No process is more critical to successful structural drying than accurate and reliable monitoring of building materials.

There is no limit to the number of variables that can influence drying, many of which are unknown to the restoration contractor. For this reason, every drying professional needs to have a full complement of meters.

The invasive moisture meter is often the most valuable instrument in the restorer's kit. Used together with a hammer probe, the invasive meter can provide accurate moisture readings deep inside materials, allowing restorers to accurately assess moisture levels, establish effective drying environments and monitor drying progress.

How it works

Sometimes called a "penetrating" meter, the invasive meter allows for moisture measurement through multiple layers of materials, such as the wood sub-floors beneath carpet and cushion (underlay) or in the studs inside walls.

The invasive meter operates on a principle similar to the moisture sensor. Electrical circuits test the conductivity of the suspect material, and higher degrees of moisture result in a reading from the instrument that indicates the target is wetter.

The primary difference between invasive moisture meters and moisture sensors is that the meter can quantify the amount of moisture present, either on a relative or specific scale. This numerical value can then be documented and monitored for progress. For this reason, the amount of moisture absorbed by structural materials is measured most accurately with a moisture meter, not a moisture sensor.

Invasive moisture meters work with a variety of pin types designed for various building materials and construction methods. These pin types vary in length, diameter and tip shapes. The shafts of the pins are usually coated with a thin layer of electrical shielding, leaving only the tips exposed. This produces more accurate readings at depth.

Depending upon the intended medium to be inspected, one probe type may offer better results than another.

Here is a list of the various pin types and some of their most appropriate applications:

Short, tapered, non-insulated pins

These small probes —1/2-inch or 1 centimeter (cm) — are the most common size, and are helpful for evaluating materials such as baseboard or structural materials in the crawlspace.

They are also very useful in monitoring through thin surface materials, such as fabric on upholstered furniture. This pin type will leave the smallest hole.

For some meters, the invasive moisture meter is manufactured with this pin type permanently mounted attached to the meter housing.

Medium, straight, insulated pins These require the use of a hammer probe. These longer pins — 2-inches or 5 cm — are used to monitor structural wood and wood flooring.

The shafts of the pins are insulated — usually with a non-stick coating — while the tips of the pins are bare. This allows the meter to detect moisture levels at varying depths in the material.

Because the amount of moisture can vary greatly from one depth to another, the hammer probe is the most accurate for evaluating actual moisture levels in materials such as wood sub-flooring, wood flooring, structural plates, wall studs and similar materials.

When using this attachment to monitor strip hardwood flooring, drive the pins into the darker, "winter" portion of the wood grain. This is because the inspection holes are easier to repair in the dark wood and in the center of the board, which will prevent splitting the strip flooring.

Long, straight, insulated pins

Although the description is similar to the medium pins described previously, longer pins — 6-12 inches or 15-30 cm — are not used on wooden materials because they lack the strength necessary to be driven into them.

Long pins are used to evaluate moisture levels behind and beneath finished surfaces such as gypsum wallboard. In denser materials, such as cabinetry, holes must be pre-drilled. This pin type is especially effective when inspecting moisture levels in wall insulation.

Long pins can also be used to measure surface moisture on sub-flooring beneath cabinetry, gypsum wallboard behind cabinetry and wall plates behind baseboards. Two-inch insulated pins on a hammer-probe attachment can be driven into the wood beneath a carpet and pad (underlay).

Universal connectors

The best kits include a tool, usually a pair of leads fitted with alligator clips, which allows the restorer to use virtually any metal fastener or hardware as an electrode, such as long wood screws and metal rods.

This dramatically increases the flexibility of the technician to reach difficult materials while allowing stationary pins to be left in place and consistently checked each day.

Pin insulation

It is important to check the insulation on these pins frequently. As the pins are driven into structural material, the insulation will begin to wear.

To test the integrity of the insulation, place your finger and thumb on the outside edge of each pin, creating a "circuit" through your hand. Don't worry… it won't hurt.

Slide your finger and thumb up and down the pins while observing the meter readings. If the meter displays elevated numbers, the insulation has been compromised and one or both of the pins must be replaced. Failure to replace defective the pins will lead to inaccurate moisture content readings.

Brandon Burton is the technical education manager for the Restoration Sciences Academy, a part of Legend Brands. He has served the restoration community for more than 15 years as an IICRC instructor, ANSI/IICRC S500 chair, RIA restoration council member, and many other industry roles. You can contact Burton at brandonb@rsa-hq.com.