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Community and Q&A

Zip R Sheathing for Cold Climate

Green_Geek | Posted in General Questions on

Good afternoon,

Does Zip R12.6 work in the winter for zone 5A?

I’m located just outside of Des Moines Iowa and want to choose an exterior insulation that will make my home more comfortable and reduce energy costs. As I understand, Zip R12.6 uses polyiso foam, and polyiso doesn’t perform well when temps fall below 50 degrees Fahrenheit.

Winter in Iowa
4 months of low temps ranging from freezing (32F) to negative temperatures of -5F.


Summer in Iowa

3-4 months of high humidity and high temps that can range from 75F to 105F.

 

I’m Intrigued by Zip R12.6, because the existing sheathing is mostly 1/2″ fiberboard. Zip sheathing would make it easy to install 1x4s  for a rain screen, and then install siding.

Context
Insurance is replacing our siding due to hail damage, and I thought this would be an excellent time to add exterior insulation like Zip R12.6 or GPS Foam.

Other house details

Framing: 2×4

Insulation R13 Fiberglass batts

Sheathing: Fiberboard (asphalt impregnated) and some OSB.

Foundation: Partially Exposed Poured Concrete

Thank you,

Ryan

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Replies

  1. Jon R | | #1

    > want to choose an exterior insulation

    Note that Zip-R isn't exterior insulation - - the OSB portion of the Zip will be cold and so more subject to moisture problems than if it were warm.

    Higher exterior perms are helpful - so consider taped plywood (if sheathing is being replaced), WRB, unfaced EPS or GPS. Furring strips can be attached through to studs.

    In any case, make sure the final result is compliant with the IRC here:

    https://www.continuousinsulation.org/content/2021-ibc-and-irc-adopt-improved-vapor-retarder-requirements

    1. John Clark | | #14

      deleted

  2. Walter Ahlgrim | | #2

    Jon in most homes I would agree Zip+R is interior insulation but not if like in this case the poster will have a layer of sheeting inboard of the Zip+R makes it exterior insulation.

    “I’m Intrigued by Zip R12.6, because the existing sheathing is mostly 1/2″ fiberboard.”

    Walta

    1. Expert Member
      Malcolm Taylor | | #3

      Walta,

      I'm not following you. if you install Zip-R you have a layer of sheathing outside the rigid insulation. What difference does it make that there is another layer somewhere else in the wall?

      1. Walter Ahlgrim | | #7

        The way I see it if the original poster keeps the existing sheathing and adds Zip+R on the outside of the existing sheathing he will need to do a dew point calculation to prevent rot.

        To me his wall looks more like this drawing with the strapping being replaced with the Zips OSB.

        https://d4c5gb8slvq7w.cloudfront.net/eyJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjg1MH19LCJidWNrZXQiOiJncmVlbmJ1aWxkaW5nYWR2aXNvci5zMy50YXVudG9uY2xvdWQuY29tIiwia2V5IjoiYXBwXC91cGxvYWRzXC8yMDE4XC8wOFwvMDgwNDM0NTBcL1JpZ2lkIGZvYW0gd2FsbCBzaGVhdGhpbmcgLSBGSEIgLSBjcm9wcGVkLW1haW4tNzAweDg1OC5qcGcifQ==

        There is a risk that the inner sheathing becomes a condensing surface and become wet over time.

        Walta

        1. Expert Member
          Malcolm Taylor | | #8

          Walta,

          Ah - got it. So the protection of both layes of sheathing need to be thought through.

        2. Jon R | | #9

          > strapping being replaced with the Zips OSB

          There is a fundamental difference in wetting and drying between intermittent and high perm exterior strapping and continuous, low perm exterior sheathing (like the OSB portion of the Zip in this case).

  3. JWolfe1 | | #4

    Before going too far make sure obtaining Zip R12 is even remotely possible in your area with the timeline you have in mind. I’d also check the price.

  4. Green_Geek | | #5

    To clarify, I plan to add a layer of insulation over the exiting sheathing. I was looking into GPS as exterior insulation, but wonder if Zip R12.6 might offer a better solution if it is not significantly impacted by the cold. My main question is whether or not Zip R12.6 would perform well in winter climates.

    I figured that Zip might cost less in labor to install. The current sheathing doesn't have any WRB.

    Thanks again for sharing your thoughts.

    1. Expert Member
      Malcolm Taylor | | #6

      Green Geek,

      Jon has given you good advice. The concerns you should address before deciding to go ahead are:
      - Making sure the sheathing on your Zip-R has a path to dry.
      - Making sure the wall assembly is more permeable on the outside than the interior so that drying exceeds wetting.
      - Ensuring it conforms to the code requirements he linked to.

  5. Johngfc | | #10

    To get to the question of insulation - yes, the polyiso will still insulate at cold temperatures, but likely not at the full R6 value. See: https://www.greenbuildingadvisor.com/article/cold-weather-performance-of-polyisocyanurate
    The second graph in that review shows that at 0 deg F, the R varied from ~4.5 t0 6.1 per inch for polyiso from a variety of sources. And keep in mind that only the outside 1/2"? or 1"? will be at the coldest temperatures - there will be a temperature gradient. So yes, using insulated Zip should dramatically increase insulation from what you have.

  6. Green_Geek | | #11

    Thanks everyone.

    I called Huber and asked what zones Zip R12.6 could be used in, and they said it can be installed in climates as cold as Zone 7.

    I also asked about polyiso thermal drift, and they said it was neutral compared to other foams; given that polyiso offers the highest R valuer per half inch.

    Thanks again,

    1. DCContrarian | | #12

      The key things to keep in mind are: 1. While polyiso loses insulating power at colder temperatures, it still keeps some. 2. Only the outside of the polyiso is at the outdoor temperature, the rest is somewhere between the outdoor and indoor temperatures and doesn't suffer as much loss in insulating power.

      You do have to heed what the other posters are saying about vapor management. Usually houses in cold climates are built with a vapor barrier on the interior side of the exterior walls, and are vapor-open on the exterior side. Any vapor that gets into the wall can dry to the exterior side, and the heat flow from warm to cold through the wall will tend to drive moisture in that direction.

      Polyiso foam has very low vapor permeance, so you'll be creating a wall that has vapor barriers on both sides (assuming your existing walls have vapor barriers on the interior). This can lead to mold and rot.

      1. Green_Geek | | #13

        Thanks again everyone.

        At this point I'm planning to go with 2" of GPS foam. Should have Net Perms of 0.72 with factoring for cavity insulation, framing, sheathing, WRB, 2" GPS foam board, and siding.

        Best,

        1. Charlie Sullivan | | #16

          That sounds great. Good decision.

  7. John Clark | | #15

    deleted

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