Definitions

sorptive

Structural insulated panel

Structural insulated panels (or structural insulating panels), SIPs, are a composite building material. They consist of a sandwich of two layers of structural board with an insulating layer of foam in between. The board is usually oriented strand board (OSB) and the foam either expanded polystyrene foam (EPS), extruded polystyrene foam (XPS) or polyurethane foam.

SIPs share the same structural properties as an I-beam or I-column. The rigid insulation core of the SIP performs as a web, while the OSB sheathing exhibits the same properties as the flanges. SIPs replace several components of conventional building such as studs and joists, insulation, vapor barrier and air barrier. As such they can be used for many different applications such as exterior wall, roof, floor and foundation systems.

History

Although foam-core panels gained attention in the 1970s, the idea of using stress skinned panels for construction began in the 1930s. Research and testing of the technology was done primarily by Forest Products Laboratory (FPL) in Madison, Wisconsin as part of U.S. Forest Service's attempts to conserve forest resources. In 1937, a small stressed-skin house was constructed and garnered enough attention to bring in First Lady Eleanor Roosevelt to dedicate the house. In a testament to the durability of such panel structures, it has endured the severe Wisconsin climate and is currently being used by University of Wisconsin-Madison as a day care center. With the success of the stress skinned panels, it was suggested stronger skins could take all of the structural load and eliminate the frame altogether.

Thus in 1947, structural insulated panel development began with corrugated paperboard cores were tested with various skin materials of plywood, tempered hardboard and treated paperboard. The building was dismantled in 1978 and most of the panels retained their original strength with the exception of paperboard which is unsuited to outdoor exposure. Panels consisting of polystyrene core and paper overlaid with plywood skins were used in a building in 1967 and the panels have performed well to the present day.

Materials

SIPs are most commonly made of OSB panels sandwiched around a foam core made of expanded polystyrene (EPS), extruded polystyrene (XPS) or rigid polyurethane foam, but other materials can be used, such as plywood, pressure-treated plywood for below-grade foundation walls, steel, aluminum, cementitious panels, and even exotic materials like stainless steel, fiber-reinforced plastic, and Magnesium Oxide. Some SIPs use fiber-cement or plywood for the panels, and agricultural fiber, such as wheat straw, for the core.

Benefits and drawbacks

The use of SIPs brings many benefits and some drawbacks when compared to a conventional framed building. A well built home using SIPs will have a tighter building envelope and the walls will have a higher insulative value, which leads to fewer drafts and a decrease in operating costs for maintaining a comfortable interior environment for the occupants. Also, due to the standardized and all-in-one nature of SIPs construction time can be reduced over building a frame home as well as requiring fewer trades for system integration. The panels can be used as floor, wall, and roof, with the use of the panels as floors being of particular benefit when used above an uninsulated space below.

An OSB skinned system structurally outperforms conventional stick framed construction in some cases; primarily in axial load strength. SIPs maintain similar versatility to stick framed houses when incorporating custom designs. Also, since SIPs work as framing, insulation, and exterior sheathing, and can come precut from the factory for the specific job, the exterior building envelope can be built quite quickly.

The EPS insulation is a closed cell insulation as compared to fiberglass insulation which is an open cell insulation. Both insulations' R-values are tested in a laboratory under steady state conditions where there is no air infiltration. When a SIP is installed as a wall, foundation, floor or roof system, the EPS is installed in a steady state environment, whereas fiberglass insulations are installed in a non-steady state environment because these wall, foundation, floor and roof systems have to be vented to remove moisture. Many research studies show that the R-values of fiberglass insulation decrease as the temperature differential of indoor and outdoor temperatures increase resulting in higher energy costs to the homeowner.

EPS foam is a non-toxic hydrocarbon. Burning it results only in water vapor, carbon dioxide and trace levels of ash, similar to paper.

The long-term effects of using wood or unconventional materials in roof applications also require further research. Wood or other bio-based materials creep over time, and the long term viability of the insulating materials is also unknown. In climates where the relative humidity of the air varies a lot, for example, wood creep may be particularly bad (mechano-sorptive effects). While the use of SIPs in residential applications has many potential benefits, caution must be used to ensure that the lack of consideration for such effects does not lead to the creation of ill-designed structures. Use of an experienced architect or designer will minimize this potential issue.

However, as a non-standard material it may be more expensive to purchase SIPs and may need to be transported greater distances. Also, despite having easier installation than framed buildings, it may be necessary to hire consultants to properly assemble SIPs.

Dimensions and characteristics

In the United States, SIPs tend to come in sizes from 4 feet (1.22 m) to 24 feet (7.32 m) in width. Elsewhere, typical product dimensions are 300, 600, or 1200 mm wide and 2.4, 2.7, and 3 m long, with roof SIPs up to 6 m long. Smaller sections ease transportation and handling, but the use of the largest panel possible will create the best insulated building. At 15−20 kg/m², longer panels can become difficult to work with without the use of a crane to position them, and this is a consideration that must be taken into account due to cost and site limitations. Also of note is that when needed for special circumstances longer spans can often be requested, such as for a long roof span. Typical U.S. height for panels is eight or nine feet (2.44 to 2.75 m). Wall panels tend to come in 125–200 mm thicknesses (US: 4.5–6.5 inches), but can be made up to 300 mm (US: 1 ft) for roofs.

EPS is the most common of the foams used and has an R-value (thermal resistance) of about 4 K·m²/W per 25 mm thickness, which would give the 3.5 inches of foam in a 4.5 inch thick panel an R value of 13.8 (caution: extrapolating R-values over thickness may be imprecise due to non-linear thermal properties of most materials). This at face value appears to be comparable to an R-13 batt of fiberglass, but because in a standard stick frame house there is significantly more wall containing low R value wood that acts as a cold bridge, the thermal performance of the R-13.8 SIP wall will be considerably better.

The air sealing features of SIP homes resulted in the Environmental Protection Agency's Energy Star program to establish an inspection protocol in lieu of the typically required blower door test to assess the home's air leakage. This serves to speed the process and save the builder/homeowner money.

External links

References

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