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interior foam/vapor barrier (EPS) presents exterior foam + siding challenge

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  • interior foam/vapor barrier (EPS) presents exterior foam + siding challenge

    After studying years of JLC siding and foamboard details and scouring forums/letters I think I have a plan for a peculiar reinsulation/siding project, but want to get some feedback.

    A poorly insulated 2x4 wall (8'H X 24'W) had the interior painted sheetrock covered with 1/2" EPS (pink stuff with the thin sheet plastic covering) and the seams taped and all the edges foamed to complete the air and vapor seal before a wall of cabinetry was installed wall to wall and floor to ceiling.

    Now it's time to replace the siding. Plan is fiber-cement planks/trim/soffits with 1x to provide the airspace behind and use the ridgevent material at the bottom with an insect screen for draining/drying. Going to take the opportunity to replace the studbay insulation with new batts or maybe cellulose.

    Desire is to add another 1" of insulation board over the studs before putting up the siding, but most of the foamboard products are air and vapor barriers, which seem unwise to put on the outside when I already have vapor barrier on the inside wall. I'm in Austin, so outside cold condensation is unlikely, but inside cool (A/C) and very hot/humid outside could condense on the batt side of the interior drywall... mold-city.

    I found some tables of insulation that show that some closed cell polyiso rigid board does have a small amount of moisture permeability when the board is not foil-faced.

    Is this permeable enough to allow drying of the stud bays if they get wet? Or will this allow too much moisture through the foam into the bay where it could sometime condense on the batt-side of the interior drywall? I did some back of the envelope calcs and think on a horrible worst-case day (105F 60% humidity) I'd still have a dew point below 88F on the exterior of the inside drywall, and seems like it shouldn't get that cool with the inside A/C at 78F and the 1/2" EPS, but looking for some validation before I proceed.

    thanks y'all!