Dear Inspector,

Moisture 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.

First, 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.

In order to use moisture meters to their fullest potential one must first understand relative humidity (RH) and the hygroscopic properties of wood.

Let's 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:

30 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

These numbers, which apply to air at sea level pressure, are based on measurements over the years.

Now, 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.

How 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.

Next, 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 colder climates.

Now 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 baseline.

The 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.

We 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.

For 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.

We 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 90+percent RH).

The 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.

Understanding the above, and reading the instructions with your moisture meter will make it an indispensable tool for the home inspector.

We'll talk next month,

Mike Casey
Kaplan Professional Schools
Now You're Ready For Business!™