Dew Point at the Slab
A 900 sq ft finished basement has 8-ft ceilings and 6-mm vinyl plank glued to a concrete slab. A supply-line leak soaked the carpet and lower drywall; carpet and pad are removed, drywall is cut 12 in above the floor, and two air movers plus one LGR dehumidifier are running. After 24 hours the logger shows 73°F and 68% RH at 4 ft, while the slab surface is 61°F. A handheld hygrometer held 1 in above the floor in a corner reads 74% RH. The dehumidifier is draining 45 pints/day and its setpoint is 50% RH. Outdoors is 58°F and 55% RH; windows and doors are closed. The customer says the floor still feels clammy and wants the equipment removed tomorrow. The project manager asks you whether the floor is actually drying or whether condensation is working against the system.
1. Estimate the dew point of the basement air and determine whether the slab is at risk of condensation. What confirming measurements would you add?
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At 73°F and 68% RH, the vapor pressure is about 0.68 × 0.40 psia, roughly 0.27 psia; the corresponding dew point is about 61°F. The slab is also about 61°F, so it is at or below the dew point - condensation is likely. The corner reading of 74% RH at the same air temperature raises the local dew point to roughly 64°F, which is above the 61°F slab, so that corner is an active condensation surface. Confirm with a surface-temperature probe at several slab points, a hygrometer held within 1 in of the floor, and a visual check for repeated beading after wiping. Also record the dehumidifier's actual outlet condition and runtime. If surface temperature is consistently below the local dew point, the floor is not drying effectively no matter what the air RH reads at 4 ft.
2. Compare lowering the RH setpoint to 45% versus adding heat to 78°F. Which would you choose, and what operational risks follow?
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Lowering to 45% RH at 73°F reduces the dew point to roughly 54–55°F, giving about 6–7°F of margin above the 61°F slab. Adding heat to 78°F at 50% RH gives a dew point near 58–59°F and only a 2–3°F margin; it also warms the air faster than the slab, so the slab may lag. The lower-RH option is more direct, but the LGR's capacity falls as RH drops, so 45 pints/day at 68% may become roughly 30–35 pints/day at 45%. I would set 45% RH, keep air moving across the slab, verify the unit is not short-cycling, and add capacity or a desiccant unit if the setpoint is not held. Do not open windows to outdoor air at 58°F/55% RH; that adds moisture load. Document the dew point margin daily.