WME/Wood Moisture Equivalent: A Home Inspector’s Guide

Protimeter MMS2 Wall Test_3

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.
 

What Is WME/Wood Moisture Equivalent? 

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. 

Say what?

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: 

  1. From species to species (think oak and maple), wood has generally a similar level of electrical conductivity -- which matters for taking WME readings. We’ll talk about that more later.
  2. It’s hygroscopic. An organic material, wood is always interacting with its environment. Depending on the conditions in the area around it, wood will absorb or release moisture until it has the same moisture content as its environment. Ever had a wooden door swell on a humid summer day? That’s wood moisture equilibration in action.  
  3. When damp, wood supports microbial growth (molds).Taking moisture measurements relative to wood helps better gauge when mold may become an issue for other materials that support microbial growth. 

 

How WME Measurements Work & Why They Matter   

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. 

Screen Shot 2021-05-19 at 1.16.26 PMThat’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; you can 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. 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. 

Measuring WME: The Best Moisture Meter for Concrete & Other Construction Materials

For measuring WME with a moisture meter, there’s only one option regardless of the substance being tested. Whether you’re using a damp 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. 

Factors That Impact WME Readings 

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:

Rising salts

Salts

Salt presents itself on materials where moisture is present. It’s most often seen as buildup on flooring 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.

Conductive Materials (metals) 

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.

WME/Wood Moisture Equivalent = Accurate Home Inspections for 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.

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