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Does Rockwool Board Follow The Same Exterior Rigid Insulation % Requirement?

D_Hallowell | Posted in General Questions on

If part of the current building science minimum exterior foam thickness requirements for different wall thicknesses and climates is partially determined by the inability of walls to dry to the outside due to the low permeability of foam, would the percentage that needs to be observed change if the exterior insulation”foam” is rockwool board that has a higher outward drying potential?

I understand that the recommended percentage values are based on increasing the temperature of the sheathing, but from what I’ve read it seems that the percentage is also partially based on the inability of the walls to dry to the exterior due to the exterior foam layer.

Perhaps the prescriptive values are not meant to be taken into account when using exterior rockwool board. Rockwool is not a “foam” afterall. It seems like this could use some clarification since rockwool board is often used interchangeably with exterior rigid foam board.

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  1. Expert Member


    Martin's advice is that the ratios only apply to foam for the reasons you suggest. I do remember Ben Bogie saying he still likes to stick to the ratios when using mineral wool boards because he feels they also impede drying to some extent, but have no idea whether that is based on anything concrete, or is just a preference.

    1. D_Hallowell | | #2

      Thanks for the reply Malcolm.

      1. Expert Member
        MALCOLM TAYLOR | | #3


        I finally found Martin's quote:

        "Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement."

  2. bpontolilo | | #4

    My understanding is that there are at least two good reasons to use continuous exterior insulation.

    One is because it is continuous and mitigates thermal bridging. That's why the current International Residential Code (IRC) now gives options for insulating walls with different R-values for cavity insulation alone, continuous insulation alone, or a combination of the two. When there is continuous exterior insulation the total required R-value is less than when there is cavity insulation alone. This is in the part of the code on energy efficiency.

    The other reason I am thinking of is because it is on the exterior of the building where it can improve durability by bringing the framing inside the thermal envelope. The idea is to keep the sheathing warm and reduce the risk of a potentially cold, condensing surface. This is where the ratios come in (in the IRC they are given as prescribed R-values for 2x4 and 2x6 walls in different climate zones, not ratios).

    These R-values are given in the section of the code on vapor retarders. There are two tables, and I believe that the code only refers to "continuous insulation" and doesn't distinguish between types. I guess they assume that low-perm rigid foam is the most common type of continuous exterior insulation. The purpose of the tables is to determine what class of vapor retarder to pair with a prescribed R-value of continuous exterior insulation. The code is trying to ensure two things: minimized risk of wetting while maintaining drying potential of the assembly.

    So, a vapor-open exterior insulation doesn't necessarily fit into the scenarios outlined in the IRC. But if a builder is going to install continuous exterior insulation, why wouldn't they want to add enough to reduce the risk of wetting by condensation? In this way, a wall with enough vapor-open exterior continuous insulation, and a class III interior vapor retarder seems pretty hard to beat. You've done everything you can to reduce wetting and the wall can dry in both directions.

    1. Expert Member
      Michael Maines | | #6

      Brian, it's nice to see you here! The IRC table that provides guidance for keeping the sheathing above the dewpoint temperature is R702.7(3): It is specifically to determine when it's safe to eliminate an interior vapor-retarding membrane, relying on painted drywall alone to slow vapor movement. But many of us use it as a proxy for determining when an assembly has enough R-value on the exterior to be safe from accumulating vapor on the interior.

      If you use a class 1 or 2 vapor retarder, including variable-permeance (aka "responsive") membranes, you only need to meet the energy-related requirements of table R402.1.2: But the higher the ratio of exterior-to-interior R-value, the safer the assembly.

    2. Expert Member
      MALCOLM TAYLOR | | #7


      I think people use below ratio thicknesses of any type of exterior insulation to practical reasons. Builders in Eastern Canada usually use 1" of exterior foam because it allows cladding to be installed without furring strips, and makes the detailing around penetrations and openings much simpler to detail. These walls are definitely not as resilient as ones with thicker foam, but seem to be doing fine.

  3. Expert Member
    BILL WICHERS | | #5

    I would say that vapor open exterior side insulation allows for drying to the exterior, which helps to deal with any moisture that makes it into the wall. If you keep to the ratios though, you limit how much moisture will get into the wall in the first place. The primary concern either way is to keep the exterior sheathing from getting wet and having mold/rot issues. If you stick to the ratios and use comfort board, you get the best of both: warmer sheathing, so less moisture getting in, and more ability for any moisture that does get in to find a way out.

    I would still stick to the ratios though to keep the sheathing from getting wet in the first place, and because the whole point is to add continuous R value to the wall. The more the merrier for R value in this case.


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