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Community and Q&A

vapor retarder/barrier use in Minneapolis

Stevie0302 | Posted in General Questions on

We’ve got 1950’s house and we are refinishing the kitchen. Wall assemblies in this home are 2×4 wood stud; interior walls are plaster on sheet rock; exterior wall is 1x ship lapped boards of varied widths (~12″?) which is covered with what looks like tar paper and then mediocre vinyl siding. No house wrap beyond the spotty tar paper.

Insulation within wall cavities is minimal (original) and in places someone appears to have blown in cellulose, however, I suspect coverage and density is spotty as our exterior walls have many cold spots.

There is not immaterial air movement through wall assemblies, which is something we endeavor to address over time, including correcting the house wrap situation when new siding is in the budget, possibly a couple years down the road.

I would like to remove existing interior wall finishing to install stone wool bat insulation, and then new sheetrock. My question is whether – given the current state of air permeability of the external wall side – we should be applying a vapor retarder or smart vapor membrane underneath the sheetrock. A number of years back I read a reference book for cold-weather building that suggested drywall glue, applied evenly and consistently along with effective air sealing around outlets, etc and a good latex pain was sufficient. 

My concern is that during the Summer months, Minneapolis experiences notably high dew points often enough, paired with high temps. We use air conditioning and I am worried about moisture buildup within the wall cavity during the summer, against the vapor retarder/membrane, given the relative air “leakiness” of the exterior wall coverage for the next couple of years. I understand that a retarder or membrane will prevent vapor during the winter months from moving to the cold outside. As such wouldn’t a correctly installed retarder/membrane beneath sheetrock allow for moisture to not condense on the back of it and pass through the interior wall structure? Or am I grossly overthinking this…

thanks for any insights folks can share. Stevie

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  1. Expert Member
    BILL WICHERS | | #1

    The general rool is that you need a vapor retarder on the interior (warm) side unless you have exterior continuous insulation of sufficient R value to make the minimum ratios of interior to exterior R value to be safe. In your case, with no exterior insulation, you should use an interior side vapor retarder. Latex paint on drywall is sometimes enough, but I have never trusted that and have always added a smart vapor retarder behind the drywall (usually MemBrain, since it's cheapest and works fine in most cases). If you were using kraft faced batts, the kraft facer is sufficient, but with unfaced mineral wool batts, I would add the smart vapor retarder.

    I typically install the batts, then the vapor retarder with a sealant around the perimeter and any penetrations to allow the vapor retarder to double as an air barrier. I install the drywall airtight too for some extra redundancy. I use a pneumatic staple gun that shoots regular Arrow-style staplegun staples, which makes things quick and easy.


    1. Stevie0302 | | #2

      Thanks Bill that makes sense. Much appreciated. I kept reading after this post and came across a suggestion to use vapor-variable membranes. I'll either use the basic membrane you suggested or one of these.

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