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Community and Q&A

Roxul Batts and Air Sealing

alex9999999 | Posted in General Questions on

Hi all, I am insulating my barn/workshop/schoolhouse gradually with whatever insulation I come across. It is in Massachusetts and has bare stud bays, board sheathing, asphalt felt building paper and clapboards. A contractor friend recently gave me Roxul batts, and suggested after I tuck them in the stud bays I staple plastic sheeting over them.

Is that necessary and/or a good idea? I have been using gun foam around my cuts and cobbles of rigid foam elsewhere, but that doesn’t seem to make sense around the snug Roxul. Maybe the plastic sheeting would accomplish the same sort of air sealing? But maybe it would just collect condensation.

For context, building code isn’t real relevant here, and this isn’t a home. I’m just trying to make the place a bit cozier when the wood stove is cranking.

Also: eventually I’m going to put up shiplap pine boards as the interior wall surface. I know that is fraught without drywall behind it, but I’m thinking it won’t be much of an issue with a space that is not lived in and doesn’t have much in the way of showers, kitchen, etc.

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  1. Patrick_OSullivan | | #1

    Unless very well detailed (stapled well, cut neatly, taped at all seams), the plastic wouldn't be a very good air barrier.

    The risk with plastic is, as you've noted, condensation under certain conditions. For example, if you ever air conditioned that space, hot humid air from the outside could condense on the relatively cold plastic sheet.

    Most folks here don't love interior vapor barriers and instead prefer either smart vapor retarders (where needed) or vapor permeable material when there's a need for a sheet-type interior air barrier.

    1. charlie_sullivan | | #3

      Patrick, this is totally off topic, but I saw you said in another thread that found a good HVAC contractor in NJ. I've been trying to find one with little success so I'd be delighted if you could point me to one. Maybe this question is a better place to reply:

    2. charlie_sullivan | | #4

      Patrick, this belongs on another thread, but I wanted to ask you about the good HVAC contractor in NJ you said you'd found. I've been trying to find one with little success so I'd be delighted if you could point me to one. Maybe this question is a better place to reply:

      Or this one:


  2. Expert Member
    BILL WICHERS | | #2

    Poly can be detailed as an air barrier, but it’s easy to damage — especially if you use something really thin like 3 mil. You also have the potential issues due to it being air barrier as previously mentioned.

    If you don’t want to use drywall, I’d use thin plywood, maybe 3/8”. This material isn’t very expensive, just a little more than drywall, and you can detail it as an air barrier too. Plywood isn’t a vapor barrier, and you can easily install your shiplap over it with little risk of damaging it.


  3. alex9999999 | | #5

    Thank you guys. Bill would the point of the plywood be that it would be an air barrier?

    1. Expert Member
      BILL WICHERS | | #6

      Yep, that's exactly what it's for. You need to tape it and seal it the same way you would if it was exterior sheathing being used as an air barrier. Plywood's advantages are it is more durable than drywall, lighter weight (a little bit anyway), and... Hmm... Maybe that you don't need to mud it :-)

      The problem with poly sheeting is that it's relatively easy to tear gashes in it if you gouge it with the end of a shiplap plank, for example. Plywood just goes "thud". Drywall goes "thud" and maybe gets a little dent that you can spackle over.


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