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Does it matter what side of batts insulation kraft paper is installed?

bmstevens | Posted in General Questions on

I’m having difficulty finding a concrete answer on this from reputable sources. There are a few blogs, including one on GBA, that indicates that it doesn’t matter. However, some industry professionals have indicated otherwise. 

I’m hoping for some opinions and input about if it is necessary to install fiberglass batts with the paper backing on the bottom between the warm side of the home and the insulation, or is it acceptable to have the paper backing on the cold side of the attic.

Thank you in advance! 

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi bmstevens.

    The kraft facing on most fiberglass batts is a class II vapor retarder, which typically goes on the warm-in-winter side of the assembly. If you are in a climate and your assembly is such that you should have an interior vapor retarder, the kraft facing would be towards the interior. Otherwise, use un-faced batts.

  2. Expert Member
    Dana Dorsett | | #2

    >"The kraft facing on most fiberglass batts is a class II vapor retarder, which typically goes on the warm-in-winter side of the assembly."

    An asphalted kraft facer is a SMART vapor retarder, that becomes fairly vapor open when the humidity of the entrained air in the insulation is high enough that it needs to dry, only a class-II vapor retarder when the proximate air is sufficiently dry.

    Putting it on the warm-in-winter side of the assembly is only valid for heating dominated climates (DOE/IECC zones 4 & higher). In climate zone 3 it doesn't matter, in climate zones 1 & 2 it's somewhat better if it goes on the exterior side of the assembly. But screwing it up by putting it on the "wrong" side of the assembly doesn't have much consequence in zones 1-4, or even very much in zone 5 if the wallboard is air tight. But it's a potential issue in zones 6 & higher unless there is some other interior side vapor retarder controlling vapor diffusion through the interior, unless there is sufficient insulation on the exterior of the sheathing for dew point control at the sheathing layer.

    You've probably already seen:

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