Moisture Meters for Inspectors

by Nick Gromicko, CMI® and Kenton Shepard, CMI®

A moisture meter is a device designed to measure the moisture content of various building materials, such as roofing, siding, insulation, drywall, plaster, wood, tile and fiberglass. Structural and safety hazards, such as mold, rot and decay are all potential consequences of elevated moisture levels in these materials. An inspector can use a moisture meter to locate moisture that would not otherwise be apparent.

Here are a few ways that inspectors may find moisture meters useful:
  • A moisture meter can be used to determine whether a material is moist enough to allow mold to grow. Mold will begin to accumulate on surfaces that contain approximately 20% moisture, although this value varies based on vapor pressure and other factors.
  • An inspector can test the moisture level of a section of building material that appears to be dry, in order to establish a baseline from which other measurements can be compared.

Moisture meters can also be useful in the following applications that are not related to inspection:
  • If a home has been vacated due to flooding, a moisture meter can be used to determine if the home is once again suitable for occupancy.
  • Before a home is purchased, an inspector can use a moisture meter to determine if the house has leaks.  
  • A moisture meter can assist a homeowner in determining whether wood is dry enough to be painted or stained.
  • Wood installers use moisture meters to make sure that wood is dry enough to be installed.

Modes of Operation
Moisture meters come in two different varieties known as pin type and search mode. They are each suited to different applications, and InterNACHI believes that the best meters contain both options.
Pin Type 
This mode is used to measure the moisture content of a material’s surface, or at an incremental depth using probes. While in this mode, the meter can measure the amount of moisture on a material by its electrical conductivity. This is often regarded as a more repeatable and accurate type of moisture measurement than the “search mode” described below, although intermittent wet spots in the wood may be missed by pin type meters. This method can be used to test for moisture on the surface of building materials, such as stucco, drywall, plaster or wood. It is especially useful for determining if the source of a stain on a wall or ceiling is active, or if it has been repaired. 
Probes of varying lengths and designs may be used to extend the reach of a moisture meter operating in this mode. They are slender metallic poles with sensitive tips that extend the reach of the meter’s electrodes. Delmhorst makes probes that can be inserted deep into the straw in straw homes to measure its moisture content. Hammer probes can be driven into wood and then extracted. Other probes can be inserted into pre-drilled holes in masonry, or pushed through insulation. Moisture content in log homes can be measured by inserting a probe two-thirds of the way from the log’s surface to its center.
Search Mode
Search mode, also known as pinless mode, detects and measures moisture content beneath the surface of a material. Meters in this mode emit electromagnetic waves (usually radio waves or an electrical current) that are affected by the presence of moisture. The meter can detect changes in the characteristics of returned emissions and then use this information to calculate moisture content. Meters manufactured by Tramex, for instance, operate by the principle that a material’s impedance (resistance) to an electrical current varies inversely with that material’s moisture content. The instrument determines the amplitude of a low-frequency alternating current, and uses this information to calculate moisture content. Other meters, such as those manufactured by Protimeter and Wagner, detect the characteristics of emitted radio waves in order to determine the presence of excess moisture. These meters detect the amplitude of returning waves, which is diminished when they come in contact with water.
The actual depth that these waves travel varies based on the material’s properties and the device’s settings, but they generally penetrate from 1/2-inch to 3/4-inch beneath the surface and are unaffected by surface moisture. Unlike the pin type, this mode of operation arrives at a relative value for moisture content that must be calibrated, using an external equivalency table in some models. The meter will display moisture content as a scale of color-coded lights that indicate whether the material is damp, dry, or in a borderline condition. In other models, such as those made by Wagner, the default setting can be used to approximate moisture content in most materials, although dense materials, such as cement, will require adjustment of the device’s controls. In addition, Wagner's meters take a three-dimensional moisture average of the wood, which decreases the likelihood that intermittent wet spots will be missed.
Search mode is commonly used in the following locations:
  • the sides and the base of a tub or shower. Any penetrations, such as faucets, shower heads and soap dishes, are likely locations of water leaks. The water can originate from internal plumbing behind the wall, or from the shower itself.
  • water that has escaped from a dishwasher into surrounding kitchen materials.
  • the sub-floor beneath a bathroom’s tile floor. Water intrusion can cause enough damage there that the toilet becomes detached.
  • peering behind a wall or floor covering, such as a vinyl floor or a tile wall.

False Readings
If metal is present within the penetrating range of the meter, it will alter wave characteristics in ways similar to water. The meter will report levels of moisture that are higher than the actual level of the material if it detects a copper wire, a metal pipe, or some other metallic substance. If an inspector suspects that the meter is sensing metal, s/he can monitor the readings as s/he moves the meter in a straight line away from the elevated reading area. The straight outline of a copper wire or metal pipe can usually be traced in this fashion.
In summary, moisture meters are capable of detecting moisture levels in most building materials. They are useful tools to have during home inspections because they can calculate the properties of inaccessible locations without causing them any damage. Two types of moisture meters are available, sometimes in the same model.