Moisture Testing Before Flooring Installation: Why It Matters

Choosing the Right Flooring

Installing flooring over an untested subfloor with moisture issues guarantees failure — moisture testing is the critical diagnostic step that determines which products can go in which locations.

Concrete slabs emit moisture vapour continuously as residual construction moisture migrates to the surface. New slabs cure over 60 to 90 days after pour but continue to emit some vapour for months. Older slabs may also emit vapour due to groundwater pressure below grade — this is separate from construction moisture and persists indefinitely.

Installation and Timeline

The calcium chloride test measures the moisture vapour emission rate (MVER) of a concrete slab. A sealed dish of anhydrous calcium chloride is left on the slab for 60 to 72 hours; the weight gained by moisture absorption is converted to a vapour emission rate. Most flooring manufacturers specify maximum MVER values for their products.

The relative humidity probe method is increasingly preferred over calcium chloride testing because it measures moisture at depth within the slab rather than just at the surface. Probes inserted to 40% of slab depth sample the moisture condition where the slab will be in equilibrium once covered.

Care and Maintenance

Wood subfloor moisture testing uses a pin or pinless moisture meter. Pin meters measure resistance between two pins pressed into the wood; higher resistance indicates lower moisture content. Target moisture content for a wood subfloor before hardwood or engineered wood flooring installation is 7 to 10% in Ontario's climate.

Moisture mitigation products — epoxy coatings, moisture membranes, or PVDC barrier sheets — can reduce vapour transmission from high-moisture slabs to acceptable levels. However, products rated for mitigation have specific MVER limits above which even mitigation is insufficient. Above those thresholds, addressing the source of moisture is the only correct solution.