throughout the attic is extremely important to preserving the service life of the roof. With careful consideration for the movement of moisture throughout the attic and the required design and ventilation, builders can successfully deliver the dramatic look and spacious feel of cathedral ceilings with proper ventilation.
Ridge and soffit venting should be installed to allow for a balanced system that ensures that each rafter space has adequate airflow. (Photo courtesy of Trimline Building Products.)
To understand the necessity of proper ventilation, it helps to have a basic understanding of water vapor movement. The principle is fairly straightforward: during the winter months or in cold regions, water vapor always migrates from a warmer environment (the interior of the building) to a cooler one (the exterior). If this flow of water vapor into the attic and ceiling cavities is not subsided, condensation can occur and eventually lead to the formation of mold and mildew. Additionally, condensation can saturate roof components, leading to problems such as wood rot and damage to the insulation, potentially weakening it.
Step 2: Sealing Bypasses - The First Line of Defense
Attic bypasses are hidden air passageways that lead from the heated space into the attic and are often the cause of moisture problems in the attic. These problems can include water-ridden insulation, cracking plaster and paint, structural rotting and ice damming. As warm air rises into the passageways, water vapor is often carried along with it where it condenses in the attic. A close inspection of the attic can help identify attic bypasses that should be sealed to ensure the insulation works properly and prevent moisture problems. To control the flow of water vapor and the potential of moisture problems in the attic, it is advised to seal all attic bypasses that may allow air leakage. Additionally, any water vapor that does get into the attic should be dispersed and/or ventilated to keep it from accumulating and causing problems.
Step 3: Vapor Retarders
Materials that are not easily penetrated by vapor are referred to as vapor retarders, or vapor barriers, and they are used to keep moist air from reaching its dew point and forming condensation. Because the vapor retarder serves to prevent the passage of water vapor contained in the air, failing to install one allows vapor to more easily travel through the ceiling and walls and eventually condense in the attic.
Vapor retarders are typically recommended for buildings in which the flow of the direction of the humidity may cause the dew point or condensation to occur under the roofing within the insulation chamber. For this reason, it is important to be mindful of the insulation being used when installing vapor retarders to ensure that they interact beneficially. The vapor retarder should be installed on the warm side of the insulation to block moisture flowing into the rafter space.
A balanced system of soffit and ridge ventilation provides continuous ventilation of hot, moist air at the hottest point of the roof.
Even with the attic bypasses sealed and effective vapor retarders installed, proper ventilation is essential to prevent moisture problems in cathedral ceilings. Proper ventilation is necessary in order to address water vapor that cannot be blocked to allow it to escape, rather than allowing it to accumulate until it causes problems. Ridge and soffit venting should be installed along the ridge and eave to allow for a balanced system and so that each rafter space has adequate airflow. Trimline® Ridge Vents, for instance, provide continuous ventilation of hot, moist air at the hottest point of the roof to promote a balanced system. For information on how much ventilation is needed, visit Trimline Building Products’ Web site (http://trimline-products.com/tools_calculator.cfm). This will help you determine the proper amount of soffit and ridge ventilation needed. Vent chutes should also be installed between the rafters from the soffit to the ridge to channel the airflow.
Ventilating Cathedral Ceilings for a Durable Roof
Considerations related to condensation and dew points are critical in controlling the intrusion of moisture in any building, and cathedral ceilings are no exception. The accumulation of water in a roof assembly facilitates mold growth and over time can contribute to bigger issues, such as physical deterioration. In today’s tightly sealed buildings, water vapor in a roof assembly needs to be managed to avoid these problems. Cathedral ceilings that are properly designed to addresses the source of water vapor, dissipate the movement of water vapor through the assembly and ventilate collected moisture will help to ensure long-term roof system performance and enjoyment of the ceiling for years to come.
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