Creep Performance Testing of Structural Insulated Panels

Photo Credit

Courtesy photo by Mississippi State University

Creep performance test setup of structural insulated panel beams weighed down with sandbags. The deflection over 90 days was measured in real time to an accuracy of 0.001 inches.

Creep occurs when building materials slowly deform under sustained loading (e.g., snow on a rooftop) without breaking. For engineered composite building materials such as structural insulated panels to be used in building construction, these products need to be tested in a way to demonstrate that their creep performance is comparable to that of solid wood. 

Structural insulated panels (SIPs) consist of two engineered wood composite layers with insulated foam sandwiched between them. These commercially available products derive from Forest Products Laboratory research from as far back as the 1940s on sandwiched panel products and have been applied in building construction since the 1950s. Despite their current use in construction, industry-wide information on SIP creep performance was limited.

Teaming up with Mississippi State University, the Structural Insulated Panel Association (SIPA), and SIP producers, the Forest Products Laboratory developed methods to evaluate creep performance in SIPs to determine compliance with the ASTM International standard. These data were used to develop creep deflection models that building designers can use when making decisions about using SIPs in their building design.

Key Findings:

• Testing of SIPs for creep performance confirmed that these products meet the acceptance criteria to comply with the ASTM D6815 standard.
• These methods and the resulting data and models increase the accessibility of SIPs as a building material.

Photo Credit

Adapted from Senalik et al. 2024.

Diagram showing the sandbag test setup. Each structural insulation panel beam was weighed down with a known weight of sandbags based on the beam’s thickness. Deflection was measured in real time for 90 days.

Approach

The SIP beams tested by the research team consisted of expanded polystyrene (EPS) foam sandwiched between two oriented strandboard (OSB) layers. Two thicknesses were tested: 6.5 inches and 12.25 inches. All SIPs were made in accordance with International Code Council (ICC) Evaluation Standard ESR-4689.

The main tests of creep performance involved three stages:

1. SIP beams were loaded for 90 days with weight (i.e., sandbags) equivalent to the amount estimated to bend the beam by 5 percent (494 pounds for the 6.5-inch beams, 566 pounds for the 12.25-inch beams) while deflection was measured in real time using a sensor accurate to 0.001 inches.
2. The weights were then removed and the beams sat for 30 days while deflection was measured in real time.
3. The beams were then bent to the point of failure (i.e., breakage in foam or oriented strandboard) to test the strength of the beams after loading.

Most of these tests were conducted at Mississippi State University.   

Photo Credit

Adapted from Senalik et al. 2024.

Deflection (in inches) over time of 12.25-inch-thick structural insulation panel beams subjected to 566 pounds of sand in a laboratory test of creep performance. Each line represents an individual beam. None of the SIP beams exhibited failure. 

Outcomes

• The 90-day deflection of 6.5-inch-thick SIPS was between 0.27 and 0.50 inches, and that of the 12.25-inch-thick SIPs was between 0.41 and 0.66 inches.
• All SIP beams tested passed the acceptance criteria for ASTM D6815, which means that SIP beams manufactured to comply with ICC-ESR 4689 are compliant with ASTM D6815.