Retrofit Basement Shower Insulation

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Birmingham, AL
Online research regarding correct basement insulation methods that avoid moisture intrusion is what prompted this post. My question is regarding the best method to retrofit a portion of insulation removed from stud framing behind the side of shower stall in basement bathroom that faces underground concrete foundation blocks.

The house is ranch style w/ walk-out basement built in Birmingham AL, 1983, basement living space finished early 2000's.

The shower encasement sits 8.5" off the concrete blocks, the stud framing around it sits 3.75" off wall. Kraft paper backed insulation was removed from only the first stud bay facing the foundation. The remaining insulation down the concrete wall is inaccessible, what's visible seems intact, no mildew odor exists.

The insulation piece was removed due to being halfway disintegrated (possibly caused by a hole in foundation grading on exterior side from misaligned gutter allowing rain water under structure). I discovered this after the sheetrock wall situated between garage & living space was taken down to upgrade electrical & water lines. The back of shower faucet faces the removed sheetrock wall / garage, not the foundation blocks.

Plastic wrapped R-13 fiberglass is what I purchased to replace in the frame studs along the removed wall. The vapor barrier side will be placed towards interior warm side, hopefully that is correct method?

Will be closing wall with Densarmor plus sheets.
Will the R-13 be sufficient to replace in the one stud bay facing concrete blocks or too much of a gamble to prevent future mold growth?

Overall, obviously whatever I use for replacement, the original insulation installed could trigger future moisture issues and may be the worrisome culprit. The Kraft paper side is faced towards concrete which I believe is incorrect. However, with no apparent moisture issue occurring over 20 years since bathroom was put in, is the 3.75" between concrete wall & framing leaving sufficient space for any moisture to dry out?

Just not sure how to move forward or if I possibly need a completely different plan. I truly appreciate knowledgeable insight, I am very confused & also new to DIY repairs!



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With 3.75" of space between the studwall & framed wall you have both a thermal bypass and convection path for dragging moisture into that cavity. The best solution is to install 1.5" of foil faced rigid polyisocyanurate foam (or 1.5" of HFO blown closed cell foam) directly onto the block wall, leaving the studs empty, and on the "warm in winter, cool in summer" side of the assembly. That would meet or exceed IRC code insulation requirements for US climate zone 2 (which includes Birmingham AL) even for above grade walls, and would allow ample drying space for the wood. With a history of flooding it's best to keep the bottom of the sheet polyiso above the high-tide mark, but with closed cell spray foam it can go all the way to the floor. Alternatively, 2.5" of EPS (white beaded foam board) can hit the same performance point with a high degree of tolerance to liquid water.

With any foam board tape the seams with housewrap tape (or HVAC tape if foil faced), and use a horizontal bead of caulk to seal the top & bottom to limit/prevent convection around the foam. Mounting the foam to the block wall with foam board construction adhesive will work in your application if the foam can rest on the floor, but cap-screws through screwing the foam to the wall with masonry screws is better if the foam needs a gap at the bottom due to flooding risk. Don't substitute standard construction adhesives- the solvents will attack the foam. Box stores usually carry the specially forumulated foam board versions.

Fiberglass batts in a studwall need an air barrier snug to the fiber on ALL sides of the fiberglass to perform to their rated R. With 3.75" of air open to the fiberglass an R13 won't perform better than R7- R8 when there's a bit temperature difference, and if there is a foil or plastic facer on either side you're setting it up to have moisture issues, but kraft facers are fine. For below-grade spaces there is a constant water vapor pressure across the foundation wall, and with a true vapor barrier on the interior side of the wall moisture is likely to condense and accumulate there, particularly during the cooling season. With a vapor barrier on the exterior side (the side adjacent to the air space to the block wall) moisture drives from the shower can accumulate and condense there. Fully encapsulated fiberglass batts designed for basement applications can keep the fiberglass from getting wet, but does nothing to keep the wood framing dry.
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