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Community and Q&A

Foam Rockwool Equivalence

Kevin Erickson | Posted in Energy Efficiency and Durability on

This chart addresses foam R-values. Can Rockwool be substituted for foam in this chart?

All you need to know

Here is the essential information from Table N1102.5.1 that applies to foam-sheathed walls:

    Climate Zone     Minimum R-Value of Foam Sheathing
  Marine Zone 4   R-2.5 for 2×4 walls; R-3.75 for 2×6 walls
  Zone 5   R-5 for 2×4 walls; R-7.5 for 2×6 walls
  Zone 6   R-7.5 for 2×4 walls; R-11.25 for 2×6 walls
  Zones 7 and 8   R-10 for 2×4 walls; R-15 for 2×6 walls

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Yes, if you use Comfortboard, which is the rigid version of the Rockwool batts. What really matters is the absolute R value in that chart though, not so much the material that is providing that R value. I suppose you could even use regular batts if you could figure out a way to get them to stay put on the exterior.

    Bill

  2. Kevin Erickson | | #2

    Thank you Bill! So, 3 inches of Rockwool (r 4 per inch) will meet rqmt. Right?

    1. Patrick OSullivan | | #3

      We don't know where you're building and/or what the local code is. :-)

  3. Kevin Erickson | | #4

    Zone 6 - 2 x 6

  4. GBA Editor
    Martin Holladay | | #5

    Kevin,
    If you are talking about a continuous layer of semi-rigid mineral wool installed on the exterior side of your wall sheathing, there is no minimum thickness or minimum R-value. Mineral wool insulation is vapor-permeable, so this type of wall dries quickly to the exterior (unlike walls with a continuous layer of exterior rigid foam).

    So you can use any thickness of mineral wool you want -- a little or a lot.

    1. Drew Baden | | #6

      Martin,
      I thought the idea was to have a certain insulation r-value inside:outside ratio based on climate zone to keep one's sheathing warm enough to prevent condensation on the inside of the sheathing?

      1. GBA Editor
        Martin Holladay | | #8

        Drew,
        No. The ratio rules were invented to prevent moisture problems in walls that can't dry to the exterior. The problem arises with continuous layers of exterior rigid foam, because the foam is so impermeable to water vapor that it limits outward drying.

        No such problem occurs with mineral wool, so there are no ratio rules for exterior mineral wool.

      2. Expert Member
        Zephyr7 | | #9

        To add to what Martin is saying, remember that the code allows for NO exterior rigid foam, as long as you meet the minimum R value requirements for the wall itself. Obviously with nothing on the exterior of the wall, you have a lot of drying potential.

        The general rule has been that more exterior continous insulation is always better, since more continous insulation means warmer sheathing, and warmer sheathing means drier sheathing. With exterior mineral wool as your continous insulation, which is very vapor open, you have both continous insulation AND relatively unimpeded drying potential to the exterior, which is a win-win. The downside is that Comfortboard is expensive as continous insulation materials go.

        I'd still shoot for at least about R8-R10 worth of exterior continous mineral wool in your climate zone though, just for good energy efficiency. I'm a big fan of exterior continous insulation. What you might consider doing is picking a thickness of Comfortboard that will be easy for you to work with when you trim out the exterior. 1.5" might be good minimum here, since it would be easy to create "hard points" with framing lumber, but that's only about R6.3 or so. 2" thick would give you about R8.4.

        Bill

  5. Deleted | | #7

    Deleted

  6. Kevin Erickson | | #10

    Thank you gentlemen! I appreciate your help!
    Being close to the ratio will reduce the risk of condensation overall - correct?

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