What Are Safe Moisture Levels in Walls?

Posted by PAUL LAURENZI on Feb 28, 2018 11:48:00 AM

One of the most common questions people have about moisture measurement is “What are safe moisture levels in walls, floors, etc.?”

The answer is: “It depends.”

Types of Wall Materials and Moisture

What constitutes a “safe” level of moisture in a wall will change depending on what the wall is made of. In most modern construction, interior walls tend to use a surface layer of gypsum (a.k.a. drywall) because it’s a relatively inexpensive, lightweight, and long-lasting material that’s easy to work with. However, older buildings may use other interior wall surfaces, such as wooden paneling or plaster.

Generally speaking, for drywall, a safe moisture content (%MC) would be less than 1% MC. Anything above 1% MC in drywall would indicate a level of moisture that could compromise the integrity of the gypsum board.

Exterior walls may use a variety of materials, from wooden siding, to vinyl, aluminum, brick, and stone. Of these exterior wall materials, wood is generally the most susceptible to moisture—which is why wood siding is usually treated to resist rain. However, water repellent finishes can only prevent moisture from getting past the surfaces where the wood has been treated. If water reaches an untreated surface, such as the backside of the wood, then it can still be absorbed and cause problems.

With wood, it can be harder to generalize what a safe amount of moisture is. This is because the humidity conditions can have an effect on what the ideal moisture content of wood can be—not to mention that “safe” moisture levels can vary from one species of wood to the next.

How Humidity Impacts “Safe” Moisture Levels

The ambient humidity and temperature conditions where your walls are located can have an effect on what could constitute a “safe” amount of moisture in the wall when dealing with hygroscopic materials like wood. If wood wall materials aren’t in equilibrium with their surrounding environment, then they will absorb or let out moisture until they are—which can cause swelling or shrinkage that impacts the look and integrity of the wood.

So, knowing the relative humidity (RH) conditions is a must when trying to determine what a “safe” moisture content for wood walls is. For example, if the temperature in the room is around 80 degrees Fahrenheit, and the RH is 50%, then a “safe” level of moisture in the wall would be about 9.1% MC.

Measuring Moisture in Walls

BD-2100-1 moisture meterSo, how can you be sure if the walls in a structure have a safe level of moisture? One way to check is to use a moisture meter that is optimized for building inspection work like the BD-2100.

This particular moisture meter is ideal for checking the moisture content of walls because it has reading scales for both drywall and wood that allow for precise, quantitative moisture measurements in these materials. A separate reference scale setting can be used for getting qualitative readings of moisture in other wall materials (like plaster).

The BD-2100’s drywall moisture meter readings are accurate in gypsum to moisture content percentages as low as 0.2% and as high as 50%. In the wood scale, the meter can detect moisture accurately over a range of 6% MC to 40% MC. The ability to detect moisture accurately in both wood and drywall makes this device particularly suited for building inspection work.

Learn more about measuring moisture by checking out Delmhorst’s restoration guide, which has information about the different kinds of moisture meters, the main causes of water damage, and how to best use moisture meters.

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