Many mold inspectors use moisture meters to find wet areas where mold may be growing. These meters measure the moisture in many types of building materials (see Mold Problems From Wet Framing Lumber During Construction). They also can monitor the process of drying these materials. Moisture meters help you figure out whether your building materials are too wet. They typically use electrical conductivity between a couple of metal pins to assess moisture content; the more moisture in the material, the greater the electrical conductivity. The moisture content is measured as a percentage of total weight.. Moisture meters can be used on carpet, wallboard, wood, brick, and concrete. Because black mold often grows where moisture is high, a moisture meter can help an investigator locate hidden areas of mold growth.
In mold inspection two types of meters are used the most, surface and pin — we recommend them for builders to identify if there is a moisture problem, a potential toxic mold problem, and how widespread it is.
Surface Mold Meters, Pinless or Search Mode
Surface mold meters are good because they don’t punch holes. These units are best for “seeing” moisture behind a solid surface like drywall or ceramic tile, and are also effective for locating defective plumbing, condensation, or even standing water accumulated somewhere like in a ceiling. It works like a stud finder and can read about 3/4 inch deep. The surface meter delivers dependable readings in many situations, which it determines through measuring the resistance in the conductivity field between the pads on the sole of the tool. Note that pipes, nail heads, and corner bead can register on the meter, and they don’t necessarily mean water is present, so double check by moving the meter to a dry stud bay nearby to get an accurate baseline reading.
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 ½” to ¾” 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, showerheads 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.
Pin Mold Meters
For reading to a greater depth below the surface use a pin meter, which provides the dependable readings needed to formulate an analysis of what’s happening to a building. Pin meters are suitable for testing the moisture content of everything from framing lumber to concrete and stucco systems. They’re also effective for finding damp spots on, in, under, or behind other materials like gypsum board, carpet, hardwood floors, and plaster and are especially useful for testing baseboards. Not only can a pin meter tell if a baseboard is wet, it can tell how wet it is based on its Percent Scale. I know that if I punch holes in the baseboard and find a reading over 16 percent, there is a good chance mold could be part of the moisture problem (mold won’t grow in moisture levels less than 16 percent). I also suggest using pin meters to make sure your framing is dry before you close it in with drywall — make sure it is less than 16 percent. You can also use it to make sure wallboard is dry.
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.
Avoiding 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, they can monitor the readings as they move 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. Meters are a small investment when one considers the potential cost of mold removal from structural materials and professional mold abatement.
You may also want to refer to How to control humidity, Understanding home moisture and What is moisture?
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