All Things Gypsum
The Moisture in Gypsum
Gypsum is a naturally occurring mineral that lends certain rock-like qualities to wallboard. Under extreme or sometimes even normal environmental conditions, gypsum board may become damp. It may also be intentionally moistened by drywall installers under controlled conditions in order to facilitate application of board to curved surfaces. Despite the possibility of encountering these conditions, gypsum board must not be repeatedly subjected to water, or prolonged high levels of moisture or dampness.
For many years, the manufacturers have cautioned that gypsum board should never be inundated by water, especially prior to application. This warning applies to the proper handling and protection of gypsum board when it is being shipped, stored, stocked or installed. Additionally, gypsum board and its systems must receive regular and routine maintenance following installation, especially in areas that may be exposed to periodic and unusually high moisture levels.
The moisture in boardThe gypsum industry’s strong recommendation that gypsum board not be exposed to water or exceptionally high levels of moisture is widely understood and generally heeded by manufacturers, builders and contractors. In spite of this warning, however, the Gypsum Association and the individual gypsum board manufacturers receive inquiries from time to time about the methodology of measuring moisture content of gypsum board that has become wet, or that has been exposed to high levels of moisture or excessive humidity.
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 gypsum board would with the same percentage of moisture. Gypsum board is generally 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.
The typical moisture content of gypsum board is fairly low (around 5 percent or less). At this low level, the normal propensity of gypsum board is for the moisture content of the core to seek equilibrium with the surrounding air. This ability to absorb moisture from the surrounding environment means the moisture content of gypsum board may fluctuate as the relative humidity changes. This characteristic of gypsum board makes accurate and reliable measurement of its moisture content very difficult, if not impossible, to attain with the average hand-held moisture meter. Because of the inherent difficulty in measuring moisture content at such low levels, the most dependable and consistent method is to adhere to a laboratory procedure that uses especially sensitive balances or scales.
Moisture metersA hand-held moisture meter can function as an excellent tool if it is properly calibrated for the specific material being tested and if the operator is professionally qualified and properly trained in the use of the meter for the material being measured. When used correctly by trained individuals, moisture meters can provide valuable information about the moisture levels in a wide variety of materials.
In addition to cautions in the discussion of moisture content above, the following reasons further support the gypsum industry’s caution concerning attempts to use traditional hand-held moisture meters to measure the moisture content of gypsum board.
Pure gypsum crystals contain approximately 20 percent by weight of chemically combined water. (This “water of hydration” is the primary property that provides the natural fire resistance of gypsum board but will not be detected by a moisture meter because it is not “free” moisture.)
Many moisture meters are intended to provide direct readings for the percentage of moisture content of a variety of specific materials. While some moisture meters have a specific setting for testing gypsum board, the moisture content typically encountered in gypsum board may make it difficult to derive the level of consistency and reliability that is necessary to produce an accurate assessment.
A moisture meter must be calibrated for the specific material being tested if it is to function properly and the trained operator must adhere strictly to the instructions if the information is 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, and especially a completely different material such as gypsum board.
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 rank ordering was reversed by the oven tests at certain moisture content levels 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 reliable results.
Convection ovens help evaporate the free water and enable the moisture content to be calculated by the difference in weight of the specimen before and after exposure to heat. However, the water of hydration (mentioned earlier) may also be driven off during the convection oven test if the oven is operated at too high a temperature or if the gypsum board specimen is left in the oven too long. This evaporation of chemically combined water in the gypsum core is commonly 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.
Moisture meter readings can also be affected by the presence of other materials, such as salts or carbonaceous materials, on the specimen being tested; salts frequently are left behind when water evaporates.
In view of the above, it is readily apparent that hand-held moisture meters are more appropriately used to provide a “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. Comparative tests of board in two different areas in the same building may determine which board is “wetter” but will not necessarily quantify “how much wetter.”
When trying to determine the moisture content of gypsum board, a hand-held moisture meter may be used to determine the relative level of moisture but not the absolute amount. For instance, hand-held moisture meters may be used to test gypsum board that has been exposed to excessive amounts of water or moisture in order to help identify the areas that are relatively “moist” and the areas that are relatively “dry.” This rank ordering can be accomplished by taking readings on areas believed to be dry and comparing the meter readings to 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. W&C