Mold can grow in the absence of visible moisture intrusion.

A source of mold may be hidden right underfoot. Under many floors are concrete slabs that give off water vapor. If this release of moisture results in high-enough relative humidity, flooring can fail and mold can grow. It is essential, then, to ensure that concrete slabs are sufficiently dry before flooring is added and that air movement is adequate to prevent high relative humidity.

One traditional and widely accepted test for water vapor emission uses calcium chloride. In this test, a plate of calcium chloride is weighed, then placed on the concrete and covered by a dome or other apparatus to trap the moisture vapor. The dome is sealed to the concrete to prevent ambient influence or loss of vapor. The testing materials are left untouched for 60 to 72 hours, during which time the calcium chloride absorbs water vapor. At the end of the test period, the plate is again weighed. The difference in the starting and ending weights is used to determine the vapor emission volume. ASTM International Standard F1869-98 covers this method.

The calcium chloride emission test can provide false results since it measures conditions only in the top one-half inch of the concrete. Even when the outside area of concrete is dry, there could be a reservoir of moisture deeper down that could later migrate to the surface later. A more robust test, which accounts for this possibility, is the equilibrium relative humidity, or ERH test (ASTM F2170-02).

In the ERH test, a hole is drilled into the concrete to either 20 percent or 40 percent of its depth. (Alternatively, concrete can be laid around a tube to create a cast hole). After the hole is cleaned, a liner is inserted and capped. After 72 hours, the cap is removed and a probe (attached to a hygrometer) is used to measure the equilibrium relative humidity at the bottom of the hole.

For both the calcium chloride and the ERH tests, ambient relative humidity and temperature should be the same as will exist during normal occupant use. Testing concrete in non-acclimated rooms will result in irrelevant data.