If a dry and absorbent material, such as wood or brick,
is placed in a very damp environment (high %rh) it will absorb
water and its moisture content will increase. Conversely,
absorbent materials with high moisture content will lose moisture
to a dry environment (low %rh).
The movement of moisture from material to environment and
vice versa continues until the vapor pressures (within the
material and the environment) have equalized. When this condition
is reached, the moisture level of a material can be expressed
in terms of equilibrium relative humidity (ERH).
ERH techniques are very useful for assessing if structures
are in a dry, borderline or damp condition. For example, when
the ERH of a concrete floor falls to 75, a flooring contractor
knows that it has dried sufficiently for a decorative floor
covering to be laid.