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What insulation works best with Zip system R-12

flatbrook | Posted in Energy Efficiency and Durability on

When asking experts what insulation works with Hubers zipsystem R-12, we are getting different answers.
The framing is made with 2×8 with zip.
Do we use closed cell, open cell or mineral wool? We are in zone 5.

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  1. Expert Member


    There are many aspects of that question that probably need dissecting first.

    1. Can you tell us what the full wall assembly looks like? sans cavity insulation is fine.
    2. What elevation or zone, (A,B,C) you're in?
    3. general house design?

    After that we'll be able to break down the topic for better discussion.

    1. flatbrook | | #3

      Hello Kyle,
      Wall assembly consists of Concrete siding, 2.5" R-12 zipsystem,2x8 wood framing, sheetrock
      Zone 5, upstate NY
      Gable wall, long house style 24' x 60', with walk out basement, attic is framed with attic trusses and is conditioned within the envelope
      Heat pump ducted, with solar panels and ERV system.
      Thank you

  2. Expert Member
    Michael Maines | | #2

    Are you using the Zip R-12 as wall bracing as well as insulation? Are you using nails that are at least 3 3/4" long?

    Will you have humidity control?

    1. flatbrook | | #4

      Yes to all of your questions.

      1. Expert Member
        Michael Maines | | #5

        There is virtually no benefit to using any sprayed foam or even a moderately high-embodied-carbon material like mineral wool in your assembly. I'd use dense-packed cellulose if available, or fiberglass batts as a second choice. With either of those options you only need drywall with regular latex paint at the interior for vapor control.

        Open cell foam has the same R-value as cellulose or fiberglass. Closed-cell foam has higher R per inch but the slight gain in total R-value would have a very long return on investment, and its negative environmental impact would never be recovered.

        Be sure to tape the seams and edges of the Zip for airtightness and roll the tape for proper performance. I would also use a rain screen gap of some sort.

        1. flatbrook | | #6

          Thank you for the answer, I have read that dense packed cellulose can loose it's volume over time?
          I have another question about the roof, that has the zip system for the roof and attic trusses, the attic will be conditioned. Cellulose there as well?

          1. Expert Member
            Akos | | #8

            Can you sketch what you think your roof assembly will look like?

          2. Expert Member
            Michael Maines | | #9

            Cellulose that is properly dense-packed and that doesn't become saturated won't settle. It's loose density is about 1.8 pcf and it should be installed at 3.5 to 4.0 pcf, though some installers skimp in the interest of easier installation.

            Like all air-permeable insulation, cellulose is not safe to use in an unvented roof assembly, but it works great in a vented roof--I use it on most projects, including ones with vaulted ceilings. Or, like your walls, you can do a hybrid approach, using enough foam on the exterior to provide dewpoint control. From a building science point of view that's an ideal assembly, for for cost, practicality and carbon emissions, it's better to do a vented roof.

        2. maxwell_mcgee | | #7

          Given the 2x8 insulated wall cavity (~R26 or so?), isn't there a significant risk that the Zip gets cold in the nasty New York winters and that vapor condenses on the inner face of the Zip poly-iso?

          And if so, wouldn't a hygroscopic material like cellulose be much better than fiberglass which tends to clump and slump (and grow mold) when it gets wet?

          Also, my understanding is that you'd want a smart vapour retarder on the inside to limit moisture getting to the cold sheathing in the first place (but a smart retarder which will open up to allow moisture to escape the cavity if it does get wet due to condensation). You're saying drywall + latex paint alone should suffice. Why?

          1. Expert Member
            Akos | | #10

            Take a look at the table here:


            Exterior R12 is good for condensation control for up to R28 of fluffy in zone 5 with only painted drywall as the interior vapor retarder.

            In climates colder than Zone 5, a Class 2 vapor retarder would be needed.

          2. Expert Member
            Michael Maines | | #11

            In climate zone 5, walls need roughly 27% of the R-value on the exterior to provide dewpoint control. Because the issue is long-term moisture accumulation, I use the aged value for polyiso, about R-5.5/in. 7 1/4" of cellulose or fiberglass is around R-26, for a whole-wall nominal R-value of about R-37, with 30% of that on the exterior. In colder climate zones it would not be a safe ratio.

            Edit: the ratios at the end of the BSC document that Akos shared are a little different than mine, which I derived from the 2015 IRC. I would err on the side of caution; the BSC article says 30% is safe. Either way, you just make the cutoff but it would be safer to add a variable permeance membrane at the interior.

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