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Community and Q&A

cathedral ceiling Insulation (with some limitations)

AFriesen11 | Posted in Energy Efficiency and Durability on

Hello, This is my first post on GBA. I am currently employed at a building department, and am constantly trying to learn. Anyways, I just bought a house and one of the rooms has a vaulted ceiling with 4×7 (actual) beams and 1.25″ sheathing. it currently has about an inch of foam on top of the sheathing, which is not enough… thus my problem. I want to keep the beams visible, which means I have about 6 inches to play with. I cant put more foam on top because the roof elevation of the vaulted room currently matches the rest of the structure, which is trussed. so my thought is to put 2″ of rigid foam up against the interior side of the roof sheathing and then get the rest of my insulation with a fiberglass batt. and then remove the 1″ of foam on the exterior and replace it with furring strips as to allow a for a 1″ of airspace and a “dry to the outside assembly” is this a recipe for disaster? I am afraid of moisture reaching the bottom of the sheathing and condensing and not being able to dry with a 1.25″ thick piece of sheathing between it and the air space.  but the system seems similar to the r-zip sheathing assemblies that are going up on walls.

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  1. Expert Member
    BILL WICHERS | | #1

    I see no problems adding a vent channel as you describe, as long as you don't have an extremely low-pitch roof that would limit that vent channel's effectiveness.

    Rigid foam on the inside directly against the sheathing is "cut and cobble", which is usually not a good idea with a roof, but in your case the ventilation channel makes it much less risky. The issue is that the perimeter seal around the rigid foam panels can't be trusted in the long term, so you risk some moisture making it to the underside of the sheathing. I think you'll have enough drying potential with the vent channel provided that you don't have a very low-slope roof, you have proper in and out venting for each vent channel, and you don't have a fully adhered membrane or other vapor barrier type of coating on the upper side of the roof sheathing that is exposed to the vent channel.

    Walls are different from roofs for this, and are a bit less critical. You can see this in the more extreme "rigid on the top to fluffy underneath" ratios for roofs than the "rigid exterior to fluffy interior" ratios of insulation for walls.


  2. NewTinyBuilder | | #2

    Hi AFriesen11! I am a brand new, first time builder who is trying to learn as much as I can about construction while building my own 8x10 tiny cabin here in Vermont. This is the first time I am posting to try and help another person - so please take my response in that context (inexperienced and learning).

    It might be helpful to share what climate zone you are located in. If you arent familiar with climate zones, I just learned about them here:

    Based on my understanding you are smart to take out the exterior foam IF you want to put foam on the interior side of the sheathing, as you do not want foam on both sides of the sheathing as it would trap moisture (Im sure you already know that based on your plans). It sounds like you want to replace 1" of exterior foam, which if the image I have in my head of your home is correct should be helping prevent thermal bridging, with insulation (foam and fiberglass) installed between the beams using the cut and cobble method. Im not skilled enough to do a full wall R analysis on both set ups but it might be worth seeing if, based on your climate zone, the 1" of foam is enough to keep the sheathing warm if you add a few inches of fiberglass on the inside of the sheathing - this might give you a warmer set up overall than removing the exterior foam, if its continuous, and replacing it with cut and cobbled pieces which would re-introduce thermal bridges in your set up. You would probably want to add air barriers on both sides of the fiberglass, which if you keep the exterior foam should not be poly but could be taped seams on your sheathing and drywall on the interior. You can make that determination using the information in this article:

    I hope that is helpful - if nothing else it helps me think through what Ive learned so far on this site!

  3. AFriesen11 | | #3

    I have attached a pdf for more clarity. the roof is a 4/12 and is in climate zone 5 central Oregon. I would install a ridge vent and some sort of venting at the facia. all my roofing membranes would be above the air space. I know spray foam would be the easiest option, but I am trying to stay away from it due to costs. from what I understand 1" of exterior insulation wont get my sheathing out of the dew point and therefore is a risk, without any clear path of drying. Are there any other assemblies that would work?

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