Installing LVP Over a Concrete Slab: Moisture, Testing, and Prep

Why concrete is never actually dry

Concrete cures by chemical hydration, not by drying out, and it retains water long after it is hard. A slab poured on grade also sits in contact with the soil beneath it, which supplies a continuing source of moisture unless a vapor retarder was installed under the pour.

That moisture moves upward as vapor and accumulates under any impermeable surface laid on top of it. Under a glue-down floor it attacks the adhesive bond. Under a floating floor it collects between the slab and the underlayment, where it can support mold growth and produce odor.

The two tests that matter

ASTM F2170 is a relative humidity test using probes inserted into holes drilled into the slab and left to equilibrate. It measures conditions inside the slab and is the test most manufacturers now specify. Typical acceptance limits run around 75 to 85 percent RH depending on the product and adhesive.

ASTM F1869 is the calcium chloride test, which measures moisture vapor emission rate at the surface in pounds per 1,000 square feet per 24 hours. It is older, surface-focused, and less predictive on power-troweled slabs, but some manufacturers still specify it.

There is also a simple pH test of the slab surface. High alkalinity accompanies high moisture and can degrade adhesives directly, so it is worth checking alongside the primary test.

A plastic-sheet taped to the slab overnight is not a test. It gives a crude yes-or-no indication at best and is not accepted by any manufacturer warranty.

When readings are too high

There are three legitimate responses, and one common non-response.

• Wait: a new slab may simply need more drying time under conditioned, ventilated conditions.
• Mitigate: apply a moisture vapor barrier coating — typically a two-part epoxy — rated for the measured level.
• Change the assembly: choose a floating installation with a sheet vapor retarder where the product allows it, which tolerates more moisture than glue-down.
• What does not work: installing anyway with a thicker foam underlayment. Foam is not a vapor retarder and does not address the mechanism.

Flatness and surface prep on slabs

Slabs come with their own surface issues independent of moisture. Power-troweled slabs can be dense and slick enough that adhesives struggle to bond. Trowel ridges, control joints, curing-compound residue, and old cutback adhesive all need handling before install.

Diamond grinding with dust extraction is the standard answer for ridges and residue. Control joints in a residential slab are typically filled with a semi-rigid filler before a hard surface goes over them. Cracks need to be assessed for whether they are static shrinkage or active movement — a moving crack will telegraph through any floor eventually.

Slab-level rooms in local homes

The situations we run into most often here are daylight basements in the Boise foothills and Eagle, garage conversions across Meridian and Nampa, and newer slab-on-grade phases in Star and south Meridian where the concrete is only a couple of years old.

In all of them the correct sequence is the same: test first, prep second, install third. Testing costs a small fraction of what a failed slab-level floor costs to tear out and redo.