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Vapor diffusion venting to the interior of the home at the ridge

Laura62584 | Posted in Energy Efficiency and Durability on

Hello, Laura here asking a variation of the question so many have asked before, I know…  But I keep getting conflicting information and Im at a standstill.  Here are a few key details on the home first:

-1963 construction
-3×6 roof rafters, shiplap on top of them
-Roof replaced two years ago with metal roof (Peel n Stick does not allow vapor through)
-Plywood was added on top of my shiplap when roof was done
-Non vented roof assy
-Location: Los Angeles

Now Im insulating my ceilings.  I thought I did all my research and had a solid plan..  but Im being told it will fail.  Im using 2” foilfaced rigid foam between the rafters sealed with spray can foam around every edge, foil tape etc.  My plan is to fill the remainder of the cavity with Rockwool and put a 1” unfaced foam Under the rafters for thermal bridging.  Ill then use a vapor retarder under that to control any air moisture that does make it in to the cavity.  Its just not possible for me to vent the space even with an exterior diffusion vent at the ridge due to my already finished roof install.  I also was talked out of foam board on the roof deck by the roofing contractor.  So this is best Ive got for my situation.  My main concern is up near the ridge in the winter, getting condensation between the shiplap and the ply.  Im trying to keep the warm air away from the roof deck to prevent any problems but im torn between trying to seal it up air tight, and creating a way for moisture to escape.  My question is if I leave the 3.5” or so at the ridge not foamed, but instead just put rockwool in that area and then wrap my vapor retarder around the area would that allow moisture that may accumulate up there to dry toward the interior?  Like an interior version of the ridge diffusion vent?  Or should I just seal the shit out of it with spray foam and vapor barrier paint or or mastic sealant or something.  There is a wall that runs down the middle of the house about half way, which i could pack the top with spray foam and seal it fine I think…  BUT the living room has a large solid wood beam at the ridge, about 15”x6.5” and i have no way of accessing the area between it and the roof sheathing.  The beam will be wrapped in drywall and ill prob wrap it with the vapor retarder and the ceilings will have drywall (might put shiplap over them later).  Any advice would be appreciated.  Im at a stand still here until I figure this out..  thanks!

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  1. Jon_R | | #1

    Air sealing is critical - so consider using open cell spray foam to replace the upper rigid foam and the mineral wool.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Laura.

    With that ridge beam and a lofted ceiling, you have plenty of room to deepen the rafter bays which will allow you to reach a decent R-value for your climate zone (IRC minimum is R-38, I believe, and a good target). One way to do that is by installing strips of rigid foam along the bottom of the rafters, creating a deeper cavity and mitigating thermal bridging, like this, but for roof rafters.

    Have you read this article: How to Build an Insulated Cathedral Ceiling? I think it will answer your questions and get you pointed in the right direction.

    1. Laura62584 | | #3

      I have read all the articles I can find on this trust me.. The articles on here are cut short and im not sure how to access them. I dont have the room to extend my rafters down or i would have added a soffit vent. Im already extending them down more than Id like with the 1” foam board on the underside of the rafters to prevent thermal bridging. My question is if I dont air seal the under side of the ridge will it allow the moisture that may get in there to dry toward the interior of the house? Over the beam I can also add a vent at the exterior end of the beam where it exits the house just under the roof when I redo the outside of the house... that’s something i thought of yesterday. It wouldnt be the best solution I dont think but it would allow air to dry the space.. my only other option at this point is to just make sure im using as many methods as possible to air seal the ceiling. I really dont think its going to be an issue but with all the different opinions on here you never know what to believe. I havent seen anything about venting the ridge to the interior so I thought id ask and see if anyone had done it before. 2part spray foam kits are only an option at the ridge as Ive already got my foam board in place in a couple rooms. I had originally wanted to do spray foam up in the rafter bays but it was going to be not as DIY friendly and messy since Im living in the house. The foam boards are just as air tight with canned foam around them. What Im mostly trying to stop is any air getting through the wood. I know ill be sealing everything else up tight.


      1. Expert Member
        MALCOLM TAYLOR | | #5


        You can get a free trial membership on GBA which gives you access to all the articles which may be useful.

    2. Laura62584 | | #4

      Minimum R value by code here is R-30. Ill have R-34 when Im finished.

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