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ventilation and vapor

Posted by tomh on March 12th, 2003 11:19 AM
In reply to Vaulted Ceiling Ventilation and Vapor Barrier ? by Mckenzie on March 12th, 2003 10:11 AM [Go to top of thread]

1 of 1 people found this post helpful

Sounds like you have taken a lot of steps to ensure adequate ventilation. Passive venting is fine as long as the vent area matches up with the attic square feet. There is a strong convective current caused by the heating in the roof that will move significant air from the soffits to the roof vent. You can enhance natural circuation with turbine vents or powered vents, but is completely unnecessary if you already have the correct vent area.

Use the vapor barrier. Pine and cedar ceilings are transparent to moisture, and you don't want it condensing on the cold surface of the roof sheeting and dripping back down on insulation and staining the wood. Moisture will not be trapped between the wood ceiling and vapor barrier, because it will not condense where R-38 insulation is above the barrier. Moisture will remain in the vapor phase and be at equilibrium with the rest of the room. Failure to put in a vapor barrier will let moisture rise through the insulation until it reaches a temperature where it condenses. At that point liquid will form and begin to percolate back down and may actually cause drips to emerge from the wood ceiling.

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