Handheld Meter Says Dry, Hood Says Wet
A 9,600 ft² warehouse conversion to a fitness center has a 5 in. unsealed slab-on-grade with old black cut-back adhesive and patches of residual VCT mastic. The owner's maintenance technician walks the slab with a handheld impedance meter set to its concrete mode; readings are 3.1–4.2% on the meter's arbitrary scale, which he reads as dry and below 5%. A flooring contractor's representative places an ASTM F2420 insulated hood at three locations and obtains 84, 87, and 89% RH after equilibration. Slab surface temperature is 68°F; ambient is 72°F and 45% RH. The owner wants to avoid adhesive removal and a mitigation system, arguing that the meter is what his inspector used for 20 years. The resilient flooring manufacturer accepts either F2170 or F2420 results, with an 80% RH limit.
1. Explain why the meter and the hood disagree, in terms of what each instrument actually measures.
Read the worked answer
They measure different things, so agreement would be a coincidence.
An impedance or capacitance meter passes a signal through the near-surface concrete and infers moisture from conductivity or dielectric response. The displayed percent is an arbitrary scale, not RH and not even reliable percent moisture by weight. The reading is strongly affected by soluble salts, carbonation, admixtures, reinforcement, contact pressure, and especially the adhesive and mastic residue under the meter. Over a black cut-back adhesive, the meter may read dry while the concrete below is wet.
The F2420 hood seals a small air volume against the surface and measures RH after the air equilibrates with the concrete surface. It responds to vapor arriving from the slab, which is closer to the driving force that affects a floor covering. A hood result of 84–89% says the surface air is nearly saturated relative to the 80% limit, even if the electrical meter looks acceptable.
The meter is a screening guide, not a compliance test, and it cannot overrule a properly run hood measurement.
2. Which result should govern the decision, and what could still invalidate the hood readings?
Read the worked answer
The hood result should govern, because it is a standardized surface RH method accepted by the manufacturer, while the impedance meter is only a comparative indicator. But a valid-looking hood number is not automatically correct.
Check the seal: the hood must seal to the concrete, and adhesive ridges or mastic patches can create leaks or trapped air pockets. Check equilibration time and sensor calibration. Record slab and air temperature, because surface condensation or a cold slab can distort the reading. Confirm the hoods were placed on representative prepared concrete, not only on wet patches, and that locations were selected across the area rather than conveniently.
Because the slab is unsealed and 5 in. thick, I would also run F2170 at 40% depth to characterize the slab interior and confirm whether the hood is reading a surface phenomenon or a genuinely wet slab. If F2170 also exceeds 80%, the case for mitigation is strong. If the methods diverge, the surface condition and adhesive residue must be investigated before selecting a product.
3. The owner wants to keep the old adhesive and proceed. What technical and contractual steps do you recommend?
Read the worked answer
Keeping the adhesive is not a moisture-control strategy. Cut-back adhesive is a bond breaker, a reservoir, and a contaminant that can prevent the new flooring or adhesive from bonding. It also makes surface RH testing harder to interpret.
Recommended sequence:
- Mechanically remove adhesive and mastic to bare, sound concrete; do not use solvent or emulsifying strippers that drive moisture into the slab.
- Re-run moisture testing on the prepared surface with the method the flooring manufacturer accepts: F2420 at representative locations and F2170 at 40% depth for confirmation.
- If results exceed 80%, specify a moisture mitigation system with a written warranty compatible with the selected adhesive and flooring.
- Require a mock-up or test area, then measure adhesion and observe after conditioning.
- Document that the meter was a screening tool only, and record the decision basis.
If the owner still refuses removal, obtain a written manufacturer position and an owner acceptance of warranty exclusion. Do not present the meter reading as proof that the slab is dry.