Swelling Proof

Evidence for continuous and repeated water exposure.

February 26, 2013 Photo

The distinction between a sudden (short-term) water release and a continuous and repeated (long-term) water release is a critical element for policy coverage decisions. Water exposure tests on various wood products have been described in previous studies, yet the application of these research efforts for water losses is limited.

Our current study supports the finding that wood materials express predictable and distinctive changes when exposed to either a continuous or a repeated water event that are different from a sudden short-term event. Gradual discoloration, deterioration, and dimensional changes can be used to estimate the duration of a water loss in a variety of materials from solid wood to laminate carpet tack strips. The results suggest that careful examination and measurement of wood products following a water loss can support policy coverage decisions.

Wood Products and Moisture

Moisture is the most important factor that influences the function and life expectancy of a wood product. When solid wood materials are exposed to water, they absorb moisture through open cells and thin sections of the cell wall known as “pits.” Pits regulate water absorption by restricting the introduction, retention, and release of moisture. Pits allow water absorbed by one cell to be transferred to another, thereby controlling the amount of water that can be absorbed and released. This feature contributes to the “elastic” properties common to solid wood when cyclical swelling and shrinking occur without physical consequences.

In contrast, moisture absorption by wood composites (e.g., oriented strand board (OSB) and particle board) is non-regulated. During the manufacturing process, the wood cell is macerated, but the fibrous cellulose structure remains hygroscopic, or water absorbent. Moisture absorption by composites results in near irreversible swelling. The swelling observed after moisture exposure is caused by the release of latent compressive stresses generated when the materials are hot-pressed during manufacturing. Swelling following moisture exposure changes the physical properties of composite materials.

Increasing the amounts and types of adhesives can limit swelling. Waterproof adhesives (e.g., phenol-formaldehyde and isocyanate) are used for exterior applications where excessive swelling cannot be tolerated. Water-resistant adhesives (e.g., urea-formaldehyde) are unsuitable for exterior exposure and are used for interior applications.

Study Materials and Methods

Five wood specimens—solid pine, seven-ply exterior pine plywood, nine-ply interior birch plywood, exterior OSB, and interior (vinyl) faced particle board—were used for continuous and cyclical wet-dry testing. Wet-dry test samples and continuously wet samples were labeled and wetted for two weeks by placing one edge in free water in test chambers with water added regularly. Sample drying (one week) was conducted in a separate drying chamber. Moisture content and dimensional measurements were obtained at varying heights above the moisture source at the end of each wetting and drying cycle. This procedure was repeated for 13 wet-dry cycles over a 40-week period. Temperature and relative humidity measurements inside the test chambers were monitored using data loggers. Measurements obtained after the first cycle (10-inch height) were used to evaluate changes induced by water vapor and temperature alone and served as controls. Two samples continuously exposed to moisture were used for comparison to five samples exposed to wet-dry cycle conditions and after two weeks served to illustrate a short-term exposure event.


All moisture measurements (0.5-inch level featured) demonstrated cyclical wetting and drying with the moisture content ranging from “normal” (8 percent to 10 percent wood moisture equivalent (WME)) following one week of drying to “saturated” (30 percent to 35 percent WME) after two weeks of moisture exposure.

For OSB, the continuously wet samples (marked as “control”) remained saturated throughout the 40-week test period, while the test samples fluctuated between “normal” and “saturated” conditions. Dimensional changes (thickness) varied widely among the five sets of wood samples (Chart 1).

The highest dimensional changes were observed among the interior-grade vinyl-faced particle board samples (FPB) (Chart 1). The FPB exhibited initial cracking of the coated surface within one wet-dry cycle followed by friable releases (2-3 cycles) and complete failure with 100 percent expansion after 4.5 cycles, whereupon it was removed from the study (Photo 1). FPB was permanently expanded nearly 100 percent of its original dimension when exposed to both wet-dry cycles and continuous moisture exposure (70 percent expansion) with limited shrinkage following drying.

Interior plywood (IP) gradually increased in dimension by 48 percent and exhibited complete laminate separation at 10.5 cycles when the material was removed from the study (Photo 2). At saturated moisture levels, IP exhibited modest “elasticity” when subjected to wet-dry cycles (Chart 1).

Solid wood exhibited gradual discoloration and cracking during wet-dry cycle exposure, and it showed only slight discoloration after two weeks of continuous moisture exposure. Continuously wet samples gradually darkened to black after 12 cycles (Photos 3, 4, 5 and 6). Solid wood exhibited remarkable “elasticity” by repeatedly returning to its original dimension after drying.

Exterior plywood exhibited increased discoloration with increased cyclical wet-dry exposure (Photos 7 and 8). During the first two weeks, exterior plywood exhibited minimal discoloration with continuous exposure. Samples exposed to continuous moisture exhibited gradual darkening to black after 13 cycles. Exterior plywood also exhibited “elasticity” with repeatable expansion (12 percent) and contraction (6 percent) rates during the study (Chart 1). No apparent delamination occurred among the test samples.

OSB wet-dry samples swelled initially by 20 percent when wetted and then exhibited slight increases in swelling after each wet-dry cycle (Chart 1). Discoloration modestly increased over the course of study, especially following the dry cycle. However, the continuously wet samples experienced gradual darkening to black over the same period (Photos 9 and 10, p. 44). OSB swelled in excess of 45 percent after cycles 10 and 11.

The extent of swelling among continuously wet samples was less pronounced than the wet-dry test results. The continuously wet samples swelled quickly following the first wetting event and then plateaued (Chart 2, p. 44)). The continuously wet samples for the solid wood and exterior plywood samples expanded 5 percent and 10 percent, respectively, at the end of 40 weeks of constant moisture exposure. Continuously wet IP samples expanded approximately 10 percent, about half the dimensional change exhibited in the test (wet-dry) samples. The OSB continuously wet samples expanded to approximately 30 percent, about 10 percent less than the wet-dry test results. Vinyl-faced particle board initially expanded in excess of 30 percent and then continued to swell in excess of 75 percent. The control (humidity only) results indicated that sustained conditions of elevated humidity will cause measureable expansion (20 to 30 percent) of wood composites.

Temperature and Relative Humidity Results

Temperature and relative humidity monitoring results obtained during the drying and wetting cycles revealed a gradual temperature increase of 80 F at the start to 142 F after a week of drying. The temperature increase corresponded to a decline in relative humidity (RH), which began near 50 percent RH and ended at less than 15 percent after Day 3 of the week-long drying period. During the wetting period, the RH oscillated with diurnal temperature ranging between 65 percent RH (day) and 95 percent RH (night). Temperatures oscillated between 95 F (day) and 75 F (night) (Chart 3).

Constant or Repeated Seepage: Legal Precedent

Denying a claim based on the “constant or repeated seepage” exclusion is controversial because it is difficult to prove. One way to enhance certainty is by using exemplars of similar or identical materials. This study encourages the use of exemplars as a comparative tool.

Current research of case precedent does not reveal evidence where exemplars were used. In Marsh v. American Family Mutual Insurance Company, the insureds reported dry rot in their bathroom caused by a shower pan leak in June 2005. The homeowners detected a musty odor in their bathroom about two years before their floor became “mushy” and the toilet rocked back and forth. The insurance carrier did not provide coverage for the damages due to the “continuous or repeated seepage” exclusion. During the trial, the plaintiffs’ contractor testified that the cause of the damage was a leak that continued “over a period of time.” Additionally, one plaintiff confirmed that they noticed the instability of the bathroom floor before June 2005. Although the trial court concluded from the evidence presented that the damage to the structure constituted a “collapse” and was caused by “continuous or repeated seepage,” the court determined that there was still coverage in the policy regarding dry rot. The Oregon Court of Appeals reversed the decision in favor of the insurer, reasoning that there was “evidence to support the trial court’s finding that the dry rot under the shower was caused by leakage or seepage of water from the shower over a period of weeks, months, or years.”

In Hoey v. State Farm, a vacant home was damaged by a toilet water supply line leak. The insureds filed suit against State Farm for failure to provide coverage for the damages. The insureds believed the damage was caused by “a continuous leakage from that toilet pipe, for a course of three weeks or so” and by a “sudden, accidental discharge of water.” An expert testified on behalf of State Farm that the leakage was a constant increase from a drip to a major failure of the supply line. The expert also presented monthly water bills as evidence that the leak occurred over a period of two or three months. Furthermore, rot in the wood near the fitting of the line and mold in the nearby drywall were consistent with leakage over a period longer than a few weeks. Based on this evidence, the trial court found in favor of the insurer, determining that the damage was caused by “continuous or repeated seepage of water from the supply line over a period of more than two weeks.” The Fourth District Court of Appeal of Florida affirmed the lower court’s denial of the insurance claim.


Wood products exposed to 13 wet-dry cycles and continuous exposure over 40 weeks resulted in the following conclusions. 

  • Wood discolors slowly and requires continuous moisture exposure to develop a dark (two-to-five months) to black (six-to-eight months) appearance. Repeated wet-dry cycles did not discolor materials to black in the observed study period.
  • Wood materials exposed repeatedly to wetting and drying will maintain elasticity if the moisture content does not exceed 23 percent WME.
  • Wood materials exposed to constant moisture swelled less than those exposed to wet-dry cycles.
  • Solid and exterior composite wood products exhibited “elasticity” when exposed to wet-dry cycles. Particle board and interior-grade plywood expressed “plasticity” by gradual dimensional expansion.
  • Composite wood materials are more vulnerable to swelling following repeated moisture exposure as compared to solid and laminate wood.
  • Composite wood products made with phenol-formaldehyde adhesives remain visibly competent for up to 13 wet-dry cycles. Composite wood products made with urea-formaldehyde exhibited swelling, friable particle release, and failure.
  • Particle board materials exhibited surface coating cracks after one wet-dry cycle. Material fragmentation and the release of friable particles (sawdust) can become evident after two or three wet-dry cycles.
  • Interior-grade plywood does not tolerate repeated moisture exposure and may begin to separate after three wet-dry cycles. Complete separation between all layers may occur following five-to-seven wet-dry cycles.   

Ralph E. Moon, Ph.D. is with HSA Engineers & Scientists and Alicia M. Moon, Esq. is with Groelle and Salmon. The authors would like to thank Nicholas Albergo, P.E., DEE, Bruce Bosserman, P.E., Bob Braun, P.E., John Marquardt, P.E., Chin Yang, Ph.D., and Jeff Wilemon for their review comments.



Photo 1


Photo 2


Photo 3


Photo 4


Photo 5


Photo 6


Photo 7


Photo 8


Photo 9


Photo 10


About The Authors
Multiple Contributors
Ralph E. Moon, Ph.D., CHMM, CIAQP

Ralph E. Moon, Ph.D., CHMM, CIAQP, is with GHD, a worldwide consulting firm. He has been a CLM Fellow since 2013 and can be reached at www.ghd.com. rmoon@hsa-env.com

Alicia Moon

Alicia M. Moon, Esq. is with Groelle and Salmon. www.gspalaw.com 

Sponsored Content
Daily Claims News
  Powered by Claims Pages
About The Community

CLM’s Insurance Fraud Committee identifies, analyzes, and offers education on emerging fraud schemes and tactics; monitors and reports on developments in case law, state fraud statutes and applicable regulations; collaborates with other anti-fraud industry organizations and associations; and seeks to provide amicus support in matters of importance in the fight against insurance fraud.

Community Events
No community events