Meters come in many styles, measurement functions and/or
displays. To help clear up some of the confusion we submit
the following home inspector's view of use of these tools.
we do not recommend using a moisture meter throughout the
house on a "moisture finding mission." We use
the moisture meter as a confirmation tool only when the
area or surface looks, smells or feels beyond normal moisture.
order to use moisture meters to their fullest potential
one must first understand relative humidity (RH) and the
hygroscopic properties of wood.
first discuss RH. The warmer air, the more water vapor it
can "hold." Dew point is a measure of how much
water vapor is actually in the air. Relative humidity is
a measure of the amount of water in the air compared with
the amount of water the air can hold at the temperature
it happens to be when you measure it. The following are
basic physical facts regarding the capability of air to
hold moisture at certain temperatures:
degrees C (86F) - 30 grams (1.05 ounces) of moisture per
cubic meter (1.3 cubic yards) of air
20 degrees C (68F) - 17 grams (.6 ounces) of moisture
per cubic meter of air
10 degrees C (50F) - 9 grams (.32 ounces) of moisture
per cubic meter of air
numbers, which apply to air at sea level pressure, are based
on measurements over the years.
let's see how dew point and relative humidity work. Imagine
that at 3 p.m. you measure the air's temperature at 30 degrees
C and you measure its humidity at 9 grams per cubic meter
of air. What would happen if this air cooled to 10 degrees
with no water vapor being added or taken away? As it cools
to 10 degrees the air becomes saturated; that is, it can't
hold any more water vapor than 9 grams per cubic meter.
Cool the air even a tiny bit more and its water vapor will
begin condensing to form clouds, fog or dew - depending
on whether the air is high above the ground, just above
the ground, or right at the ground. Back at 3 p.m., when
we made the measurements, we could say that the air's dew
point is 10 degrees C. That is, if this particular air were
cooled to 10 degrees at ground level, its humidity would
begin condensing to form dew.
about relative humidity? At 3 p.m. the air has 9 grams of
water vapor per cubic meter of air. We divide 9 by 30 and
multiply by 100 to get a relative humidity of 30%. In other
words, the air actually has 30% of the water vapor it could
hold at its current temperature. Cool the air to 20 degrees.
Now we divide 9, the vapor actually in the air, by 17, the
vapor it could hold at its new temperature, and multiply
by 100 to get a relative humidity of 53% (rounded off).
As you can see, the cooler the air, the higher the relative
humidity at the same moisture level.
we need to understand that wood is hygroscopic (having the
characteristic of drawing moisture from the atmosphere).
Wood will typically be at a moisture percentage about 1/5th
that of the relative humidity of the air surrounding the
wood. It usually takes the wood 2-4 days to reach its equilibrium
in its environment. This is why wood flooring contractors
always have the material delivered inside the house days
prior to installation to allow the wood's moisture content
to equalize with the house environment. As an example, wood
measured with a percent content (pin) type meter reading
12% would indicate its surrounding environment is at a 60%
relative humidity. The humidity in conditioned spaces of
most typical homes is about 50-60% when cooling and 10-40%
when heating. The heating of the house tends to dry out
the indoor air, the reason why humidifiers are popular in
we need to understand the two basic types of moisture meters.
The high frequency type (non-pin) moisture meters (such
as the Protimeter Aquant) use radio waves to determine moisture
in a material. These meters use relative scales to show
the difference between wet and dry materials. This is comparative
from wet and dry rather than percent moisture content. These
meters are good to determine if an area is wet or dry, or
to search for moisture through a surface such as vinyl or
tile. These meters can be used on a wet surface to search
the material below as the amount of moisture on the surface
is minimal when compared to a dry substrate. Always take
a reading from the material in a location known to be dry
prior to searching a suspected wet area to have a comparison
pin type moisture meters (such as the Protimeter Mini) use
a small electrical current sent into the material between
the pins to provide actual percent moisture content, most
meters are typically calibrated for wood. Pin type meters
that do not indicate actual percent moisture content are
not valuable to home inspectors other than to verify moisture
on a comparison basis such as with the frequency types.
prefer the combination type meter, (such as the Protimeter
Survey Master) that provides both high frequency and pin
(current) type modes. These meters have a pin mode extension
for reaching difficult locations and providing visual access
to the percent meter. We use the high frequency mode on
the back of this meter to confirm wet stains or for searching,
and the pin mode for more advanced investigation of potential
high moisture conditions in exposed wood areas, such as
crawlspaces, basements and attics.
Example: If an inspector suspects inadequate attic or crawl
space ventilation, the inspector would use the pin (current)
mode on a Protimeter SM to investigate his suspicions. If
the wood moisture content is more than 18%, (remember wood
is hygroscopic) ventilation is not adequate or the ambient
air has a very high RH (possible in very humid climates).
Just stick the pins in the wood anywhere in the crawl or
attic to determine actual moisture content.
have recently heard of a story where an inspector, knowledgeable
about moisture meters and their use, used their moisture
meter to find a cracked heat exchanger. The inspector noted
condensation on windows in a basement. Knowing that wood
is hygroscopic, the inspector stuck a pin (current) type
moisture meter into a floor joist and it was only 12%. Something
was recently different as the condensation indicated a very
high relative humidity, and the wood had not yet reached
its normal moisture content for such an environment (probably
inspector inquired with the sellers if they had been doing
anything different in the basement in the last few days.
The sellers indicated they never run the furnace down in
the basement (normally they heat with wood) but had it on
because of all the people coming through the house. Knowing
that the products of fuel gas combustion included large
amounts of water vapor, the inspector went right for the
furnace. It turned out the furnace heat exchanger had large
holes and was a potential hazard to the occupants.
the above, and reading the instructions with your moisture
meter will make it an indispensable tool for the home inspector.
talk next month,
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