Plastic at bottom of exterior walls cavities: a partial vapor barrier?
Longtime lurker but and first-time poster here. I have a 1960s brick ranch on a crawlspace in the midst of a partial remodel. Some details about the construction:
Foundation: CMU, crawlspace
Sheathing: Celotex fiberboard, no housewrap
Insulation: Originally dense-pack cellulose installed through sheathing, installing Roxul
Siding: Brick, 1″ gap to sheathing, no brick ledge, no weep holes
The dense-pack holes were all patched with stapled-on cardboard when the house was built, and I have caulked the patches to reduce air leakage. My goal is to make the home as airtight as possible with a reasonable DIY effort. I am partway through insulating with Roxul and had, after agonizing over the issue and combing through this site at length, intended to omit an interior-side vapor retarder due to the vapor-open nature of the assembly. The home is located in southwest Ohio in Zone 5 but only about 50 miles north of the Zone 4 border. We run the A/C six months out of the year.
Black poly sheet is present in the stud cavities along the bottom 4-8″, depending on the wall, on the inside face of the sheathing. This poly extends down to the sill in the crawlspace but is hidden between the band joist and the brick. (I encountered it while working on the framing in the crawlspace.) The gap between the brick and sheathing breathes near the crawlspace vents. (Don’t get me started on the crawlspace–that’s a work in progress!)
I would guess the purpose of this plastic is to shield the wall cavity from wind-driven moisture or splashback, but it’s odd to find it inside the wall rather than as flashing on the outside. The home has 24″ overhangs, so rain hitting the brick is rare. Ground splashback was happening when I bought the home but has since been addressed with new gutters.
When a wall has plastic as shown in the pictures, does it create a localized vapor barrier that will collect condensation cause moisture concentration on nearby framing? In particular, I have a wall junction that contains horizontal blocking that partitions the vertical extent into four sections, one of which is almost entirely lined by poly on the sheathing.
The framing is in mint condition except in spots where there have been bulk water leaks, but the original insulation was cellulose, which adsorbs moisture, and not rockwool which resists moisture. I have never seen walls detailed like this and am concerned that installing rockwool instead of cellulose with this plastic present may create a localized problem at the bottom of the walls that will rot nearby lumber. Am I overthinking this, or should I be concerned?
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