Moisture can be a troublesome thing, especially in buildings.
Able to travel through and saturate substances, it can cause plenty of problems inside a structure. Sometimes, a moisture issue is immediately apparent -- there’s no mistaking a wet piece of drywall. Other times, it takes years before realizing moisture’s damaging effects, such as mold growth and decay in out-of-sight areas.
For home inspectors trying to determine if excessive moisture is present and root out its source, it’s critical to understand the readings from a good quality moisture meter.
When measuring moisture’s presence within building materials, WME, or wood moisture equivalent, readings set the standard.
The technical answer: WME is a measurement of the (theoretical) percentage of moisture content that would be attained by a piece of wood in contact with, or in close proximity to, a moisture equilibrium across a host of materials.
In layman’s terms, WME is a scale of measuring moisture content in materials as if they were wood.
Why is wood the basis for this type of moisture measurement? Three reasons:
Inside buildings, moisture is mobile.
When it enters a space, whether as naturally occurring humidity or via a leak, moisture will interact with most building materials. Like wood, construction materials such as plaster, drywall, or concrete absorb or release moisture to equilibrate with the moisture present in the immediate surrounding area.
That’s where WME readings are helpful for measuring moisture levels. With wood as the barometer for moisture readings, the playing field is level for moisture content measurements across all materials. You won’t need to have a material-specific chart handy to effectively gauge moisture’s presence when conducting a survey; a wood moisture equivalent chart is more than sufficient. You're able to report the moisture condition of these materials directly by quoting their WME values.
Wood moisture equivalent measurements also help you understand what moisture presence means for other building components in a space.
The impact of moisture content within a material is relative. Acceptable moisture levels in drywall is not the same as what's considered acceptable moisture content in other building materials.
Or, what’s considered an acceptable
moisture content reading level in one material may be irreversibly damaging to another.
Take concrete, for instance -- one of the hardest building materials. Made using water, concrete already has moisture present in it. After curing, its composition is porous, meaning moisture is able to move through it.
While it may appear dry, a higher WME reading in concrete can mean trouble for other materials around it. Wooden baseboards or structural supports may be damp and starting to grow mold while drywall with the same moisture level is visibly saturated.
WME Vs. Equilibrium Moisture Content
When evaluating how building materials interact with moisture, another term frequently comes up in the same conversation as WME: equilibrium moisture content (EMC).
Though both are related, the two shouldn’t be confused.
Again, WME measures the percentage of moisture content that would be attained by wood in contact with, or in close proximity to, a moisture equilibrium across a host of materials. You’ll use a traditional professional moisture meter (more on this in the next section) to take WME readings.
EMC, however, is the point at which a material stops absorbing or releasing moisture to reach a balance with the humidity in its immediate environment. Taking EMC readings requires using a hygrometer.
For measuring WME with a moisture meter, there’s only one option regardless of the substance being tested. Whether you’re using a moisture meter for concrete floors or a wall’s drywall, you’ll need a pin-type moisture meter.
WME measurements are determined by measuring electrical conductivity within a material. Quantifying moisture content by measuring electrical resistance between its probes, pin-type moisture meters are calibrated using moisture content on wood derived from oven tested wood samples.
Water is naturally conductive. The more moisture present in a material, the less electrical resistance there is. With wood having an almost universal conductivity across species, it sets a reliable base standard for measuring electrical conductivity in other substances. Note there are some small differences in base conductance between wood species but this is very small and not considered too important when measuring a building moisture levels and if the building is wet enough to support microbial growth (i.e. mold).
Pin meters are the most accurate and repeatable measurement system so when documenting readings this type of meter should be your primary tool.
For home inspections, it’s still essential to have a non-invasive meter in your tool kit. During the initial stages of a survey, a pinless moisture meter lets you work quickly to locate where moisture is present in a space. With a pinned moisture meter in hand, you’ll be able to confirm initial readings by a non-invasive meter and determine the moisture content in a material.
Getting accurate WME measurements isn’t as simple as sticking a pin-type meter’s probes into a substance and waiting for a reading. Be aware: there are two common substances in buildings that can skew measurements when taking readings:
Salt presents itself on materials where moisture is present. It’s most often seen as buildup on flooring (salt efflorescence) where rising moisture has occurred or along walls where moisture has leached through.
Salt is problematic for taking moisture readings for two reasons: It increases water retention within a substance and affects water’s conductivity. In both cases, salt impacts a moisture meter’s ability to take an accurate reading.
There’s rarely a building that’s only constructed with wood. Metal pipes, wires, nails, and screws are all components of modern construction, and they all conduct electricity. When in contact with a pin-type meter’s probes, these materials will appear to be wet. This will throw off its measurement of electrical resistance and give what is referred to as false/positive readings of moisture.
As a standard for gauging moisture content, wood moisture equivalent measurements provide you the right data for assessments. Using one of the world’s most common construction materials as a basis for comparison, WME readings help you understand what moisture content in a space means across a host of building materials.
Explore our selection of meters designed to make home moisture surveys easier: