How To Use Moisture Meters

The increase of water intrusion and other moisture related problems in construction have made the use of moisture meters more popular than ever. However, one must understand that a moisture meter is merely a tool and provides one piece of information to assist the user in reaching an assessment. It is also critical for the user to be aware of how moisture meters work.

There are two basic types of moisture meters: surface (scan type) meters and probe meters.

Industry professionals generally agree that scan-type, or non-intrusive, meters are not terribly reliable and can give readings that may mislead the user or give a false sense of security. They are popular with home inspectors because there is no need to probe or penetrate the cladding material to get a reading. This method is also appealing to building owners, who do not want holes put into their walls.

The probe-type meter is much more reliable. As the name implies, the probes must touch the material that is to be tested for a reading. These meters work on the basic principle of electrical conductivity. In other words, the more moisture in the wood, plaster, cement or gypsum, the more conductive it is.

Probe type meters can also give misleading information. For example, if a probe-type meter were to touch a metallic object, the reading would be 99.9 percent, which could lead an investigator to assume the wall is completely soaked. With cement plaster, it is not uncommon to accidentally touch the metal lath and get a 99.9-percent reading while the wall is dry.

Most probe-type meters come with a separate attachment of long probes to get behind thicker cladding materials. An important factor often overlooked with the long probes is the insulating coating on the probes. This insulation protects the probes from getting false readings from materials that could touch the sides of the probes.

What is Dry?

Wood is considered “dry” at 19 percent or less moisture content per the Canadian Wood Council. Wood that has a 28-percent moisture content will have wood fibers that are completely saturated. When the moisture content of wood is above 28 percent for prolonged periods of time, decay will begin. Indoor wood will stabilize at 8 to 14 percent, while outdoor wood stabilizes at 12 to 18 percent.

On a recent investigation into an EIFS project, readings were 99.9 percent, and some assumed the walls were saturated. Further investigation discovered that a drainage mat that incorporated a foil type backing had been used on the building. This produced intermittent readings of 14 to 16 percent (normal) and then suddenly a 99.9-percent reading when the probes touched the metallic foil.

Another concern is the use of the long probes. The length of the probes provides the opportunity for the sides of the probes to touch objects and give false readings.

The probes come from the factory with a thin insulating paint coating with only the tips exposed. However, the thin coating will wear off relatively quickly, and you can get false readings. To re-insulate the probes, some inspectors use the same plastic coating used to protect electrical wires or a thin straw. Slip on the protective cover that is slightly larger that the diameter of the probe and apply some heat. This will cause the plastic to melt and shrink and to adhere to the probes to protect them.

Courtesy of EIFS Alliance