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Zip R Sheathing and Condensation

JWolfe1 | Posted in Building Code Questions on

Something that is endlessly confusing to me for climate zone 7 is that many insulation manufactures or general discussions about insulation cite that you need R-value 20+5 in a 2×6 wall to meet code in climate zone 7. However, there are also references (especially here on GBA) on needing to have at least R15 in exterior foam to prevent issues with condensation. What gives? R5 can easily be accomplished with just an inch of various products, but R15 almost always gets into much more expensive and complicated assemblies from both a material and labor perspective.

Is the condensation concern when having less than R15 foam only for when you want to use a class III vapor retarder like drywall with latex paint or is it across the board? Here on GBR there are you would think that it is a given that your house will rot and mold in a few years if you do anything less than what the IRC 2018 Chapter 7 calls for (R15). What about if you do a well detailed class II vapor retarder such as Certainteed’s Membrain with a R20+5 assembly?

https://codes.iccsafe.org/content/iecc2018/chapter-4-re-residential-energy-efficiency

https://codes.iccsafe.org/content/IRC2018/chapter-7-wall-covering

I am interested in using Huber Zip R12 for my upcoming build and reached out to Huber’s technical team to inquire about whether my proposed wall assembly will have issues. In fact I communicated with two people because these days it is hard to get anybody to respond to anything. Below are their answers:

Hi Huber,

Is ZIP R12 with 2×6 framing a viable option for climate zone 7 in Park County Colorado at 11,000 feet if a class II vapor retarder such as Certainteed’s membrain is used? https://www.certainteed.com/building-insulation/products/membrain/

I’d really like to be able to use your product, but don’t want moisture issues.

Planned wall assembly will be:

Hardiboard siding or similar
Zip R12
2×6
R23 Roxul mineral wool
Certainteed Membrain
1×6 shiplap.

https://www.huberwood.com/zip-system/insulated-r-sheathing

Thanks!

 Here was Huber’s response:

 Gents –

 I don’t see any issue with that assembly. The newest 2021 version of the International Residential Code will actually have more supplemental information on this design, but will basically suggest exactly what you’re doing. What you want to avoid is using a Class I vapor retarder (i.e. 6 mil poly) on the interior.

 Regards,

 XXXHuber’ employees name was removed XXX

 I asked the exact same question above to another person at Huber and received this basic response:

XXXXXPerson’s name removedXXXXX

I dont see any issues with the described wall assembly.

Sent from my iPhone

 If I follow what Huber says am I ok or being set up for trouble? I can’t imagine  that they would persistently lead people astray.

I have also considered the same wall assembly but using a product called Thermatight that has slightly lower R-value at R10. https://thermaltight.com/. I just came across this product and need to look into it more.

Thanks everyone.

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Replies

  1. Expert Member
    Akos | | #1

    Take a look at the Table 2B here:

    https://www.appliedbuildingtech.com/system/files/abtg_rr_1701-01_moisture_control_guidelines.pdf

    You can extrapolate based on your local HDD to see where exactly you fall in the ratio of needed exterior insulation VS interior vapor retarder.

    At least for walls, even in very cold climates you can usually get away with a smart VB on the inside and a relatively thin layer of somewhat permeable insulation (unfaced EPS/GPS, rigid mineral wool or permeable polyiso). Important part is always to air seal the walls and have decent warm side air barrier.

  2. JWolfe1 | | #2

    Thank you Akos. According to that document my potential plan iterations I'm currently considering will likely be quite safe given my HDD days.

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