To Use Moisture Meters
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
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
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.
of EIFS Alliance