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Community and Q&A

Zip-R and a vapor barrier?

Cheebs127 | Posted in Green Building Techniques on

Hello GBA community,

Our home build is closing in on the insulation phase and I’m looking for information to make sure we get it right. We have been building it all ourselves so far. We are also building in a small town (in Massachusetts) and many people who have visited the house that are in trades haven’t even seen zip-r. It seems pretty common for people to stick with what they know and not be too familiar with newer methods. I worry that our inspector will push non compatible building methods when he comes for our framing or insulation inspections.

Our house is a single story ranch. 2×6 framing with Zip R6. Our sill plate is  2×8 so it overhangs the foundation and the Zip R terminates into the PT to leave no foam exposed. No corners were cut and extra time has been spent making sure it is air and water tight.

I plan to use Rockwool batts for the walls and blown in cellulose in the attic. The basement slab  and interior walls are insulated. My question is where do I need a plastic vapor barrier?

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  1. Patrick OSullivan | | #1

    According to the IRC, you need a class II vapor retarder unless you have a vented cladding, in which case a class III vapor retarder (e.g. latex paint) is sufficient.

    If you used kraft-faced fiberglass batts instead of Rockwool, they would suffice.

  2. Jon R | | #2

    IMO, the conservative option is a Class II smart retarder also detailed as an air barrier. Some support for this here.

    Not exactly the case here, but it's an interesting question as to why the IRC authors think that vented cladding with 10 perms on the interior side and as little as zero perms on the exterior is an acceptable Z5 wall. It doesn't even come close to the perm ratio recommendations in Table 2A below, so I'd expect lots of moisture accumulation.

    1. chicagofarbs | | #3

      The Intello smart vapor barrier seems to be the cat's pajamas right now, for interior application.

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