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Vaulted ceiling insulation

diy_Nate | Posted in Energy Efficiency and Durability on

Hi all.  Nate here. 
I’ll describe my situation then post my question at the end. 
I have an empty addition room that I am turning into our kitchen. It orginially had flat ceiling, but we don’t want that and are turning it into a vaulted ceiling.  2×6 roof rafters were used. I want to get near R30 insulation.   I plan to combine (R6) 1″ Rmax polyiso & (R15) 3.5″ Roxul (R21) in the cavity,  and give it a 1″ air gap between the sheathing and polyiso. On the inside of the rafters I plan to use additional whole sheets of (R6) 1″ Rmax polyiso to cover the entire interior of the ceiling. That should put me around R27.  I am going to use ship lap to cover the interior of the ceiling. 
       My question is.
 Do I need to have a slight air gap between the back side of the shiplap & polyiso boards or can they be in direct contact with each other?
  I thought about using 1/4″ sill gasket if a small gap is needed.  
Thanks for your time & any info given.
Have a good one.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Nate,

    I'm sure you will get some feedback on your proposed assembly, I'm just going to answer your question the best I can right now. I can't see why you would need an air space between the rigid foam and the paneling, but you may need a thermal barrier (drywall) over rigid foam since this is in a living space. Check out section R316 of the International Residential Code (link below) and check with your building inspector.

  2. Jon_R | | #2

    A good rule to follow in cold climates is 5x more perms to the exterior than the interior. Allow your 1" vent to function with something like EPS or plywood below it. Always avoid moisture traps and air seal well.

    Hopefully the ship lap wood will serve as your thermal barrier?

    1. Expert Member
      Dana Dorsett | | #3

      >"Hopefully the ship lap wood will serve as your thermal barrier."

      Normally plank layers wouldn't meet code. Even if that ship lap is 2" thick the seams leak air/flame, as do any loose knots, splits, etc.

      Sheets of 3/4" plywood or OSB would qualify (but not half-inch), as would a layer of half-inch sheet rock between the ship-lap and foam.

      1. GBA Editor
        Brian Pontolilo | | #4

        Hi Dana.

        The IRC language reads "...not less than 1/2 inch (12.7 mm) gypsum wallboard, 23/32 inch (18.2 mm) wood structural panel or a material that is tested..." I assume that plywood and OSB are "wood structural panels" as you suggest.

  3. diy_Nate | | #5

    Thanks for all the feed back. I read the IRC, I was hoping that the Ship Lap would count has the thermal bearier, it would be atleast 3/4". I may shift gears and just hang drywall or find a dealer and use Roxul ComfortBoard instead of the Rmax polyiso. If I did use drywall as the barrier for the ship Lap, would I have to mud the seams to seal the gaps of the drywall? Cuz I don't really want to drywall AND ship lap. Any other ideas on insulating 2x6 rafters for a vaulted ceiling?
    Again. Thanks for the info.

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