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Zip R-sheathing minimum R-value

G_Achilles | Posted in General Questions on

Is zip r 3 with 2×6 frame insulated with rock wool a slippery slope or half inch of foam for a continuous thermal break is better than no exterior insulation!?

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Replies

  1. Expert Member
    Peter Engle | | #1

    Achilles,
    Please tell us where you are located. It makes a difference.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Achilles.

    The section on vapor retarder requirements in the International Residential Code is a good place to start.

    In that section, the code lessens requirements for interior vapor retarders based on a few details, one of which is continuous exterior insulation. That's because with sufficient continuous exterior insulation the sheathing stays warm and reduces the likelihood of condensation forming in the wall (and because that exterior continuous insulation is lowering the perms on the outside of the walls so inward drying becomes important).

    So, like Peter said, it depends on where you are, but here are the values listed in the IRC Table R702.7.1:

    Marine Zone 4: R-2.5 for 2×4 walls and R-3.75 for 2×6 walls
    Zone 5: R-5 for 2×4 walls and R-7.5 for 2×6 walls
    Zone 6: R-7.5 for 2×4 walls and R-11.25 for 2×6 walls
    Zones 7 and 8: R-10 for 2×4 walls and R-15 for 2×6 walls

    In the warmer climates not listed here, no minimum value is necessary and of course, these are minimums and you can always be cautious and increase these R-values. This is how at least one Huber rep suggests builders proceed when choosing the right R-value for ZIP R-sheathing for projects, as well.

    You may find these articles helpful: Working with ZIP R-sheathing and Walls that Work.

  3. Jon R | | #3

    Don't apply the "continuous insulation" portions of IRC Table R702.7.1 to Zip-R - because the insulation isn't "over" the wall (as required) and doesn't keep the wood portion of the sheathing warm (it actually makes it colder).

    When it comes to moisture damage, be conservative - don't a) build right to code minimums or b) violate the exact wording of code.

    Good advice is here: https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers#Side_02

    In most of the US, a smart vapor retarder is a good way to be conservative.

    1. GBA Editor
      Brian Pontolilo | | #4

      Hi Jon.

      Achilles' walls will wet and dry differently whether he decides to go with continuous exterior insulation over sheathing or with ZIP R. No doubt. Fine tuning interior vapor control will be important and climate dependent.

      But to answer his question about choosing the right R-value for ZIP R, I've talked to the folks at Huber, architect Steve Baczek, and some others and they all agree that the values listed in IRC Table R702.7.1 (in the section on vapor control) are a good place to start. Those R-values can always be increased.

      So, to be clear, I'm not suggesting following the vapor retarder requirements in that section of the code, just that it is a starting point for R-values for continuous exterior insulation and ZIP R, which was the original question. A vapor retarder or not can only be specified once the assembly type and R-value of insulation is determined.

    2. Matt McLagan | | #5

      If I understand correctly, warm sheathing is a way to help keep sheathing dry, but warmth is not a requirement for dry sheathing.
      Continuous insulation as with the Zip-R will still keep condensation from occurring on the interior side of the sheathing. Table R702.7.1 should then still be followed.

      1. GBA Editor
        Brian Pontolilo | | #6

        Hi Matt.

        See my reply to Jon above. I just added an additional paragraph for clarity.

        Adequate continuous exterior insulation keeps sheathing warm to prevent condensation on the sheathing, but that isn't the only way walls get wet. So, because many walls with continuous exterior insulation cannot dry to the exterior, it is important that they can dry inward. This is what Table R702.7.1 specs and allows for by apporoving the use of class III interior vapor retarders in climates that would otherwise require class I or II.

        ZIP R has the insulation inside the sheathing. It does not keep sheathing warm. And the polyiso insulation on the product is low perms so, if the OSB gets wet, it will need to dry outward (a rainscreen detail is important with ZIP R). Still, many people use the continuous insulation minimums in the vapor retarder section of the IRC to spec R-values for ZIP R.

        I believe that the coming IRC, 2021, which will be out this fall is going to be even more nuanced when it comes to vapor retarder requirements. Looking forward to reporting on that.

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