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Fix for Unvented Cathedral ceiling issue

BPortnoy1 | Posted in General Questions on

Hello. I previously wrote about my cathedral ceiling which was insulated incorrectly. We are in 4a and have an unvented cathedral ceiling with no vents and also no spray foam or rigid board. Just fiberglass. We will have condensation issues if we don’t fix the problem.  We can’t add ventilation as we don’t have room on the eaves to do so and it doesn’t make sense for how the ceiling was done  

To avoid taking down the ceiling that’s there (it’s a bedroom) and spray foaming, we are debating using blow in cellulose and doing it though the roof sheathing from outside. We have a large space under the sheathing (between sheathing and fiberglass). The finished assemble would be an unvented cathedral ceiling that would have cedar shingles, cedar breather, tar paper, sheathing, blown in cellulose, finger glass (Kraft covered to the inside ), Sheetrock and then the shiplap  that’s already there.  We are thinking of maybe added a ridge vent as well. 

do you think this could work for a fix? It’s defiantly not a typical suggested unvented roof assembly but I have limited options now. 

thank you 

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Replies

  1. Expert Member
    Michael Maines | | #1

    That would still not be a code-compliant assembly, since cellulose is vapor-permeable. Because you have a large space between the sheathing and fiberglass, a code-compliant and safe option would be to introduce a ridge vent with a net free area equal to or greater than 1/150 of the floor area. If there were corresponding vents at the eaves, the required ratio drops to 1/300.) Wind-washing of the fiberglass would impact your thermal performance but building durability is more important.

    There are ways to introduce venting at the eaves other than typical soffit vents: https://www.cor-a-vent.com/in-vent.cfm,
    https://airvent.com/index.php/products/intake-vents/vented-drip-edge

  2. BPortnoy1 | | #2

    Thank you. My issue is that the Sheetrock has many holes in it due to penetrations from recessed lights and the shiplap that was nailed to the Sheetrock. Blower door showed the room to be very very leaky. If I introduce vents I'm worried that the room will be even more leaky. Is there a way to air seal the inside? See pic (yes the recessed light was also done off centered :-) )

    1. Expert Member
      Michael Maines | | #3

      Some people have had luck using dense-packed cellulose to slow air flow, but it's not a reliably safe approach, especially if your ceiling is full of penetrations. How new is your roofing?

      1. BPortnoy1 | | #4

        It's only a year old. It's new construction.

        When you say dense packed cellulose, are you are referring to the original fix I mentioned of dense lacking under the roof?

        Is there a good way to air seal from the is inside? I can remove the recessed lights and hypothetically Sheetrock over all this shiplap to make an air barrier but I don't feel that's the best option.

        Do you think venting with soffit and ridge vents without fixing there penetrations is a bad idea? Thank you for the help

        1. Expert Member
          Michael Maines | | #8

          Yes, it's not code-compliant and I would not recommend it, as it is not a reliably effective approach, but some people get lucky.

          The only way I know of to fix your issue properly is to expose the framing cavity, from either the interior or exterior. I understand that you don't want to do that; any half-measures come with higher levels of risk.

  3. Expert Member
    AKOS TOTH | | #5

    Any proper fix is about as disruptive as a replacing most of the finishes. Not worth it unless you have a problem.

    I would put a vapor diffusion vent at the ridge and call it a day. Make sure the membrane is sealed to the roof deck with a quality tape not to the shingles to minimize airflow. This will allow for moisture to move out of the roof without allowing for airflow. You can read more about it here:

    https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports

    1. BPortnoy1 | | #6

      Thank you. I thought about this but in other climate zones. I'll check it out. Thank you. I feel like by the time you know it's a problem it's a huge one. Was trying to be pro active. Thanks!

      1. BPortnoy1 | | #7

        Just read the article. He says it's not safe for cold Climate climates unfortunately. Or else it would be a great fix

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