The study was conducted using six samples of each building material and a control placed in a polyethylene box (measuring 18 inches by 18 inches by 18 inches) equipped with a plastic lid. The control sample was elevated above the moisture source for exposure to elevated humidity only, while the test samples were placed vertically on a wet towel and individually supported using a wood frame. Water was added regularly to maintain wet conditions in the towel. The moisture content (Tramex Moisture Pro) and dimensional changes (Westward Caliper) in width were obtained at specific height intervals throughout the test period, and the interior test chamber conditions of relative humidity (93 percent RH) and temperature (76 degrees Fahrenheit) were recorded. Only measurements obtained at six inches above the moisture source are reported for simplicity. The moisture and dimensional measurements quantified the precise location of increasing moisture content and the corresponding change in width following moisture absorption from the base upwards to the top of the sample via capillary attraction.

The test conditions were intended to evaluate the worst-case scenario created during a single water-loss event. Test conditions were intentionally selected to represent on-going seepage or leakage over the entire test period.

1. Plywood

According to the Forest Products Laboratory, plywood is manufactured by bonding thin sheets of wood (veneers) together with adhesives so as to create a product with improved mechanical and physical properties. The classification of plywood is determined by the quality of its glue bond. The number of wood adhesives that provide an acceptable application for high moisture environments is low. When wood becomes wet, the critical factor that sustains the bond is the dimensional strains that occur between different layers of wood laminate. In this experiment, the 3/4-inch thick, 5 ply, B-C plywood chosen was an exterior grade underlayment, Type 1 with fully waterproof glue (phenol resin or melamine-urea resin) as specified by the American Plywood Association and The Engineered Wood Association.

After the 34-day experiment, the maximum dimensional change was approximately 9 percent (0.06 inch increase) and was attained after approximately two weeks. The width of the plywood samples remained relatively unchanged after two weeks of moisture exposure. The moisture content (Moisture Equivalent Concentration) increased from 9.5 percent (initial) to 35 percent (final) at six inches after six days of exposure.

The results show that efforts to measure dimensional changes must be made promptly after a water loss in order to confirm whether the loss is short-term (within 14 days). Moisture measurements showed that saturation was achieved quickly within 5 days of exposure. These results indicate that plywood is not a favorable material to evaluate dimensional changes or moisture content following a water loss because it increases rapidly in width; achieves limited expansion (4 percent); and absorbs water rapidly.

2. Oriented Strand Board (OSB)

Developed 25 years ago, OSB is an engineered wood-base sheet material where three layers of long strands of wood are bonded together with a synthetic resin adhesive. Usually two of the three layers are oriented in the same direction. Most manufacturers use melamine fortified urea (MFU) resin or isocyanate (PMDI) resin. In this experiment, the 7/16-inch thick, OSB (Georgia Pacific, Blue Ribbon, PS-2 bond), exterior grade was used in this study. OSB is typically used for flooring, flat roof decking, and wall sheathing.

This experiment yielded a maximum dimensional change of approximately 17 percent (0.07-inch increase) at the end of the study period. The moisture content increased gradually from an initial moisture content of 6.5 percent to 22 percent at a sampling height of 6 inches. Moisture saturation (+35 percent) was not achieved after 34 days of exposure. The results show that OSB is a good candidate for measuring dimensional changes following a moisture release because it gradually increases in both dimension and moisture content.

3. Particle Board

The Forest Product Laboratory identifies two general types of particle board panels. Type 1 panels are manufactured with an adhesive (urea-formaldehyde) that is water resistant, but not water proof. Type 2 panels are manufactured with a waterproof adhesive (phenol-formaldehyde). Within these two types, there are three grade or density specifications: H (high), M (medium) and L (low). Finally, a third number is used to designate the panel screw holding strength. A particle board panel with a grade of 1-H-1 denotes a panel manufactured with urea-formaldehyde glue, a high density (>50 lb/ft3) and a screw holding value of 400 pounds (board face). In contrast, a grade panel of 2-M-3 would designate a panel manufactured with phenol formaldehyde, medium density and a screw holding strength of 450 lb/ft3). Particle board is commonly used as a floor underlayment, shelving, stair treads and in furniture.

In this experiment, the non-faced, 5/8-inch, Type 1-M, particle board experienced an initial moisture content of 9 percent and gradually increased to 28 percent at a sampling height of 6 inches. The maximum dimensional change was approximately 11 percent (0.07 inch increase). The particle board width continued to increase slowly up to the end of the study. The results show that moisture saturation (+35 percent) was not achieved after 34 days of exposure. These results show that particle board as used in kitchen and bathroom cabinetry is a good candidate to measure dimensional changes and moisture content after a water release.

4. Faced Particle Board (Shelving)

The maximum dimensional change observed in the medium density, faced (melamine) 3/4-inch particle board (Type 1-L) was approximately 16 percent (0.127 inch increase). This faced particle board continued to increase slowly in width to the end of the study.

The initial moisture content of 7.5 percent gradually increased to 32 percent at a sampling height of 6 inches. The results indicate that moisture saturation (+35 percent) was not achieved after 34 days of exposure. This material is also a good candidate for duration estimates following a water loss because it increases slowly and consistently in moisture content and thickness when exposed to a continuous source of moisture.

5. Faced Particle Board (Cabinet)

The maximum dimensional change observed in this high density, faced (melamine, 3/4-inch particle board, Type 2-H) was approximately 5 percent (0.127 inch increase) and was attained at the end of the study period. The particle board width increased throughout the study period. This faced (melamine) 3/4-inch particle board had an initial moisture content of 7 percent that gradually increased to 12 percent at a sampling height of 6 inches.

Cabinet grade particle board is an excellent candidate for estimating the duration of loss because it absorbs moisture slowly and creates an easily distinguishable bump or swelling on the exterior of the cabinet that corresponds to the rising moisture level. Type 2 particleboard commonly found in high quality vanities and cabinets make this a preferred material to examine after a water loss.

6. Sheetrock (Gypsum Board)

Sheetrock is rapidly absorbent to both free water and water vapors. The maximum dimensional change experienced in the 5/8-inch sheetrock (National Gypsum) was 0.017 inch (4 percent) after 3 weeks of exposure. The moisture content increased from 12 to 36 percent and achieved saturation at the 6-inch level within 2 days. Similar research investigations published by gypsum board manufacturers (NORDEST) tested moisture absorption by gypsum board used in bathrooms. In these studies, boards from six manufactures were subjected to relative humidity values ranging from zero to 100 percent. The results showed that when exposed to relative humidity values above 90 percent, the moisture content in the gypsum boards attained equilibrium at 20 percent moisture content and above. This investigation revealed similar results and support field investigations where the sheetrock walls in residential homes attain moisture content ranging from 17 to 19 percent when exposed to humid outside air .

Sheetrock is an excellent material to examine for moisture change, and estimates of the duration of loss are less than 14 days — especially when residual stains following the loss provide a direct measurement of the duration.

In summation, when the materials used in this study are exposed to a single, continuous moisture event, the extent of water absorption and changes in dimension are measurable and predictable. The authors conducted this particular study as well as others to more accurately determine the duration of a water loss. As made evident by this particular case, careful measurements will support insurance coverage decisions. K

Ralph E. Moon, Ph.D., CHMM, CIAQP, is a shareholder and principal scientist at HSA Engineers and Scientists with 19 years of consulting experience in the areas of risk assessment, project management, and industrial hygiene. He may be reached at [email protected].

Rick Price, CIAQP, is also a shareholder at HSA Engineers and Scientists. He may be reached at [email protected].