The Future Build

Green Materials Knowledge Series: Structural Insulating Panels



Structural Insulating Panels (SIPs) are typically factory produced, arriving at the construction site pre-engineered for the specific building with door and window openings cut. Wiring chases also can be predrilled. As a result, SIP structures tend to generate less wood waste than a stick built building, lessening the impact on already full landfills, and reducing the builder’s dumping fees. Oriented strand board (OSB), which forms the exterior of most SIPs, is made generally made from relatively small, plantation-grown trees, a more efficient source of wood fiber than large or old-growth trees. Overall, SIP buildings require less sawn timber than their stick-built counterparts.

Energy Efficiency

The R-4 value of SIPs per inch of thickness translates to a 2 x 4 wall offering a rating of R-16; a 2 x 6 wall carries an R-24 value. In addition to these R-values, SIPs are highly energy-efficient because their construction has fewer seams, in the thermal envelope an SIP home is about half that of a standard stick-built home. It should be noted that the expanded polystyrene insulations used in most SIPs is itself a petroleum product. However, an SIP house reportedly saves 19 times the amount of energy required to make the EPS foam cores in one year, much less its total life span.

Indoor Air Quality

The use of SIPs does pose some interior air-quality issues, although none of them is so serious as to exclude SIP from the list of “green” building materials in view of its resource and energy efficiency. These IAQ issues include out gassing of VOCs from certain types of insulating foam and facing materials used in SIP, as well as the ventilation characteristics of buildings constructed from SIPs. The polystyrene foam used in most SIPs is inert and not susceptible to out gassing. The urethane foam used in some SIPs can produce out-gassing if the surface of the foam is exposed to atmospheric pressure. However, the OSB or other “skin” that forms the outer surface of the SIP effectively seals the foam from the effects of the atmospheric pressure differential. As a result, urethane-foam SIPs reportedly do not experience significant out gassing or loss of R-value from the foam. Most of the OSB facing materials used in SIPs is made with phenol-formaldehyde binders that are subject to out gassing, although to a much lesser extent than the urea formaldehyde that is used in other engineered wood products such as particle board. About a third of the OSB manufactured in the west is made using a no formaldehyde containing binder, i.e., polymeric diphenyl methylene diisocyanate or MDI. Once cured, MDI does not off gas. The very air tightness of SIP construction that gives SIP construction its excellent thermal efficiency can contribute to reduced internal air quality, unless mechanical ventilation is used to ensure that fresh air is circulated throughout the structure. Inadequate ventilation can also lead to moisture condensation and mold or mildew problems.

Fire Resistance

SIPs are moderately fire resistant. Many manufacturers have conducted tests of 20-minute and 1-hour rated wall assemblies with SIP cores. Fifteen minutes is generally recognized as the time needed for building occupants to exit at the outbreak of a fire. Foam insulation is slow to ignite but when lit, it burns readily and emits a dense smoke that contains toxic gases. Foams used for construction require a ½" gypsum wallboard covering as a fire thermal barrier in addition to the OSB skin of the SIP. The drywall has a 15-minute fire rating that also provides the fire protection for conventionally built structures. Certain insurers have voiced concern about the structural integrity of a partially burned SIP floor or roof panel. SIPs’ moderate fire resistance is one of the reasons why they are not used more widely in high-rise and large public buildings.

Acoustical Properties

SIPs have excellent acoustical insulation properties. Their ability to insulate the interior environment from outside sound results from their closed construction, which allows no air movement inside the panel wall, and their extremely tight joint connections.

Technologies

SIPs consist of two outer skins and an inner core of an insulating material which have been bonded to form a monolithic unit. Most structural panels use either plywood or oriented strand board (OSB) for their facings. OSB is the most common facing material, in part because it is available in large sizes (up to 12' x 36'). Other materials, such as drywall, sheet metal, or finish timber are often laminated onto the OSB at the factory. This further speed up the building process by eliminating another step that otherwise would have to be carried out at the building site. The cores of SIPs can be made from a number of materials, including molded expanded polystyrene (EPS), extruded polystyrene (XPS), and urethane foam. Expanded polystyrene is most commonly used because of its low cost. However, EPS cores must be made thicker to be equivalent to the higher insulating properties of other foams. Some SIP producers use isocyanurate foam as the core material. The insulating core and the two skins of an SIP are generally not capable of bearing heavy loads by themselves, but when pressure-laminated together under strictly controlled conditions, they act synergistically to form a composite that is much stronger than the sum of its parts. Panel manufacturers supply splines, connectors, adhesives, and fasteners to erect their systems. When engineered and assembled properly, a structure built with these panels needs no frame or skeleton to support it. SIP manufacturing techniques vary among companies. The most common assembly processes are adhesive bonding and foam-in-place. Adhesive bonding involves the application of a structural-grade adhesive to both sides of the unfaced, pre-formed foam core. After the adhesive is applied, the rigid foam core is placed on top of a clean sheet of facing material and the second panel is positioned on the opposite side of the insulation core. Pressure is applied to the newly formed panel, which is then set aside to cure. Panels are then set aside until the adhesive has completely cured, i.e., about 24 hours.

End Users and Applications

SIPs are used mainly in residential and light commercial applications. Specific applications include walls, roof, and floor assemblies that provide strength and stiffness to a structure and facilitate longer clear spans than can be achieved with conventional framing. SIPs generally are not used in large or high-rise commercial or public buildings. While the foams used in SIPs are not particularly flammable (see above) they will burn when exposed to flame. As a result, their use in high-rise or large buildings requires extensive use of fire suppression technology. Most buildings higher than three stories are subject to a different set of building regulations due to the loads applied to the walls and floor systems. The current standard for this type of building is to construct the frame using structural steel members, then to infill the walls, floors, and partitions.


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