Structural Insulated Panels (SIPs) are an option for part of a building’s assembly that can help achieve the goals of achieving an energy-efficient and high-performance building enclosure. SIPs systems can be highly effective in slowing down the transfer of heat, air, and vapor through the assembly. Detailed and built correctly, SIPs offer an airtight assembly with great thermal resistance resulting in a high-performance and durable enclosure.
How SIPs Work
Structural insulated panels are composed of an insulated foam core between two rigid board sheathing materials. The foam core is generally one of the following: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyurethane foam (PUR). With EPS and XPS foam, the assembly is pressure laminated together. With PUR and PIR, the liquid foam is injected and cured under high pressure.
The most common sheathing boards are oriented strand boards (OSB). Other sheathing materials include: sheet metal, plywood, fiber-cement siding, magnesium-oxide board, fiberglass mat gypsum sheathing, and composite structural siding panels. Each sheathing material and foam type has its benefits and drawbacks. The type of SIPs selected by the architect depends upon the building type and site conditions.
Structural Design and Construction
SIPs behave similarly to a wide flange steel column in that the foam core acts as the web and the sheathing responds as the flanges. Under axial loads, the sheathing responds similarly to a slender column, and the foam core acts as continuous bracing preventing the panels from buckling. Just as wide flange sections increase in strength with increased depth, thicker cores result in stronger panels in compression and bending.
SIPs are designed to resist not only axial loads, but also shear loads and out of plane flexural loads. The panels’ ability to resist bi-axial bending and lateral shear allow them to be used as roofs and floors. SIPs panels are acceptable to use as shear walls in all seismic design categories. A structural engineer should determine if a secondary structural system is required based on the design loads.
To date, the tallest structure constructed exclusively of SIPs is four stories. Taller structures are possible; however, design limitations are due to the fact that SIPs are bearing walls and therefore open spaces at lower floors are more difficult to achieve. Often large SIPs structures rely on a secondary framing system of steel or timber to satisfy requirements for unobstructed spaces. Unique screw connections are available to attach SIPs to wood, light gage steel, and structural steel up to 1/4 inch thick.
The quality of a building’s envelope is measured by its ability to prevent infiltration of outside air. Recent energy code standards require an air tight building envelope, and a SIPs building with properly sealed panel joints is inherently airtight. The results of blower door tests on a room with SIPs walls and ceilings, one window, one door, and pre-routed wiring chases and electrical outlets compared to a identical room of 2×6 studs, OSB sheathing, fiberglass insulation, and drywall showed the SIPs structure to leak 90 percent less than the stud structure.
The Whole Wall R-Value of a wall assembly is currently the most accurate method of quantifying its thermal performance. The Whole Wall R-Value takes into account the resistance of heat flow through an opaque cross sectional area of the insulation and structure while accounting for the loss of energy at the interfaces of the wall with the roof and floor and at corners and fenestrations. The Whole Wall R-value of a 4-inch SIP wall is 14. The Whole Wall R-value of a 2×4 wall is less than 10. The Whole Wall R-value of a 2×6 wall is between 11 and 13.7 depending on the quality of the installation of batt insulation. The elimination of thermal bridging and a more air tight envelope contribute to the higher Whole Wall R-Value of SIPs walls when compared to conventional metal and wood stud walls.
Since the SIPs’ foam core acts as a vapor barrier, the weather barrier must be permeable in order to allow the SIPs sheathing panels to dry outward. A continuous air space, between the drainage plane and the exterior cladding, and vented openings at the top and bottom of the walls to allow for convective air flow is recommended to ensure adequate drying of the SIPs. This also applies to SIPs used as roof structure. Air should be able to flow under the roofing material between the eave and the ridge. In addition, all panel joints, openings around windows and doors, and other chases should be properly sealed and/or flashed to prevent moisture infiltration.
The fastener requirements for exterior cladding and interior finishes are specific to the panel manufacturer; consult the manufacturer’s specifications for this information. It is recommended that a ventilation space is created with furring strips between the exterior face of the panel and the exterior cladding. This allows for the panels to dry out when water vapor enters the panel.
The quality of SIPs is set in the manufacturing stage. Proper lamination and smooth surfaces and edges will ensure that the SIPs can endure long-term use as long as the structural skins are properly protected from degradation. It is important to note that if moisture causes deterioration of the skins, then there is a structural issue that must be repaired. Repairs can require replacement of a much larger area than the just the deteriorated portion.
According to the National Resources Defense Council, using stressed-skin panels can reduce the time to frame the building envelope by more than one-third. This time savings can improve a builder’s productivity and profitability by 16 percent. In addition, the end product is energy- and wood-efficient, generating operating savings for the owner and minimizing negative forest impacts. NRDC estimates that a builder with ten crews can increase profits by $60,000 using stressed-skin panels.