Moisture in Gypsum Panel Products
The number of inquiries to the Gypsum Association about water-damaged gypsum panels always increases during the winter, early spring, after hurricane season or after any other significant water exposure event. Answers to many of these questions are found in an Association document revised in 2019, GA-231 Assessing Water Damage to Gypsum Board.
Gypsum panels can occasionally be subjected to moisture. Sometimes, this exposure is unintentional and due to a flood or hurricane. On occasion, a panel is intentionally dampened so that it can be used to create a curved surface. In each case, gypsum panels must not be overexposed to excessive moisture levels.
Once exposed to moisture, a panel must be evaluated. It is the recommendation of the Gypsum Association and its member companies that panels exposed to water should always be replaced unless all of the following conditions are met:
• The source of the water is identified and eliminated.
• The water to which any gypsum panel was exposed was uncontaminated.
• The gypsum panels can be dried thoroughly before mold growth begins (typically 24 to 48 hours depending on environmental conditions).
• The gypsum panel is structurally sound and there is no evidence of rusting fasteners or physical damage to the framing that would diminish the physical properties of the gypsum panel or system.
All of this is immaterial, however, when it comes to panels that have been exposed to floodwater. Since floodwater is almost certainly contaminated with extremely high levels of bacteria and pollutants, the industry recommends that gypsum panels exposed to floodwater shall be replaced. The same is true if it is believed that a panel has been exposed to sewage or wastewater.
All of the above is contained in Gypsum Association document GA-231 Assessing Water Damage to Gypsum Board, along with recommendations for creating acceptable drying conditions and a list of additional information resources. Quite simply, if there is ever doubt about whether to keep or replace gypsum panels that have been exposed to water, replace them.
Beyond a flood or natural disaster, gypsum panels can be exposed to water in a number of ways, including improper installation, missing flashing, ruptured pipes, bathtub overflows, and general water leaks. The Association does periodically receive requests for instructions on how to establish the moisture content of gypsum panels if they have been subjected to potentially damaging levels of water exposure. These generally involve the use of moisture meters.
Moisture meters operate on the principle that the electrical resistance and dielectric properties of materials vary consistently with moisture content changes. Calibrating a moisture meter to a substance such as wood is relatively straightforward because wood has relatively uniform composition throughout the thickness. Moreover, calibration curves are often quite consistent for different species of wood, (i.e. between spruce, pine, or fir).
Using a moisture meter on gypsum panels presents a significantly different physical situation. In this case, volume includes two materials with quite different responses to moisture. It is important to recognize that, unlike lumber, a gypsum panel consists of an inorganic core comprising the bulk of panel thickness with thin paper or glass mat facers laminated to the front and back of the panel. The measurement volume includes a thin hygroscopic layer (in the case of a paper facer), and a much larger volume of inorganic mineral (the gypsum core). These materials have quite dissimilar moisture adsorption and electrical properties. As such, a moisture meter calibrated using the techniques commonly applied for wood can yield misleading results for quantifying the moisture content of a gypsum panel. Similarly, the facer of glass-mat gypsum panels affects moisture meters differently than the paper facer on gypsum board. Further, the hydrophobic additives incorporated in the paper and core of water- and mold-resistant gypsum panels can introduce even more complications.
The use of a hand-held moisture meter for field measurement of gypsum panel moisture content can provide a useful means of qualitatively mapping out the wet versus dry areas to identify areas of concern in a moisture-damaged building. These meters can also be used for determining when the drying process is complete. However, unless extremely careful laboratory calibrations have been conducted for the specific types of gypsum panels being tested, a moisture meter will not provide reliable readings of the actual gravimetric moisture content of a panel.
The moisture content of gypsum panels will vary with the temperature and percent relative humidity of their surroundings. Until equilibrium is established between a gypsum panel and the environment, the moisture content of a gypsum panel will fluctuate. Paper and glass facings will have a different response to changes in the environment than the gypsum core resulting in different moisture levels for the materials. As each component seeks its own equilibrium, the total moisture of the panel will fluctuate. Consequently, acquiring an accurate and reliable moisture reading from gypsum panels using a hand-held moisture meter is a challenge. For this reason, following a laboratory testing method that uses very sensitive equipment is the most reliable method to determine the true moisture content of gypsum panels.
However, a hand-held moisture meter, operated by a qualified and properly trained professional, can be a useful tool. A moisture meter properly calibrated for the specific material being measured can provide useful data on a wide variety of materials. However, several additional factors support the gypsum industry’s cautions concerning attempts to use traditional hand-held moisture meters to measure the actual moisture content of gypsum panels. Although there are moisture meters available on the market that offer a setting for gypsum , as explained above, local fluctuations in ambient relative humidity can result in readings that more reflect the moisture content of the surrounding conditions than that of a gypsum panel. In addition:
• A moisture meter shall be calibrated for the specific material being tested to provide an accurate reading, and the operator shall carefully follow the instrument’s operating instructions for the information to be considered valid. For example, it is generally recognized that a moisture meter calibrated to measure the moisture content of a specific species of wood will not necessarily provide an accurate reading for any other species of wood. Certainly, a meter calibrated for wood will not provide accurate readings for gypsum panels. The reverse scenario is also true.
• Standard laboratory tests using convection ovens have been conducted to measure the actual moisture content of gypsum panel specimens to specifically compare the results to moisture meter readings on the same specimens. In some of the comparison tests, the original rank ordering established using the moisture meters was reversed by the oven tests when the actual moisture content difference was very slight. For a high degree of accuracy when measuring the moisture content of gypsum panels, it is generally recognized that laboratory test procedures provide superior results.
• Standard convection oven tests must be conducted under carefully controlled conditions. The ovens cause the free water in gypsum panels to evaporate thus enabling the moisture content to be calculated by the difference in weight of the specimen before and after its exposure to heat. If the ovens are improperly operated, causing the temperature to go too high or the sample to heat for too long, the water of hydration that is part of the gypsum molecular structure are also released, invalidating the results. Driving off the chemically combined water in the gypsum core with heat is technically referred to as “calcination.” An understanding of the chemistry of gypsum and the calcination process is necessary to accurately measure the moisture content of gypsum panels, even under laboratory conditions with trained technicians.
The presence of other materials, such as salts or carbon-based materials, can influence the results of moisture readings because they will absorb/desorb water at different rates than pure gypsum. In addition, these materials may be dissolved in the water and will be left behind as the water evaporates, altering the measured weight change in the gypsum specimen.
Refer to ASTM C1789 Standard Test Method for Calibration of Hand-Held Moisture Meters on Gypsum Panels for the appropriate systematic procedure to calibrate a hand-held moisture meter for use on a specific type of gypsum panel product.
When considering this information, it should be clear that hand-held moisture meters provide “relative” moisture content or a “rank ordering” of moisture contents between gypsum panels in one area of a building when compared with that in another area of the building. Moisture readings of gypsum surfaces in separate areas of the same building can indicate which surface is “wetter” but will not reliably indicate “how much wetter.” Relative rankings may not reflect the actual moisture content of the gypsum core and the facings in the test areas.
A hand-held moisture meter can be used to determine the relative moisture content of a gypsum panel surface but not the absolute amount. For instance, hand-held moisture meters can be used to identify areas of gypsum panels that are relatively “moist” and areas that are relatively “dry” after a building has been exposed to excessive amounts of water. Such rank ordering may be done by measuring areas believed to be dry and comparing readings areas that are assumed or known to have elevated moisture. Because they seem to be most accurate and reliable when sizeable deviations in moisture levels occur, moisture meters shall be used with caution when attempting to differentiate between small variations in moisture content.