Vapor Diffusion Port for Cathedral Ceilings
This is reference to Martin Holladay’s Vapor Diffusion Port Article from 2018 that has been updated a few times.
First, I wanted to thank Mr Holladay’s wonderful article outlining the reasoning behind Vapor Diffusion Port for cathedral ceilings. It has helped me tremendously talking back and forth with my general contractor who is also willing to learn and be open to feedback.
Background= My house is in Southern California, Climate zone 2/3. Planning to make my master bedroom traditional ceiling to a cathedral ceiling.
Roof area will be approx 260 sq ft. Ridge of the master bedroom is 13ft. And the rake/gable length is 10 ft each side. Slope of the roof at least 3-in-12.
Article states the vapor diffusion port area should be at least 1/600 of the ceiling area. Using the 1/600 minimum requirement, then the area should be at least 260 sq ft x (1/600) = 0.43 sq ft. Is this correct?
Article states the material covering the vent slots in the roof should be airtight and have a vapor permanence of 20 or more.
My question. My contractor will be installing the traditional off-the-shelf ridge vent. Given the vent slot minimum requirement area is small (0.43 sq ft), my contractor will cut out the roof decking/sheathing needed for the ridge vent slot and install the ridge vent on top. And this completes my vapor diffusion port for my master bedroom cathedral ceiling?
Or is making the vent slot area AIRTIGHT with an exterior gypsum board sheathing that meets the vapor permeance of 20 perms necessary?
Given the small minimum area requirement, it seems like it will be a very narrow strip of exterior gypsum board sheathing (1-2 inch strip that is 13 ft long?).
Can someone explain why the vent slot needs to be airtight? The vent slot just being open to air with the off-the-shelf ridge vent installed will meet the vapor permeance of 20 perms requirement in my particular case.
Thanks for the community and help in advance!
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