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Community and Q&A

Retrofitting a Vented Cathedral Ceiling

davetrouble | Posted in Energy Efficiency and Durability on

Could this possibly work as an interior retrofit to a 1984 vented cathedral ceiling? To eliminate the effort of removing the existing drywall could I add interior rigid foam over the existing cathedral ceiling drywall in zone 5? I noticed in a previous Q&A about adding rigid foam over existing drywall in a wall application in Zone 5

Rigid foam over existing drywall?


Martin Holladay responded:

Ben,
Adding interior rigid foam over the existing drywall is fine. It won’t create moisture issues.

There is no scientific or technical justification for John Clark’s statement that “you could trap moisture within the drywall and generate mold.”

Keith’s suggestion — that “You could have the moisture content of the wall modeled with WUFI” — is unjustified. That would be a waste of money.

So, I was wondering if there was a way to make this work in my cathedral ceiling application, which I realize is a different application

The existing ceiling is 2×8 rafters with plywood roof deck and asphalt shingles. Ventilation is with soffit and ridge venting. It is insulated with R19 pink fiberglass batts which I believe are kraft faced. 1/2″ drywall with a textured finish.

What I would like to add is rigid foam, 1×3 strapping, and 3/4″ thick tongue and groove wood with the intention of making the room more energy efficient

If it could be made to work, would it be better to add additionally a poly barrier on the interior of the rigid foam or not? Is there a type of rigid foam that would work better than another?

Thanks,
Dave

 

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dave,
    Adding a continuous layer of rigid foam on the interior side of your existing drywall ceiling will, indeed, improve the R-value of your ceiling.

    I advise you to install the rigid foam in an airtight manner (by taping the rigid foam seams). There is no need to include a layer of polyethylene.

    Most green builders would choose polyisocyanurate or expanded polystyrene (EPS) for this application.

    For more information, see these article:

    "Choosing Rigid Foam"

    "Walls With Interior Rigid Foam"

  2. davetrouble | | #2

    Martin,

    It is nice to hear that this idea might not be totally insane. The reason being that it is a lot easier to do an interior retrofit like this in the middle of the winter without removing the existing drywall. What can you say about the possibility of the existing drywall absorbing moisture due to it's temperature dropping below the due point? Along this line of thinking I have some follow up questions?

    1. Is it possible to add too much interior rigid foam insulation so that the existing drywall is left too cold?

    2. I understand your suggestion to employ an "airtight manner (by taping the rigid foam seams) " is because you consider air exfiltration to be the greatest risk for water vapor to compromise the assembly?

    3. On the perimeter of the rigid foam where the interface is rigid foam to wood or rigid foam to painted wallboard is there a type of caulk which would be preferred to produce a durable seal? In particular would dynaflex 230 provide adequate performance?

    4. I understand your suggestion that all rigid foams would be acceptable (even though they have a wide range of water vapor permeance) and that there is no need for an interior layer of polyethylene is because you consider water vapor diffusion to be a negligible risk to compromise the structure? Is it possible that too low a permeance (as with polyethylene) might somehow make things worse?

  3. GBA Editor
    Martin Holladay | | #3

    David,
    Q. "What can you say about the possibility of the existing drywall absorbing moisture due to it's temperature dropping below the due point?"

    A. Not an issue. Interior moisture won't reach the drywall, because your rigid foam will be airtight and will be a strong vapor retarder.

    Q. "I understand your suggestion to employ an 'airtight manner (by taping the rigid foam seams)' is because you consider air exfiltration to be the greatest risk for water vapor to compromise the assembly."

    A. Good. I'm glad you understand that.

    Q. "On the perimeter of the rigid foam where the interface is rigid foam to wood or rigid foam to painted wallboard is there a type of caulk which would be preferred to produce a durable seal?"

    A. Not really. Anything better than latex painter's caulk should be fine. Workmanship matters more than product selection.

    Q. "In particular would Dynaflex 230 provide adequate performance?"

    A. Yes.

    Q. "I understand your suggestion that all rigid foams would be acceptable (even though they have a wide range of water vapor permeance) and that there is no need for an interior layer of polyethylene is because you consider water vapor diffusion to be a negligible risk to compromise the structure."

    A. I'm glad you understand that.

    Q. "Is it possible that too low a permeance (as with polyethylene) might somehow make things worse?"

    A. No.

  4. davetrouble | | #4

    Martin,

    For a while I was thinking telling other people about this idea might lead to men in clean white coats coming to take me away. Now, I am starting to feel a little better about it. One of my questions was

    "Is it possible to add too much interior rigid foam insulation so that the existing drywall is left too cold?"

    based on your responses to my other questions I think your answer to this will be "no".

    I notice in your 2012 Fine Homebuilding article "Insulating a Cathedral Ceiling" you do mention the possibility of adding rigid foam from the interior (see attached image) but you do not mention that it may be possible to do this without removing the drywall. The image seems to show that the original drywall has been removed. Would you say that leaving the original drywall intact is a more acceptable retrofit at this time in 2022 than it was 10 years ago in 2012? I looked around but I could not find anyone saying it would be ok to leave the original drywall intact which is why I started this thread.

  5. davetrouble | | #5

    Martin,

    In my original post I mentioned that I don't know for sure if the fiberglass insulation in the ceiling is faced or not. I had assumed that it would be but I haven't confirmed that. I could check if it were important. Based on your earlier responses I'm thinking that your answer would be that it would not matter but I wan't to double check this before crossing the Rubicon. Would it matter?

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