The number of inquires to the Gypsum Association about water-damaged gypsum board always increases during the winter and early spring. What is interesting is how answers to many of these questions can be found in the content of one of the first columns we wrote for Walls and Ceilings and in an association document last revised in 2003.

The number of inquires to the Gypsum Association about water-damaged gypsum board always increases during the winter and early spring. What is interesting is how answers to many of these questions can be found in the content of one of the first columns we wrote for Walls and Ceilings and in an association document last revised in 2003.

Gypsum board surfaces are occasionally subjected to moisture. Sometimes, this exposure is unintentional and due to a flood or hurricane. In other instances, the board is intentionally dampened so that it may be used to create a curved surface. In either case, gypsum board must not be overexposed to excessive moisture levels or prolonged dampness if it is to perform as intended.

Once exposed to moisture, the board needs to be evaluated. It is the recommendation of the gypsum board industry that board exposed to water should always be replaced unless all of the following conditions are met:

  • The source of the water or moisture is identified and eliminated.
  • The water or moisture to which the gypsum board was exposed was uncontaminated.
  • The gypsum board can be dried thoroughly before mold growth begins (typically 24 to 48 hours depending on environmental conditions).
  • The gypsum board is structurally sound and there is no evidence of rusting fasteners or physical damage that would diminish the physical properties of the gypsum board or system.

All of this gets tossed out the window, however, when it comes to board that has been exposed to floodwater. Since floodwater is almost certainly contaminated, it is the recommendation of the industry that gypsum board that has been subjected to it must be replaced. The same is true if there is a suspicion that the board has been exposed to sewage or wastewater.



Exposure levels

All of the above is contained in Gypsum Association document "GA-231-03, 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 a doubt about whether to keep or replace gypsum board that has been exposed to moisture or water, replace it.

Beyond a flood or natural disaster, gypsum board can be exposed to moisture via a variety of situations, not the least of which are improperly installed or missing flashing, pipe ruptures, bathtub disasters and general water leaks. Warnings to avoid exposure of gypsum board to elevated moisture levels are normally followed by builders and contractors; however, the association does periodically receive requests for instruction on how to establish the moisture content of gypsum board once it has been subjected to potentially damaging levels of water or moisture. These generally involve the use of moisture meters and their relationship to the typical readings obtained when used with gypsum board.

Because of the internal chemistry of gypsum and the measurement of moisture content in gypsum board, some background information on moisture content and comparisons with wood may provide insight. Furthermore, in order to respond judiciously to the implications behind questions about moisture in gypsum board, a brief discussion of what is meant by the "normal" moisture content of gypsum board should also be helpful.

Common construction lumber products may have up to 19 percent moisture content and still be considered "dry." This moisture content percentage is determined by weight. Because of wood's lower weight (by volume) when compared to gypsum board, wood that holds 19 percent moisture contains a relatively smaller amount of water than would gypsum board with the same percentage of moisture. Gypsum board generally is denser than dry lumber; hence, 19 percent moisture content by weight for gypsum board is a considerably greater magnitude of water than is in the same volume of wood with 19 percent moisture content. (In fact, it is very likely that the gypsum board would no longer resemble a rigid panel at such a moisture level.)



Moisture content

The moisture content of gypsum board is quite low. Because of this normally low moisture level, the moisture content found in gypsum board's core will fluctuate as it attempts to maintain equilibrium with the moisture content present in the surrounding air. As the relative humidity changes, so will the moisture content of gypsum board as it seeks to maintain this equilibrium. Consequently, acquiring an accurate and reliable moisture reading from gypsum board 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 board.

This is not to say that a hand-held moisture meter operated by a qualified and properly trained professional is not a useful tool. A moisture meter properly calibrated for the specific material being measured can provide useful data on a wide variety of materials. But several additional factors support the gypsum industry's cautions concerning attempts to use traditional hand-held moisture meters to measure the moisture content of gypsum board. Although there are moisture meters available on the market that offer a setting for gypsum board, as explained above, localized fluctuations in ambient moisture or humidity can result in readings that more reflect the moisture content of the surrounding conditions than that of the gypsum board. In addition:

  • A moisture meter must be calibrated for the specific material being tested for it to provide an accurate reading, and the operator must 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 other species of wood. Clearly, a meter calibrated for wood will not provide accurate readings for gypsum board. The reverse scenario is also true.
  • Standard laboratory tests using convection ovens have been conducted to measure the actual moisture content of gypsum board specimens specifically to 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 board, it is generally recognized that laboratory test procedures provide superior results.
  • However, even standard convection oven tests must be conducted under carefully controlled conditions. The ovens cause the free water in gypsum board 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 cook too long, the water of hydration that is part of the gypsum molecule is 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 measure accurately the moisture content of gypsum board, even under laboratory conditions with trained technicians, if the results are to be valid.
  • The presence of other materials, such as salts or carbonaceous materials, on the specimen being tested can have an effect on the results of a moisture reading. Salts or carbonaceous materials will release water at different rates than pure gypsum or may be contained in the evaporating water, altering the measured weight change in the gypsum specimen; salts frequently are left behind when water evaporates.

When considering this information, it should be readily apparent that hand-held moisture meters provide "relative" moisture content or a "rank-ordering" of moisture contents between gypsum board 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 board core and the surface paper in the test areas.

A hand-held moisture meter may be used to determine the relative level of moisture content of a gypsum board surface but not the absolute amount. For instance, hand-held moisture meters may be used to identify areas of gypsum board that are relatively "moist" and areas that are relatively "dry" after a building has been exposed to excessive amounts of water or moisture. Such rank-ordering may be done by measuring areas believed to be dry and comparing readings areas assumed to have elevated moisture. Because they seem to be most accurate and reliable when sizeable deviations in moisture levels occur, moisture meters must be used with caution when attempting to differentiate between small variations in moisture content.


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