Creep Performance of Structural Insulated Panels (SIPs)

Few places in the United States can understand cold like the Midwestern United States. In Wisconsin, where January’s low temperatures average around 13 degrees Fahrenheit and snowfall is typically measured in feet, a well insulated home can spell the difference between enjoying the winter and enduring the winter. At the Forest Products Laboratory (FPL) in Madison, Wisconsin, helping to evaluate effective yet structurally sound insulation methods for wooden construction is a small but important part of the laboratory’s vast repertoire.

Modern construction efforts have incorporated more and more structural insulated panels (SIPs) into designs—and with good reason. SIPs are a panel-shaped building material that consist of two wood-based composite boards with an insulating layer of foam plastic in between them. The end result is extremely strong, energy efficient and cost effective.

Creep test setup for a structural insulated panel.

Creep test setup for a structural insulated panel.

Although SIPS have been used in floor and roof applications for years, little is known about their long-term structural integrity. Recognizing this gap, researchers at FPL set to work investigating the “creep performance” of these systems, or the tendency of the panel to deform when subjected to a sustained load. The rate of creep deformation is a function of material, time, temperature, moisture, and applied load—and the right combination could spell structural failure.

The project, in cooperation with The Engineered Wood Association (APA) and the Structural Insulated Panel Association (SIPA), began in 2013 when 12-inch-wide panels were brought to FPL to be subjected to 90 days of shear and bending tests. The SIPs performed very well, and all of the 24-foot-long sections survived without any bending failure. In addition, after a recovery period, researchers retested the panels to determine the residual strength of each panel. Again, the SIPs remained strong, unfazed by the 90-day test.

Based off the positive results from the first test, the team launched into the next phase of the project. The second phase kicked off in 2014, and was much more comprehensive than 2013’s efforts. This round of tests involved as many as 28 specimens per test configuration, several different configurations, and lasted into spring of this year. Although the research is completed, the final report has yet to be released—but when it is, building professionals will have a new body of knowledge that will allow them to incorporate SIPS into designs with added peace of mind.

When the days get darker and the snow begins to pile up, SIPS have always been trusted to keep the cold air out. Now, consumers will be able to rest assured that bending, shearing and other deformation won’t be creeping in at night either.