Home Certified, Inc. Moisture Testing, Drying and Temporary Heat
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Moisture in Building Product Guidelines

Below we have provided information regarding the acceptable levels of moisture content in building products to prevent decay and methods to test the building products and its related information.

At what level of Wood Moisture Equivalent (WME) is a material in a wet or dry condition?

<18% (Green Zone) - The material is in a Safe Dry condition, moisture related problems of decay/deterioration will not likely occur.

18% - 20% (Yellow Zone) - The material is in a Borderline condition, decay/deterioration may occur under certain conditions.

>20% (Red Zone) - The material is in a Wet condition, decay/deterioration is inevitable in time unless the moisture level of the material is reduced.

What is Relative Humidity (%rh)?

Relative humidity (%rh) is the degree of saturation – or the amount of water vapor that air contains – relative to the amount it would contain if saturated. This is often expressed as a percentage, so saturated air at a given temperature has 100%rh.

If air is warmed, the amount of water vapor it could hold at saturation is increased. Consequently, the %rh value falls, even though the actual amount of water remains constant. Similarly, if the air is cooled, the amount of water vapor it can hold is reduced. Consequently, the %rh value rises.

Condensate forms when there is an excess of water in saturated air.

What is Equilibrium Relative Humidity (ERH)?

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.

What is Dewpoint?
A: Dewpoint is the temperature at which a sample of air becomes saturated. Condensation forms on surfaces whose temperature is equal to or lower than the prevailing dewpoint temperature.