In 3500 B.C., ancient Egyptians as well as the Romans understood that wood swelled when exposed to water. Applying this knowledge in construction, they would drill a series of holes in granite cliffs and then drive wooden wedges into the holes which were then soaked with water. The resulting swelling of the wood created pressure that was sufficient to break off linear slabs of granite for use in building construction. Evidence of the use of this creative method is still in existence (Photo 1). 
Today, the consequences of moisture absorption and the concurrent effects of swelling pressure are evident in every water loss claim. In fact, one of the principal responsibilities in a water loss investigation is to determine whether changes in the building materials and contents were related to the reported water loss. This article provides insight into the consequences of moisture absorption—in particular, the impact of swelling pressure and how the changes can influence the interpretation of water-related damage. To this end, our study measured one-dimensional pressure created when building materials were exposed to liquid water.
Materials and Methods
Eight building materials were selected and tested for the study—pressure treated pine (one-inch thickness), interior- and exterior-grade plywood (3/4 inch), oriented strand board (OSB, 7/16 inch), medium density fiberboard (MDF, 3/4 inch), faced and unfaced particle board (5/8 inch), and gypsum board (5/8 inch).
Expansive pressures (pounds per square inch or psi) were measured using a swelling pressure measuring device (SPMD) that consisted of a load cell (similar to a small bathroom scale) placed inside an aluminum pressure cooker with round, 1/4-inch thick steel plates and a vertical rod that directed the expansive pressures from wet material samples onto the load cell. The pressures created by the samples were displayed on a digital monitor connected to the load cell (Diagram 1).
This device was derived from the American Society for Testing and Materials (ASTM) standard for establishing the linear coefficient of thermal expansion from heated materials (ASTM E831-13) and the ISO standard for a thermomechanical analyzer (ISO 11359-2). The original standard for measuring the expansion or contraction of materials with temperature change was modified to measure the expansion pressures of saturated wood products.
The building materials were cut into one-inch square samples using a scroll saw and held at room temperature (75 degrees F) and standard relative humidity (50 percent) until testing. Before testing, the moisture content (less than eight percent wood moisture equivalent, Tramex Moisture Pro) and the dimensional measurements were documented in each sample. After sample placement into the SPMD, 600 milliliters of water were added. A HOBO (data logger) then was placed inside to monitor the temperature and relative humidity with the lid properly seated. All samples were tested in triplicate for periods up to 16 days.
Results
Most test samples expressed their maximum pressures by the second day (Figure 1). The highest average pressure measurement was obtained from MDF (144 psi) while the lowest was gypsum board (five psi). A comparison of the peak values showed the highest pressures derived from composite products (78-144 psi for MDF, particle board, and OSB) as compared to laminate wood products (57-65 psi for interior and exterior plywood). Pressure treated pine exhibited upper limit pressures of 52 psi.
Why does moisture absorption in wood create pressure? In solid wood products, the moisture is absorbed through the vascular tissues (xylem) by capillary action. Once the moisture content achieves wood fiber saturation (cell walls remain saturated with absorbed moisture), no further moisture absorption occurs.
Measurements of the dimensional change in width were made for various wood products before and after moisture saturation. Solid and laminate wood products expressed a thickness swell (TS) of five percent to seven percent. A cross section of a laminate wood product shows the natural configuration of the wood cell structure (Photo 2).
Moisture absorption in composite wood products is different because the composition has been radically changed. This product consists of a large volume of hygroscopic wood fibers, particles, chips, or flakes that are mechanically compressed as compared to natural wood materials (Photos 3 and 4).
Following compression, the final product contains more surface area for moisture absorption than natural wood of the same thickness. During OSB panel manufacturing, wood chips three to five inches in length are separated, tumbled and heated, and sprayed with an adhesive. The wood chips are then layered, compressed (650-800 psi), and heated (400-425 degrees F) to create the product.
Composite wood products generate pressure from moisture absorption because water molecules can bond to more exposed areas of cellulose. A cross section of MDF shows the compressed configuration of the wood cell structure after the manufacturing process (Photo 5).
A previous study in the February 2013 issue of Claims Management (“Swelling Proof”) examined TS following moisture saturation. The results showed that composite wood materials expressed a dimensional increase from three to 18 percent. The increased density created by compressing the wood fibers explains why the pressure created in composite wood products is highest. This interpretation was substantiated by plotting the percentage width expansion measured after moisture saturation against the average maximum pressure (Figure 2). The results show the relationship between the percentage of TS and the average maximum pressure.
How much pressure can be created from wet wood products? Using the pressures documented for MDF, for example, an eight-foot section of base trim that abuts the cross-section of floor tile (3/4-inch height) could collectively produce as much as 3,456 pounds of force or about 1.7 tons.
Pressure Relationships
In this study, pressures that are created when wood products are exposed to moisture were examined. Two examples taken from field observations demonstrate the effects of this pressure. The first example illustrates the pressure created between a piece of interior trim (made of MDF) and an adjacent piece of travertine tile after a water loss (Photo 6). This observation confirmed that the orientation and proximity of the MDF door trim to the tile can result in substantial pressure following a water loss.
The second example shows the expansive pressures created after a composite material (OSB) used to construct a roof soffit was exposed to moisture. Water penetrated the sealant and paint causing the underlying OSB to swell (Photo 7) and exert pressure on the drainage plane. Both examples emphasize the hygroscopic properties of wood and the importance of keeping it dry to prevent swelling.
Discussion
Composite wood products are predominantly used in residential construction. According to the American and Canadian Wood Councils, 16 million square feet of 3/8-inch OSB were manufactured in 2012 with increased usage anticipated. Several considerations are proposed to better understand pressure relationships of wood products when water damage claims are reported.
- Composite wood products express higher moisture sensitivity than solid wood and must be protected from liquid water exposure when used both outside and inside the building envelope. Poorly installed vapor barrier materials and sealants allow liquid water exposure to roof, wall, and floor sheathing.
- Examine water losses for visible evidence of cracking and separation in barrier wall construction (e.g., stucco, cementitious coatings) where movement and moisture damage are likely to occur.
- Investigate and identify the components of the wall assembly for vulnerability to moisture exposure and movement.
- Recommendations made by the Tile Council of North America Inc. (TCNA) for movement joints are well founded. Their recommendation for movement joints every 20-25 feet of continuously tiled surface and around the perimeter of a tiled floor abutting any restraining surface is particularly relevant when moisture contacts wood materials, such as cabinets and trim.
Strategies
- Encourage competent installation of moisture barriers and drainage planes to prevent contact with liquid water.
- Elevate composite wood materials such as cabinets above the floor to avoid direct contact with moisture.
- Allow for movement of floor tiles abutting any restraining surface.
This study looked at the pressure exerted by wood materials as moisture content increased to saturation. The results showed that: 1) pressure increased as moisture content reached saturation; 2) maximum pressure occurred within the first two days of moisture exposure in most samples, so you should anticipate a “pressure punch” to adjacent flooring and other adjoining materials several days after a water loss; and 3) the closer the proximity of adjacent building materials, the higher the pressure applied when moisture is introduced.
The authors wish to thank the following people who contributed editorial comments to the paper: Dr. Chin Yang, Ph.D.; Robert Braun, P.E.; John Marquardt, P.E.; Jeff Steger, P.E.; Jeff Wilemon; Don Nehrig; and Chris Martinez, MME, EIT.