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Community and Q&A

Vapor Barrier with Rockwool and Zip Sheathing

mclola | Posted in General Questions on


We are replacing the siding and plywood on the exterior of our home (rotting due to moisture intrusion in some areas), and want to replace all of the remaining fiberglass insulation on the exterior walls with Rockwool mineral wool batts surrounded by exterior Zip insulated sheathing.    (R6 Zip and R15 mineral wool)

Do we need an interior vapor barrier if using this in zone 5 (Syracuse, NY)?

I am hoping we don’t have to tear down all of the exterior wall drywall to re-insulate the home if an interior vapor barrier is needed ….  (Spray foam is not an option due to chemical sensitivities in the home) – and would appreciate any thoughts.


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  1. Robert Opaluch | | #1

    No. A vapor barrier (e.g., poly sheeting) is NOT recommended behind drywall for your remodeling work. Although it would prevent water vapor from entering your wall in the winter, sometimes condensing inside your wall, it also would prevent water vapor from escaping back into the interior. You don't want moisture trapped in your wall, and Zip-R sheathing isn't ideal for allowing water vapor to migrate out to the exterior, either, especially with no rain screen planned.

    A rain screen would be recommended. Vertical (or horizontal) strapping every 16" (or 24") depending upon your choice of cladding, or a rainscreen product that puts a thin air gap between your cladding and your Zip sheathing. Whatever water gets behind your cladding will drain down and out. Helps insure your sheathing stays dry.

    Good that you are upgrading your insulation from about R-19 to R-29, reducing your completed wall's heat loss by about 30%. Others here may recommend you use Zip, then add exterior insulation greater than R-6 for Syracuse's cold climate. Personally, that's what I would do but it adds cost and complications that your contractor may not want to do if they have not done that work previously. R-21 isn't a lot of insulation for your very cold winter climate, but certainly a big improvement over your current level of insulation!

    1. mclola | | #2

      Hi Robert, thanks so much for this information, it makes a lot of sense as I try to learn more about this aspect of our home. I am a novice and jumping in due to necessity....

      This project has become complicated (at least to me). It started out as a project to replace the cedar siding and turned into one of restoration for multiple areas of water damaged plywood with entrance points for various vermin to destroy the existing fiberglass insulation. Some of the water-damaged areas make sense to me (around windows and areas of poorly flashed roof/siding junctions) - however the water damage in the middle of the home likely occurred from rain/wind, I think - and I would like to frame the home in a way that this is less likely to occur in the future - (Tyvak and asphalt/felt used before, I don't think a rainscreen was in place).

      The cladding we are thinking of using to replace the cedar boards is called LP SmartSide - if you are familiar with this which type of rainscreen do you think would work best?

      Also, if I understand correctly, some may recommend using the uninsulated Zip boards surrounded by exterior insulation boards ----- would mineral wool boards work best here, or are there other choices that you might recommend? OR, is there something different that might work better?

      If you (or others) were to design your ideal wall structure for this zone 5 property working from the outside in (not removing all of the interior drywall) using any products, what would you choose?

      So many questions (sorry!!) -- I truly appreciate your time and expertise!


      1. Robert Opaluch | | #3

        Hi Molly,

        Deteriorated siding, critter damage, and water intrusion around windows likely accounted for all of your water-damaged sheathing and insulation. However, there are other potential sources of moisture getting to your sheathing, that might be a possible problem after your repairs.

        One question some of us may have, is about possible water leaks through the roof, not just around windows or deteriorated siding. You didn’t mention anything about the conditions of your roof, so we’ll assume your roof is in good condition and no ice dams have formed on your roof. (Ice frozen on the roof above your exterior wall, due to snow melting on your roof then refreezing nearer the edge of your roof, creating a “dam” that can hold water on your roof.). Water sometimes can leak in, and move through complicated paths to end up in unexpected places. Hopefully no problem in your case.

        Some of this information described below, you probably already know, but I’ll describe just in case you haven’t considered some of these potential problems and solutions. Sorry if you already know this, or its too detailed or confusing.

        Deteriorated wall sheathing could be due partly to condensation of water vapor that escaped from your home’s interior, through small holes and cracks in your walls, then condensed on your sheathing. Usually, this moisture would evaporate later as temps warm up. But it’s a possible cause of wood sheathing rot. The solution is to make the walls more airtight. When you install Zip-R or other replacement sheathing or exterior insulation, the sheathing edges should be taped, and window edges should be sealed airtight to the surrounding sheathing. If minimal air migrates through your walls, there will be little or no condensation. So an airtight building enclosure is important as well as additional insulation. Airtight walls will reduce interior warm air escaping to the outdoors, reduce cold winter air leaking into your home interior, and will reduce any potential condensation of water vapor inside your walls during winter months. Zip-R (or other) sheathing provides a potential air barrier if the edges of the sheathing create an air and water seal by taping to any adjacent Zip-R, window, door, or foundation edge. That would create an excellent air barrier at the sheathing, to prevent water and air infiltration. There are many products to do this, but Zip tape is a great product (except needing additional products for attaching to concrete foundations). Here’s a short video illustrating what this Zip sheathing and taping looks like:

        Windows have been a problem for you and there’s lots of articles on GBA that show details of how to (install and) seal windows to create a robust water and air barrier. Since you’ve already had problems with water (and air) leakage at windows, please be sure to follow those recommendations carefully. For examples, you can search GBA or other instructional videos on YouTube like these:

        The advantages of Zip-R is that in addition to adding insulation R-value, it will save labor time vs. using plain Zip sheathing plus adding exterior insulation. So unless you will add more than R-6 exterior insulation, you might as well choose Zip-R6. Zip-R doesn’t provide as much racking resistance as having the plain Zip sheathing directly nailed to the wood frame stud walls, but unless your home were in an area subject to very high winds, earthquakes etc. it shouldn’t matter much.

        Not sure what you (or your contractor) prefer to do. If you consider exterior insulation instead of Zip-R, a good choice is polyiso board insulation, which is about R-6 per inch (depends upon manufacturer). Foam board insulation is about R-4 per inch (EPS, XPS). XPS foam is avoided due to the blowing agent with high global warming potential escaping from the foam over time. GPS is graphite infused EPS with R-5/inch R-value. All these choices are available in various sizes from ½” to 2”. Polyiso also chars in place when burned, unlike other foams which could melt. Zip-R is polyiso attached to the interior of Zip sheathing (so it’s a good and convenient choice).

        You could consider one or two layers of 1.5” or 2” polyiso, with the seams of each layer offset, and use Zip or other good quality sheathing tape to seal the sheets to each other, just like with Zip. Polyiso 4” would give you R-24 approximately. Plus R-23 nominal Rockwool batts gives you an extremely well insulated wall. Two layers of 1.5” polyiso would yield R-12. However, your contractor may balk at installing exterior insulation if they have not done it previously.

        Regardless of the Zip-R vs. exterior insulation choice, you should install a rain screen between your Zip-R or exterior insulation and your cladding/siding. Any water that gets past your new siding will drain down and out, and the rain screen airspace will speed drying. Typically this rain screen is constructed with 1x4 or 1x3 vertical wood or similar size plywood battens, attached every 16” to wall studs, held in place by screws that go through the battens, the exterior insulation, and into your existing wall framing (studs and top & bottom plates). Once the battens are installed, your siding is attached to the battens or attached through the assembly to the existing wall framing.

  2. mclola | | #4

    Hi Robert,

    Again, I appreciate your detailed and thoughtful response - and have learned a lot of new information from you. (and thanks for the attached links)

    Our contractor isn't familiar with exterior continuous insulation, but has worked with the Zip products before, so this is probably the best choice in this situation.

    I am now realizing the benefits of exterior continuous insulation, and believe using more would be better -- and am thinking perhaps using the Zip R-9 or R-12 would help a bit in this regard.

    Now that I understand the benefits of rain screens and a drainage plane/ventilation behind the siding - several questions/thoughts come to mind:
    using the batten method with 1 x 4 or 1 x 3 vertical wood/plywood - how can we prevent hornets/wasps etc. from creating nests in this space, and/or would use of one of the rainscreen products on the market work as well for purposes of drainage/ventilation -- and if so, are there any that you (or others) might suggest?

    Thanks again for sharing your expertise!

  3. Robert Opaluch | | #5

    At the bottom of the rain screen, there can be perforated metal stock or screening products to block access to the rain screen cavity. Water and water vapor can pass down and out, but mice, hornets/wasps could not enter. Options include:
    -- perforated metal J-channel stock
    -- a combination of screen stock plus rigid metal mesh
    -- cor-a-vent products

    There are also special sheet products that can be used that function like a rain screen. Benjamin Obdyke is a good example:
    So instead of a rain screen as described, this mat is attached to the wall, then covered by your cladding. It creates a small gap that works to drain moisture.

    See Cor-a-vent products and rain screen articles or questions on this site for more ideas, or maybe others will suggest their best ideas on your question here.

    Some examples:

    Malcolm Taylor mentioned he has a supplier create perforated stainless steel J-channel stock to his specifications.

  4. mclola | | #6

    Thank you for this additional information - I will pass this on to my contractor.

    1. Robert Opaluch | | #7

      Best of luck with your project, your home should be more comfortable and energy efficient after your efforts. Please submit a new question if you or your contractor could use more information or ideas on some topic.

  5. mclola | | #8

    Thank you for all your help!

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