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Community and Q&A

2 inch Roxul Comfortboard on interior of cathedral ceiling

ar_lilly | Posted in Energy Efficiency and Durability on

Hello Everyone,

We are building a small house here in North Idaho (CZ5) with a cathedral ceiling framed with 11 7/8″ I-joists. The roof is vented and 3:12 single pitch. The roof/ceiling assembly is as follows from outside-in:
*Standing seam metal roof
*Solitex UM underlayment with embedded 3-D mesh (vapor open, for drainage/drying of backside condensation)
*5/8″ CDX plywood sheathing
*1 1/2″ site-built ventilation baffles (between top flanges of I-joists)
*1″ XPS attached to bottom of top flange of I-joists (to create bottom of baffle)
* R-30 (7 1/4″) Roxul comfortbatt in rafter bays
* R-15 (3.5″) Roxul comforbatt slightly compressed to fit in remaining depth of rafter bay
* Intello Plus smart vapor retarder/interior air barrier membrane from ProClima attached to bottom flange of I-Joist.
*** Proposed 2″ rigid insulation to address thermal bridging
* 2x furring
*1.5″ service cavity
* Plywood ceiling

This assembly gets us to +/- R-50 in the ceiling while staying vapor permeable (though I know there’s a slight reduction if we compress the batts). My question is: We are looking to address the thermal bridging through the I-joists (and pick up another R-8 or so) and are considering using 2″ Roxul comfortboard on the interior here. We’d like to avoid using foam for IAQ and vapor concerns. Does anyone know whether the comfortboard would be appropriate here? The Roxul application guide shows using comfort board on the interior of basement walls, but not anywhere else on the interior. 

Thanks in advance for your responses!

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  1. Expert Member
    Dana Dorsett | | #1

    It's fine to use in that application.

    Since rigid rock wool is considerably more compressible than rigid foam you may have to adjust the screws on whatever layer is retaining it to achieve a flat ceiling.

  2. Expert Member
    Akos | | #2

    Because the webs of I joists are thin, they are not much of a thermal bridge. Generally it is not worth bothering with it.

    You are in warm enough climate, that anything much above R30 is not worth it. The cost of the 2" comfort board VS bare TJI would never pay in energy savings in your climate.

    If you want higher R value roof, I would strap out the TJI and go with R23 batts instead of the R15. It would be way cheaper and much easier than getting a ceiling flat over 2" rigid mineral wool. This would get you an R47 roof vs R49 with rigid mineral wool.

  3. ar_lilly | | #3

    Thanks for the responses. I hadn’t considered the difficulty in achieving a flat ceiling. Consider me sufficiently scared off the idea for now. Also good to know about the joist webs not constituting a significant thermal bridge. Akos, were it not for the fact that we already have the R-15 batts from another project, I would totally take your suggestion of strapping out the joists and using thicker batts. Thanks for the advice, as always!

    1. Expert Member
      Akos | | #4

      If my math adds up correctly you are compressing the batts a bit. What you can do is install rigid foam strips along the bottom flange to increase the depth so you are not compressing the batts. This would get you a bit of R value bump without too much cost or work. You can hang drywall directly through rigid foam strips with longer drywall screws.

      If you don't want to use foam, plywood strips would work as well with a slight reduction in R value.

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