Mitigation Coating at Its Limit
A 10,000 ft² hospital corridor and patient-room wing has an 8 in. slab-on-grade over a vapor retarder, 14 months old, conditioned at 70°F and 50% RH. Ten ASTM F2170 tests give 84–91% RH; the flooring specification limit is 85%. The specified moisture mitigation is a two-component epoxy with a published maximum of 90% RH, a required primer over the slab, and a CSP 3 surface profile. The applicator proposes to skip the primer because the slab is tight, and to apply mitigation only in the four locations that exceeded 85%, saving about 60% of material cost. The construction manager supports the idea to hold the schedule. The resilient flooring and adhesive have a 90% RH limit when installed over mitigation with a manufacturer warranty.
1. Evaluate the two proposals: omitting the primer and applying mitigation only in the high-reading areas.
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Both proposals should be rejected.
Primer omission. The primer is part of the manufacturer's published system. Skipping it changes adhesion, film formation, and vapor resistance, and it normally voids the warranty. Tight concrete is not a justification; it is an opinion. Only a written manufacturer approval for a specific primer-free system on this substrate would change that.
Spot application. Moisture is not confined to the four tested holes. Untreated areas remain pathways for vapor, and the boundaries between treated and untreated concrete create differential movement, adhesion stress, and edge failures. A mitigation system is a continuous barrier over the entire floor area, detailed at perimeters, penetrations, and drains. Spot coating also makes future testing meaningless.
The bigger problem: 91% exceeds the coating's 90% maximum. Even full-field application with the primer would be outside the product's stated limit at the worst location. The schedule cannot override a product limit.
2. How should the measured RH values be interpreted relative to the coating's limit and to future risk?
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The 90% figure is a maximum, not a design target. A slab reading 91% at one location is outside the system's limit, and a reading 84–89% elsewhere leaves little margin.
At 14 months the slab may still be drying, especially if the vapor retarder is discontinuous or the concrete is thick. Internal RH can rise seasonally when the HVAC shifts to humid conditions or when the floor covering reduces drying from the top. The flooring and adhesive add their own limits, and a warranty claim can fail if the installed system was outside the manufacturer's moisture range.
The right interpretation is that the slab is not ready for this coating system as specified. Either continue drying and conditioning until all locations are comfortably below the coating limit, or select a mitigation system rated for the measured RH and approved for the adhesive and flooring. Retesting should use the same method, depth, and layout, with a documented margin, not a single favorable average.
3. What would you require before installation, and how would you document the decision?
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Requirements:
- A written manufacturer review confirming the system is acceptable at the measured RH, including primer and any surface preparation requirements. If no system is rated for 91%, the slab must dry further or the design must change.
- Full-field mitigation, not spot application, with CSP 3 profile achieved and verified, primer applied per data sheet, and installer certification.
- A pre-installation meeting with the owner, construction manager, applicator, flooring installer, and manufacturer's representative.
- Quality control: wet-film and dry-film thickness, adhesion pull tests, pinhole inspection, and cure verification before flooring.
- Post-mitigation moisture verification and a warranty naming the owner.
Documentation: a written technical position memo stating that spot application and primer omission were rejected and why; the 91% result and the product limit; the manufacturer's written response; meeting minutes; and photos of surface preparation and coating. The construction manager's schedule preference should be recorded as a request, not as approval. If the owner overrides the recommendation, require written acceptance of the risk and warranty exclusion.