Ventilating Cathedral Ceilings
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.
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.
Step 1: The Flow of Water Vapor
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.
Step 4: Proper Ventilation
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.
Photo courtesy of ©iStockphoto.com
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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Cathedral ceiling with a beam in the middle
November 21, 2020
I have a mono-slope cathedral ceiling where there are two runs of rafters: one below and one above a central beam from which the rafters hang (the rafters are about 20 feet in length, for the total of 40 foot long ceiling). Therefore, I cannot vent these rafter spaces since this central beam sits athwart the air path. The ceiling is insulated with fiberglass insulation and has a two inch space between the insulation and the roof deck. What do you recommend? (More curiously, the ceiling has been unvented for 60 years without a problem. I do not know why.)
lawsuite over vaulted ceiling vevtlation
July 13, 2023
I roofed a residense in Dec of 2019 in Bangor Calif. I will not disclose the homeowner. Aftyer we completed the roof and it was time to get paid. Here came the complaints. The cust said we watered her roof during the reroofing project. because it rained after we tore off. The roof was completly dried in and the cust called my office and said there was water pouring into her house. I got in my truck drove to her home inspected inside to find the dwelling dry inside. When i was done afterr many complaints she saif her roof sheathing was watered and warped due to me neglagence.
I went there with two of my crew men. Removed a couple of sqs of shingles. The shingles were not butted to tight. There were no bubbles in my paper. So we removed the roof sheathing in the presense of the homeowner. She has a vaulted ceiling. 2X4 roof rafters run horizontally. with batten insilation filled to the top of the 2X4 cavity. There was evidense of moister being trapped in the vaulted ceiling witch would certainly cause deflectioin in the roof sheathing from moister i her attic.
since the rafters and roof sheathing were both run horizontally there were areas where seams in the roof sheathing did not fall on a roof rafter. This caused deflection as well. She asked if we could come back in Oct to do some blocking for the plywood. It was 100 degrees is why she chose Octopber. After we left she proptly turned me into the contractors state lic board. And went after my bond. She hired my local competitor Butte Roofing to come out and inspect my roofing job.
This is what Bobby Starns
of Butte Ropofing wrote in his inspection. I question that he alone during the inspection even removed any roof shething to know what was there. I qoute him. shingles were installed in the rain. water became trapped in the sheathing without letting it evaporate. Mr Starnes stated this would have caused the roof sheathing to warp. bowing and swelling. mr starns said the sheating showed signs of water intrusion. Funny all the photosbus. Not one sheet lookled wet or did they have water stains on it. Mr Starnes agreed that the rafters run horizontally that was true. but stated with a vaulted ceiling this was not inappropriat.
Mr Starnes
Also stated there was blocking to carry the seams of the plywood that did not land on a rafter. Seams i obsurved not to be blocked at all. Mr Starns stated that this type of application does not require ventingh. He said it is not nessisary. he also stated that there was 3-3-1/2' of ridged insiulation in each space not stacked on top of each other. what does that mean. he states none of the insulation was withion 1 inch of the roof sheathing. doesent the fact that the rafters were full of batten insulation and the rafters rub horizontally.eliminate the posabilaty of venting this roof the way its put together any way.
This guy is an idiot.
there is an upper story on a step shingle run on one side of this roof deck. And a gable on the otherside. How can you get any thermal pull that way by venting yourt soffit and installing a continous ridge vent. The bad part about this is after his roopf inspection full of lies and no knoledge of how to vent and what its for. I was never paid for my $10,000.00 roof. And my bonding company bought his bullcrap. I was forced to pay the homeowner an additional $15,000.00 from my bond. Now i am back in court being sued for another $7K plus for gutter butte roofing wants to install. and flashing. Yes they tore off my brand new roof.
I hope a seasoned veteran roofing contractor has something to say about his understanding of venting. Cannot do much about his lies.
