Slab-on-Grade Moisture and Finish Failure in a Warehouse Conversion
A 1970s warehouse is converted to a conditioned records-storage and light-assembly facility. The existing 6 in. slab-on-grade sits on a compacted subbase with no vapor retarder and no capillary break. The design adds a vapor-permeable epoxy coating and a new vinyl composition tile (VCT) finish in the assembly area. Six months after occupancy, the VCT is cupping and adhesive is oozing at the joints; a plastic-sheet test taped to the slab shows heavy condensation within 24 hours. The mechanical engineer reports the space is held at 50 percent relative humidity and 72 F. The contractor blames the flooring manufacturer; the owner demands a fix without demolishing the slab.
1. Diagnose the moisture source and explain why the floor finish failed.
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The plastic-sheet test result indicates moisture vapor transmitting upward through the slab. With no vapor retarder and no capillary break beneath the slab, ground moisture moves through the concrete by vapor diffusion and capillary action. The epoxy coating is vapor-permeable, so it does not stop the flow; moisture accumulates at the adhesive and VCT interface, which is a low-permeability layer. The adhesive re-emulsifies, the tile edges swell and cup, and adhesive extrudes at the joints.
The interior condition of 72 F and 50 percent RH is not the cause; it is the condensing surface relationship that reveals the problem. The slab surface stays near ground temperature, below the dew point of the interior air, so vapor reaching the slab underside condenses at the finish. The failure is a slab moisture issue, not a flooring defect, and the contractor's claim against the manufacturer is misplaced. The fix must address vapor transmission at the slab, not replace the tile alone.
2. What remediation options are available short of slab demolition, and how should the designer verify success before reinstalling finish?
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Options, in order of preference: (1) apply a moisture-mitigation coating or topping system rated for the measured vapor emission rate, such as a high-build epoxy or urethane vapor barrier installed per manufacturer limits after surface preparation; (2) use a dimpled drainage mat or capillary break topped with a new self-leveling underlayment to create a decoupling and drainage layer; (3) install a raised access floor or a sleeper system with a vapor retarder, isolating the finish from the slab; (4) select a finish tolerant of moisture, such as a polished or sealed concrete or a moisture-tolerant resilient product, abandoning VCT in the assembly area.
Verification is essential. The designer should obtain quantitative vapor emission testing (for example, calcium chloride or in-situ relative humidity probes) rather than relying on the qualitative plastic-sheet test, establish the emission rate the selected system can tolerate, and require post-installation testing before finish is laid. The specification should require the flooring and coating manufacturers to warrant compatibility with the measured rate, and the contractor should be required to document slab preparation and cure. A mock-up area tested and monitored before full installation reduces risk. The designer should also confirm that the added topping does not create a step or violate accessibility at transitions.