Extending the Reach of a Moisture Meter

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Typical pins on moisture meters are ½ inch long, meaning you can only determine moisture content by weight near the surface of building assemblies and materials (including wood, gypsum wallboard, and concrete). But I often find myself needing to assess moisture content of first condensing surfaces in walls and ceilings or well below the surface of basement slabs. This article looks at ways to extend the reach of a moisture meter. (For introductory information on moisture meters, see Tools of the Trade: Moisture Meters.)

Extending reach in finished framing cavities

You can buy meter accessories that extend your reach; the Delmhorst 21-E/001 accessory has 6-inch Teflon-coated pins. (The Teflon coatings ensure that the only conductivity is at the exposed tips of the pins.) But sometimes I need to reach even deeper than 6 inches (see Image #2, below). And the 21-E/001 is not inexpensive; it retails for $105.

I checked in with Delmhorst on my own approach: two 3/16-inch rods (about $1.50 each) sharpened on a grinder wheel that I drive exactly parallel to each other. Tom Laurenzi, president of Delmhorst, agreed that this approach could work (see caveats and conditions below).

In Images #3 and #4 you can see that we are driving through the closed-cell spray foam till the pin-extenders hit the rim joist. In this case it does not really matter that the length of the pins is not insulated; the spray foam is non-conductive.

But when driving through gypsum wallboard (GWB) and cavity insulation to measure exterior sheathing moisture content, it is possible for the GWB and cavity insulation — particularly if either is at all wet — to affect the measurements because they can have their own conductivity. (See Images #5 and #6.) For the roof in Image #6, I needed 20-inch pin extenders: 15.25 inches of penetration and the remaining 4+ inches to make sure that the pins were actually parallel and the pin tips contacting the sheathing the same distance apart as the pins on the Delmhorst meter.

One way around this is to drive the pins just into the GWB and switch the meter to Scale 3, the setting for GWB, and assess the impact of the GWB based on that reading. Similarly, while driving the pins into the cavity insulation, you can switch the Delmhorst meter to Scale 2 (more on this scale below), and use this relative scale to assess just what, if any, impact the conductivity of the cavity insulation has on the ultimate moisture content reading — the reading on on Scale 1 — of the pins in the exterior sheathing .

The coat hanger trick

Another way to qualitatively assess the moisture content of the cavity insulation is to bend a hook into an opened coat hanger and insert the hooked coat hanger through one of the 3/16-inch holes in the GWB and twist the coat hanger until it snags some cavity insulation, then pull the coat hanger hook out to assess — by eye and by touch — the moisture content of the cavity insulation.

Another way to deal with the possible conductivity of the GWB and cavity insulation is to very carefully and tightly wrap the length of the rods with electrical tape. This is pretty tedious and you are going to have to re-wrap maddeningly often unless you penetrate the GWB with unwrapped rods first and round the holes out a bit to accommodate the slightly thickened tape-wrapped rods.

I spent some time looking for some sort of Teflon coating or spray paint so that I could coat the rods myself with this insulating material, but now conclude that Teflon coating or painting is strictly an industrial, not a do-it-yourself, process.

Extending reach in concrete

The Delmhorst BD-2100 has Scale 2 (also called the 0 – 100 Reference Scale); information on Scale 2 can be found on page 9 of the Delmhorst BD-2100 manual. While Scale 1 (wood) and Scale 3 (gypsum) yield quantitative measurements as a percentage of moisture by weight, Scale 2 is a unit-less relative scale. You would think that % moisture content by weight for concrete could be had, but in fact concrete mixes are so variable that only qualitative assessment of moisture content is possible with pin-type moisture meters. But this also means that Scale 2 can be used on a pretty wide variety of materials.

Sticking with concrete: you can’t drive the meter pins directly into concrete without damaging them, so to get a surface reading you drive 2 masonry nails 1/8-inch into the concrete, ideally first in an area you know to be relatively dry to get a “baseline” comparative reading, and then range from there to see how the moisture content compares with all readings being between 0 and 100 (see Image #7). The problem is that if the rate of moisture movement up through the concrete is slow, the top layer of concrete can be quite dry, masking the moisture deeper in the concrete. In fact, ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes), requires that moisture measurements in concrete be taken 40% of the depth of the concrete. For a typical basement slab, that means taking measurements at least 2.5 inches deep (see Image #7). But how do you keep the masonry nails from contacting the sides of the drilled hole, short-circuiting your slab moisture measurements?

My solution is to use powder-actuated gun masonry nails, with their plastic sleeves ensuring that the nail only contacts the concrete at its tip (see Images #8 and #9). Note that dust in the hole will inherently be dry from the drilling, so it is important to make sure that the nail tips are contacting the solid concrete at the bottom of the drilled hole. I often wait an hour after drilling to allow the concrete at the bottom of the hole to “recover” to a more representative moisture content. It's possible to quantitatively measure the moisture content of concrete using concrete RH sensors and ASTM F2170. For more information on this approach, see this blog by Paul Laurenzi of Delmhorst.