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Rigid foam plus mineral wool or fiberglass in cathedral ceiling?

rcke2 | Posted in Energy Efficiency and Durability on

I have 2×8’s for my cathedral ceiling and 4.5 rigid foam to install. Planning 5/8 drywall when done. Planning 1/2 inch gap above foam to roof as I do have ridge went…. so there is room left still. 

This is So Calif, near airport. Could I add 2 inch rock wool under the foam to block sound? Add fiberglass batt??  

Or seal the foam and be done? 

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  1. Expert Member
    AKOS TOTH | | #1

    Sounds like you are looking to do a cut and cobble foam install.

    Generally, this is a waste of money and effort as the higher R value of the foam is short circuited by the thermal bridging from the rafters.

    Since you can vent your roof, you are much better off with insulating with the correct thickness of high density fiberglass or mineral wool batts.

    Once taking the thermal bridging into account, either option would come out to pretty close to your foam cut and cobble and would be much simpler/cheaper to do.

    For noise, I would recommend installing the ceiling on resilient channel. This makes a big difference for higher frequency noise such as you would get from an airplane.

    Both for noise and thermal performance, make sure the ceiling is well sealed, the best is a thin bead of spray foam covered by a flex sealant (I like Green Glue's sealant product for this) before you mud and tape the drywall.

  2. Expert Member
    BILL WICHERS | | #2

    Mineral wool is a LITTLE better than fiberglass for sound, and is what I’d use myself. It is not the night and day difference you sometimes see advertised. I would go with Akos’s Suggestion of hanging the ceiling drywall on resilient channel which will make a BIG difference in sound transmission. You can do a double layer of 5/8” drywall for even better performance, but be sure the resilient channel you use is rated for that much since it’s a lot of weight.

    Build your walls the same way and you’ll drastically cut the airplane noise.

    Regarding the rigid foam, cut and cobble is not worth the effort, and can lead to moisture issues in unvented assemblies, so you do need to vent it. You need more than a 1/2” vent channel too —- code is 1” minimum and more is often recommended.


    1. rcke2 | | #3

      Thanks for the reply

      My city Code requires r-30 for attic and cathedral ceiling insulation. Seems that r-30 fiberglass is thicker than my 2x8 rafter bats allow without compressing it.

      Hence, figured foam makes them happy. And figured I have leftover r-9 fiberglass for 2x4 walls ... just didn’t know if it creates a problem somehow under the foam

      Mentioned the rock wool in case someone wanted to offer advice on that as an alternative to rock wool. It’s a small area (300 sq ft).

      1. Expert Member
        AKOS TOTH | | #4

        If you must have the R30, it is better to install 1" paper faced plyiso spaced away from the roof deck to create a vent space followed by R24 mineral wool batts. This is still less work than the all rigid install.

        You can also strap out your rafters by with 2x2 and install R30 mineral wool batts. These are 7 1/4 thick, with the strapping you would still have the proper vent gap. This is even less work than the above suggestion, but you do loose a very small amount of interior height.

        1. rcke2 | | #5

          thank you very much

          living near the coast, not too concerned about thermal efficiency of this portion of the house except I guess to avoid getting heat from shingled roof coming IN. Rarely use heater and dont need or even have A/C here

          Really just hoping to see responses that mention that it is fine to use combo of plyiso and mineral wool - so i am thinking that i will go with this and the resilient channel for drywall

          The conflicting requirement to the 2x2 strapping is to keep the ceiling as high as possible.

      2. Expert Member
        Dana Dorsett | | #6

        >"My city Code requires r-30 for attic and cathedral ceiling insulation. Seems that r-30 fiberglass is thicker than my 2x8 rafter bats allow without compressing it."

        R30 rock wool batts are designed to fit perfectly in 2x8 framing.

        In SoCal as little as R5 above the roof deck is sufficient for dew point control at the roof deck.

        A combination of an inch or two of foam above the roof deck and R30 rock wool in the rafter bays HAS to be cheaper than a foam + fiber cut'n'cobble between the rafters, and will substantially outperform it too.

        If it has to be all under the roof deck, in SoCal R30 rock wool and a layer of Intello Plus detailed on an air barrier on the interior side of the assembly will work pretty well. If it's possible to build in a diffusion vent it could even get away without the Intello:

  3. Expert Member


    Whatever insulation stray you settle on i would try and increase the depth of your ventilation channel. 1/2" is very difficult to maintain, and provides very little air movement to dissipate moist air.

  4. rcke2 | | #8

    thanks Dana - I will look into the trade offs between keeping my ridge vent and doing this intello plus product (honestly, not sure about it - never even heard it mentioned) ............and ask my roofer about taking off my ridge vent and replacing with this vapor diffusion vent

    thanks malcolm - i will go with at least 1 inch for the ventilation channel. Because of the way I hung the 2x8's off of the ridge beam and sistered the existing 2x4 rafters with them ...i actually do have a full 8 inches. But after 1 inch of air to roof deck, then 4.5 for the rigid foam, i would only have 2.5 for mineral wool batts

    1. Expert Member
      Dana Dorsett | | #10

      Don't be surprised if the roofer give you a blank stare when asking for a diffusion vent. You may need to do some education on the topic, and design the diffusion vent yourself. This is a pretty good quickie:

      As a "belt AND suspenders" approach, Intello + diffusion vent along with some prior education of the roofer & inspectors should be enough. I'm not sure what CA code has to say about diffusion vents, but under IRC 2018 it's a valid approach for US climate zones 1, 2, & 3, which covers pretty much all of SoCal below 5000' of elevation.

      What is your CA Title 24 climate zone?

      If you're the type who likes the details, this recent CA sponsored study includes a couple of different approaches to unvented roof assemblies:

  5. rcke2 | | #9

    i just learned that there is a 2.5 inch thick rock wool batt

    perhaps i am good to go

    1. Expert Member
      BILL WICHERS | | #13

      The thinner batts are often stocked as the "fire and sound" version. Same material, different thickness. If your inspector gives you some trouble because of the lack of a formal R value rating on those products, you can write to the manufacturer and they can send you technical info with a stated R value for the thinner sound absorbing variants of mineral wool.


  6. rcke2 | | #11


    i am in zone 6

    (btw - are you dana1 at the greenbuilding talk? saw a reply similar to my needs from 2017 there)

  7. Expert Member
    Dana Dorsett | | #12

    CA zone 6 crosses IECC zones 3B & 3C(marine zone) boundaries, but it's all zone 3. IRC 2018 allows diffusion venting in all zone 3A/B/C locations.

    And yes, I am that person on that other forum.

    1. rcke2 | | #15

      Thanks everyone. Much appreciated.

      Dana - I already have my 4.5 inch thick rigid foam on hand, and it was if I wish to avoid the roofer costs from a new diffusion vent ( just stay with my ridge vent) .... in your opinion am I good with air gap/ 4.5 in Rmax polyiso/2.5 in rock wool then drywall? (with resilient channel).

      Is the Intello a must in my case?

  8. frasca | | #14

    .."If you must have the R30, it is better to install 1" paper faced polyiso spaced away from the roof deck to create a vent space followed by R24 mineral wool batts."

    This is essentially site-built baffles, right? And the prevailing advice would be to cut these polyiso baffles about 1/2 narrower than the rafter bay and fill the 1/4" on either side with canned spray foam, presumably not expanding out to where it deformed the rockwool?

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