In the August issue of Cleanfax magazine, we discussed how to select an appropriate meter type for the measurement needed in a water damage restoration project.

In this article, we will tackle the more advanced application of the most critical instrument: The invasive moisture meter (also referred to as “penetrating” or “destructive”).

For many reasons, invasive meters are the most important instruments in your kit — the most significant of which is that they are the most accurate. Not only do invasive meters reflect the most reliable moisture measurement, but they also allow the user to get a “feel” for the structure and how moisture has affected it.

Proper use of an invasive meter can reveal the true depth of the material, expose hidden layers and structural anomalies and provide a clear understanding of the drying challenges at hand.

Invasive meters: Basic operation

Before we discuss the application of this critical instrument, let’s first explore the technology.

Invasive meters measure electrical resistance. In most materials, resistance varies consistently with moisture content. Generally, the more conductive the material, theinvasive meter more moisture is present. This is based on the simple fact that most building materials have virtually no ability to conduct electricity when dry. Because water is a good electrical conductor, electrical resistance will decrease when water is present in a material.

Meter manufacturers have spent significant time analyzing the relationship between this electrical resistance and the actual moisture content of specific building materials, especially many species of wood. Most invasive meters display moisture content values based on this analysis.

It is important to note that the electrical properties of building materials vary from one type to another, at different temperatures and even with variation in species, static electricity and orientation of the pins against grain and other material properties.

Remember that the moisture content value is only an approximation.

Note that some probes are electrically shielded along their shafts, leaving only the tips exposed. This allows the user to measure moisture content at a specific depth in the material.

Invasive meters: Advanced application

Once you understand the basic operation of your invasive meter, you can maximize its use with some very simple steps. Remember that invasive meters use metal rods or pins to measure electrical resistance, which is commonly translated to wood moisture content. But, why be limited to the metal pins and rods that come from the manufacturer?

In many cases, materials that need to be inspected or monitored are located in hard-to-reach areas, such as beneath cabinets, behind shower enclosures, or worse — only accessible through confined spaces like a crawlspace.

In these cases, having metal probes with greater length or flexibility can be significantly advantageous.

Here’s a list of a few common pieces of hardware that could be used as the “metal pin” to help you get more leverage from your invasive meter.

Wood screws

Common wood screws come in a good range of length, and can readily penetrate many building materials. A pair of inserted screws allows easy access to wall plates, subflooring and a broad variety of structural assembly components.

Even better, once inserted, they allow a technician to easily measure an identical location on each subsequent monitoring visit.

To ensure success, insert the screws approximately one to two inches apart using non-coated screws. Make sure the screws do not become a trip hazard.

Connect to the wood screws by making contact with your manufacturer-supplied pins (screws will have to be the exact distance apart as the probes), or use alligator clips (more information on this is supplied later in this article) to attach to them.

Drill bits

For greater length and easier penetration, consider using drill bits. These are available in a variety of lengths and bit types, even for use in concrete.

As with wood screws, ensure they do not become a hazard if you plan to leave them in place for multiple monitoring visits. Mark them clearly and don’t insert them into floors in traffic areas.

Speaker wire

To leave wood screws in hard-to-reach areas, use speaker wire with crimp-on ring terminals. I’ve tested this using 18 gauge wire at up to 100 feet in length, with great success.

Insert two wood screws through the ring terminals, and run the speaker wire to an easy-to-access area. Strip the wire insulation off the last one-half inch of wire, and you can now check that spot under the tub in the crawlspace the next day without re-entering the crawlspace!

Alligator clip accessory

alligator clipsIf this accessory didn't come with your meter, you can manufacture your own quite easily.

Meter manufacturers use commonly available connectors on the meter to connect meter attachments. These are either 1/8-inch mono headphone jacks, RCA connectors or standard screw-down CB radio connectors. Find out what kind of connector your meter has, and then visit your local hardware store.

Your shopping list will include the male counterpart of the connector on your meter, a few feet of 18 gauge or better
wire, and a couple of alligator clips.

Assembly is pretty straightforward, but if you need help, my e-mail is in my biography.

Valuable time management

These are just a few of the hardware examples I’ve investigated.

It’s truly amazing how much time and energy these techniques save over the duration of the monitoring project, and how often they’ve helped me find moisture while minimizing strucutural dissasembly.

Happy monitoring!

Brandon Burton is the technical education manager for the Restoration Sciences Academy (RSA), a part of Legend Brands. He teaches IICRC-approved classes in the categories of Applied Structural Drying (ASD) and Water Damage Restoration (WRT). Burton has served the restoration community for more than 15 years as an IICRC-approved instructor, ANSI/IICRC S500 chair, RIA restoration council member, and many other industry roles. You can contact him at