Structural Insulated Panels (SIPs) in Residential & Commercial Construction

To view the figures and tables associated with this article, please refer to the flipbook above.

In the last couple of decades, the need for durable and sustainable building technologies has come to the forefront in the global dialogue on the future of the built environment. These innovations are especially crucial in areas prone to extreme weather. According to the United Nations Office for Disaster Risk Reduction (UNDRR), between 2000 and 2019, 7,348 major recorded disaster events affected 4.2 billion people and resulting in approximately US$2.97 trillion in global economic losses. These staggering figures underscore the importance of reliable and safe buildings.

Just like a skeleton supports the body, the structural system makes a building’s core, providing strength and endurance. Naturally, paying close attention to the selection of advanced building technologies for these systems can help structural engineers create code-compliant and enduring structures that can stand strong.

One proven way that structural engineers can deliver dependable buildings in seismic zones and regions prone to heavy wind velocities is by specifying forward-thinking building envelope solutions such as structural insulated panels (SIPs). SIPs can be specified in walls, roofs or floors of residential and light commercial projects to improve durability while also elevating performance by creating a nearly air-impermeable envelope.

SIPs are load-bearing panels most commonly composed of expanded polystyrene (EPS) rigid foam insulation sandwiched between two sheets of oriented strand board (OSB). SIP design mimics the structural principles of a steel I-beam, where the OSB serves as the flange to resist bending and the foam core acts as a continuous bracing to absorb shear forces.

Manufactured under factory-controlled conditions, SIPs are precisely assembled using techniques similar to traditional wood construction but with a consistent bonding between the insulation and the board. All these factors give SIPs their structural strength to shoulder in-plane loads, offering an equivalent load-bearing area that is possible when building with 2 x 10 studs. For example, a 6 1/2-inch-thick SIP offers an equivalent load-bearing capacity to a 2×10 at 2 feet on center of equal height.

SIPs can be manufactured in large sizes up to 8 feet X 24 feet and in thicknesses ranging from 4-1/2 inches to 15 inches. Just as wide flange profiles have higher strength with increased depth, thicker SIPs have higher compression and bending capacities. Consequently, they are typically used for walls and roofs to resist both out-of-plane and in-plane forces induced by gravity, wind or seismic loads. The variation allows structural engineers to meet project specifications and performance requirements even in areas prone to natural calamities.

In application, SIPs provide significant advantages in terms of load-carrying capacity and support member spacing. For example, in residential and light commercial projects, a 12-1/4-inch-thick SIP spanning 8 feet can carry up to 106 pounds per square foot (psf), which is greater than many traditional systems of similar dimensions. Structurally self-sufficient SIPs require less support from framing members and allow structural engineers to have wider member spacing compared to stick framing.

Consequently, fewer members can result in lighter structures, helping reduce seismic inertia. For example, a floor constructed with a relatively thick 1-3/32-inch wood structural panel (WSP) sheathing using stick framing would require support members placed every 4 feet to be able to carry a 55 psf load. In contrast, a 6-1/2-inch SIP floor panel with support spacing at 8 feet can carry the same load. This equates to SIPs’ framing factor of about five percent, compared to nearly 25 percent in traditional framing. Similar comparisons can be made for roofs, demonstrating that the use of SIPs compared to stick framing requires far fewer support members to carry equivalent loads.

SIPs are ductile, which means they can bend and deform under stress without breaking. In application, SIPs are fastened similar to traditional wood construction, which offers additional flexibility because nails provide less resistance to external forces compared to adhesives or screws. These factors set up SIP structures to allow small amounts of flex and movement, absorbing even multi-directional vibrations and reducing their impact.

To develop performance test data on the response of SIP shear walls with high-aspect-ratio segments, the United States Department of Agriculture’s (USDA) Forest Products Laboratory (FPL) and Structural Insulated Panel Association (SIPA) commissioned a study. They tested fully anchored shear segments with aspect ratios of 1:1, 2:1, 3:1 and 4:1 using the perforated shear wall (PSW) method. The findings revealed that the unit shear capacity of these segments ranged from 1,400 pounds per foot (lb/ft) to over 2,100 lb/ft, depending on the aspect ratio. Similarly, the measured unit shear stiffness varied up to a factor of two. Additionally, it was observed that both unit shear wall capacity and stiffness decreased as the number of panels joined with a spline connection increased. For instance, a 20-foot wall with four spline joints exhibited a 25 percent decrease in unit shear compared to an 8-foot wall with a single spline joint. Moreover, the unit shear wall capacity decreased with higher aspect ratios, showing a 16 percent reduction for a 2-foot segment compared to a 4-foot segment. In the same vein, unit shear wall stiffness saw a maximum decrease of 33 percent for a 2-foot segment compared to either 8-foot or 4-foot segments. The results indicated that perforated SIP shear walls align closely with the PSW method trends in terms of both strength and stiffness.

Additionally, engineers also often look closely at a material’s ability to resist creep to ensure long-term structural integrity. In engineering, creep is the increased strain or deformation of a structural element under constant load, which can cause significant displacements in a structure. This, in turn, can result in serviceability problems, stress redistribution, prestress loss and even failure of structural elements. When a study at the FPL tested SIPs’ behavior under long-duration gravity loading, the building system stood strong . Not only was there no significant deflection but they also showed no loss in their load capacity. Owing to its strength and resilience, a 6-1/2-inch-thick SIP can carry up to 80 psf in roofs at a deflection limit of L/180, which is adequate for most roof snow load situations in the U.S.

Furthermore, SIPs demonstrate seismic equivalency to light-frame wood shear wall construction, meeting ASTM D7989 Standard Practice for Demonstrating Equivalent In-Plane Lateral Seismic Performance to Wood-Frame Shear Walls Sheathed with Wood Structural Panels for in-plane lateral seismic performance. SIP walls are code-approved by the International Residential Code (IRC) to be used in seismic design categories A, B and C for aspect ratios as high as 4:1 as well as D, E, and F for common wall aspect rations 1:1, 2:1 and 3:5:1. SIPs’ strength and durability allow engineers to minimize the risk of structural failure and provide a very comfortable safety factor to buildings. In fact, after carefully evaluating its proven performance benefits, many residential and light commercial projects in Nevada, Montana and Alaska, where seismic considerations are especially important, have utilized SIPs in their structural design.

High-velocity winds pose a challenge to the structural stability of a building. Due to SIP’s strength, the IRC provision R610.2 permits exterior SIP walls in sites where the ultimate design wind speed is not greater than 155 miles per hour in Exposure B or 140 miles per hour in Exposure C. This allows SIPs to create dependable structures even in regions that experience high-velocity winds and hurricanes.

In some cases, SIPs can resist wind loads exceeding 180 mph, making them suitable for coastal and hurricane-prone regions. Many manufacturers offer SIPs that meet the stringent standards set by the Florida building code for all High-Velocity Hurricane Zones (HVHZ), which includes hurricane-prone Miami-Dade County. To further bolster its performance, SIP screw connections can be strengthened to resist the additional concentrated loads.

Even when exposed to wind-driven rain, SIPs can maintain their structural integrity. In rigorous testing by APA- The Engineered Wood Association to illustrate their performance upon exposure to moisture, SIPs were subjected to transverse, lateral and axial load testing after they dried and regained their original weight. The results found that SIPs easily recovered from the condition, demonstrating excellent structural stability and the ability to regain their load capacity after moisture exposure.

Leveraging SIPs’ proven performance in areas prone to wind and rain, the Joann A. Alexie Memorial K-12 School in Atmautluak, Alaska utilized them as exterior walls (Figures 1 & 2). Because the school was situated in an area devoid of natural windbreaks like trees or mountains, the design team created an elevated aerodynamic building to face gusts straight from the Bering Strait. In addition to using 8-inch SIPs as standard walls, the project team specified 12-inch-thick SIPs as angled walls. These large, angled walls face the spring wind direction, standing strong against the uninterrupted force and breaking its speed. Besides bearing the brunt of the winds, SIPs also helped create an airtight envelope that supported the project’s energy-efficiency goals— a priority among many owners and design professionals alike.

While sidelined as a concern for energy-efficient envelopes, air and moisture infiltration can have negative consequences for a building’s structural system. If not addressed, uncontrolled air penetration can lead to condensation on the interior surfaces. This moisture build-up can promote mold growth, which can damage building materials and compromise the structural integrity of the building.

Among various reasons causing condensation is temperature fluctuation due to thermal bridging. Metal components within a traditional wall assembly such as fasteners enable heat transfer and can cause condensation risk. But factory-made SIPs have a consolidated makeup with insulation and facing in one unit. As a result, the structure is not vulnerable to thermal bridging. In fact, this property allows SIP structures to be nearly air impermeable.

When the Department of Energy’s Oak Ridge National Laboratory (ORNL) tested a SIP building compared to a stick-framed building, it found the SIP structure was 15 times more airtight. This illustrates SIPs efficiency in limiting air intrusion, which can play an important role in the long-term durability of the building. Additionally, this ability is instrumental in a SIP structure’s performance in events of fire. Typically, SIP walls and roof/ceiling assemblies with the appropriate amount of gypsum board can achieve a 1-hour fire-resistance rating. But the air-sealing capabilities prevent the circulation of air and smoke within the structure, minimizing the risk of fire spreading through the building. After all, as basic fire science tells us, without oxygen, fire will not burn.

The nearly airtight nature of the SIP structures, while beneficial for long-term durability and fire resilience, necessitates mechanical make-up air to prevent mold and dampness, which can compromise structural integrity if left unaddressed. Proper HVAC sizing as per the ASHRAE Manual J guidelines is critical in SIP structures because an oversized system will not reach its intended operating rate, leading to “short cycling,” where the system runs briefly but fails to dehumidify the air effectively. Conversely, a properly sized HVAC unit within the conditioned SIP envelope may require half the British thermal units (BTUs) compared to traditional framing to efficiently take in and redistribute air, enhancing a structure’s durability.

SIPs have been specified for use in seismic and hurricane zones, and they have historically performed very well. After knowing its properties, structural engineers can rely on this construction technology and create buildings that can stand the test of time.

To help structural engineers create technically sound structures, the Structural Insulated Panel Association (SIPA) offers an in-depth analysis of SIP design principles through several resources, including “SIP Design Best Practice BP-3: SIP Structural Capabilities.” Some key considerations elaborated in this document include:

For more SIP design considerations, structural engineers can refer to SIPA’s free content, available for download at www.sips.org/resources/design.

Tom Moore, a SIPA Board Member and the Principal Structural Engineer at Pinnacle Engineering shared his expert input for this article.

About the Author

Jack Armstrong is the Executive Director/COO for the Structural Insulated Panel Association (SIPA). He’s been on the SIPA board since the mid-2000s and transitioned to leadership in 2014. Armstrong worked for the BASF chemical company for 24 years, focusing on energy efficiency and durability for sustainable construction in the built environment. He can be reached at jack@sips.org.