It's Called Drywall For a Reason

Donald E. Smith, CCS

June 2006

We have an inspector on a residence that is telling us that the moisture content of the drywall is 5 percent when his data indicate that the moisture content should be 2.5 percent. He has determined the moisture content using a non-destructive moisture meter. Is there an industry standard that specifies the moisture content of drywall panels?

In researching your problem I spoke with several technical experts who work for drywall manufacturers, most of whom serve on ASTM Committee C11, which deals with gypsum products. I also had a conversation with the technical director of the Gypsum Association. The question I posed to them was, Is there a technical standard or industry standard defining the acceptable moisture content of drywall? They responded with the same answer: No. In fact, one of them responded by saying, "Why do you think we call it drywall?”

I know that this response does not answer your question in the manner you expected, so let’s see if I can come up with some background to help you and your general contractor.

First, I would inquire about the document the inspector used to determine that 2.5 percent moisture content in drywall is acceptable, since there is no such ASTM standard that specifies acceptable moisture content.

Second, you say the inspector used a non-destructive type of moisture meter. These types of meters use radio frequency waves to determine the presence of moisture. One of the problems with these meters is the possibility of operating in an environment where there is a lot of stray radio frequency energy. An example could be cell phones or two-way radios operating in the immediate vicinity where the testing is being conducted. Even if a probe-type meter was used, there are still potential problems in using these devices to determine moisture presence specifically in drywall.

After researching the Web sites of several moisture meters manufacturers, the one common factor is that they all refer to a "relative reading” for drywall. This means that regardless of the type meter used, the inspector must have a control sample of the material being tested. Also, when relative readings are used, they bear a relative relationship to each other and have no empirical or absolute value. The person taking the reading must make an interpretation about the difference of the reading between the control sample and the reading taken on a wall in place. When the control sample reads 0 and the wall reading is 10, it only means that the device determined that the wall reading is 10 times the control sample.

Another factor to consider is the moisture content of the space where the readings are being taken as well as the makeup of the components of the wall assembly. All too often a material is singled out as a culprit when a problem occurs. An example is drywall over wood studs. What happens to the drywall when it is installed over wood studs? An industry standard specifies that wood studs have a moisture content of 19 percent. The drywall installed directly over the wood stud will absorb moisture from the wood stud because of the drywall’s lower moisture content. That said, in the correct environment the drywall and wood will balance out and not present a problem on the finished surface or affect the integrity of the drywall.

Attempting to determine the moisture content of installed building materials is a direct result of mold problems. Mold problems generally are caused by the introduction of water or water vapor into a building without a way to evaporate into the atmosphere.

The root of your problem, I think, is an overzealous inspector using what he considers to be the state-of-the-art in determining the presence of moisture. If there really is moisture in the wall assembly, it will manifest itself in ways that are very apparent to the unaided eye, generally in the form of discolored finishes or deteriorating wall surfaces.

About the Author
Donald E. Smith, CCS, is AWCI’s director of technical services.