Slab-on-grade moisture issue — Mississippi CZ2A, looking for building science input – GreenBuildingAdvisor

Single-story slab-on-grade, brick veneer, Meridian MS (CZ2A). Spray foam attic original to the build (9 years old). Had glue-down LVT fail with moisture and mold at the perimeter.

During mold remediation we flood-cut and opened walls to studs in two rooms. Framing, studs, and bottom plates were all clean — no rot, no elevated moisture, no mold. Damage was limited to baseboards, quarter round, and the bottom few inches of sheetrock. Moisture was found under the LVT almost exclusively near exterior walls and interior walls that connect to exterior walls — not in the field of the slab.

Field observation I’m fairly confident in: with a dehumidifier maintaining high 40s RH and AC vents closed (warm slab), the floor stays dry. The moment AC vents are reopened and the slab cools, moisture reappears at specific locations that correspond with the most pronounced gaps between the sill plate and the slab. Currently testing with positive room pressure (supply open, return covered, containment door bulging outward) to isolate whether negative pressure is a driver.

Suspected contributing factors: HVAC possibly oversized and short-cycling with poor latent removal, undersized return ducts possibly creating whole-house negative pressure, brick veneer acting as reservoir cladding, sill plate gaps without capillary break.

Questions:

1. Does the wall-line moisture pattern — exterior walls and connecting interior walls, not field of slab — point clearly to air-driven condensation rather than sub-slab vapor emission?
2. Best approach for sealing the sill plate joint in a hot-humid climate — interior face seal only, or fill the void?
3. Any experience with this specific failure mode in CZ2A slab-on-grade with original glue-down LVT?