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Community and Q&A

Vapor Diffusion Port for Cathedral Ceilings

dno713 | Posted in General Questions on

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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  1. Expert Member


    From Martin's 2018 article:

    "Builders are generally familiar with vented attics. In a vented attic, air flow through the attic is encouraged. Wide soffit vents, generous ridge vents, and random air flow cracks are all good.

    By contrast, an attic with vapor diffusion ports at the ridge needs to be airtight. For builders familiar with vented attics, this approach may seem odd. If you plan to install vapor diffusion ports, soffit vents are not only unnecessary — they are counterproductive.

    Here’s how Peter Yost, GBA’s technical director, explained the need for airtightness when constructing this type of attic: “Note that while ridge diffusion venting could work, it would only work if there were a continuous roof assembly air barrier; otherwise that vent will make the air leakage and subsequent wintertime wetting worse. The vent needs to be for diffusion only, not an air vent.”

    1. dno713 | | #2


      I should of re-read the article more closely. Thank you for pointing out the need for airtightness.

      I will plan with my contractor to make sure the vent slots are AIRTIGHT. Thank you!

      1. Expert Member
        MALCOLM TAYLOR | | #3


        It's all new to me too. It was a good opportunity to read more about it.

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