Structural Insulated Panels

Introduction

Structural insulated panels (SIPs) are a ‘composite’ building material consisting of two structural board layers with an insulating layer of polyurethane foam. Usually the structural board is oriented strand board (OSB) and the foam is one of the following:

-Expanded Polystyrene Foam (EPS)
-Extruded polystyrene foam (XPS)
-Polyurethane foam

Structural insulated panels may act as I-beams or I-columns, and the rigid insulation core acts as a web while the oriented strand board sheathing acts as the flanges. Structural insulated panels can be used in lieu of several conventional building components can be replaced including insulation , studs, vapor barrier, joists, and air barrier. Structural insulated panels can be used on roofs, foundations, walls, and floors.

Below: An SIP panel comes off an assembly line in Arizona.

Background

The concept of using stress skinned panels in building structures dates back to the 1930s, although such panels did not become popular until the 1970s. Forest Products Laboratory (FPL) in Madison, Wisconsin performed research and testing for skinned panels as a government funded measure to preserve wilderness areas and reduce the need for commercial logging. The first home built with stressed-skin panels was built in 1937, although later when the building was torn apart, it was found that those panels which contained “paperboard” had decayed, while the other types of stress skinned panels retained their original strength. Further testing in 1947 including plywood, treated paperboard, and hardboard along with other materials was conducted. In 1967 a structure was built with polystyrene core panels that included paper overlaid with plywood. This building still stands today.

Materials

Most Structural insulated panels (SIPs) are made of two oriented strand board layers with a foam core in the center (with EPS, XPS, or Polyurethane foam as mentioned in the introduction). Sometimes plywood, steel, aluminum, stainless steel, fiber reinforced plastic, or other materials are used instead of OSB board, depending on the application, and where the board will be used in the structure of a building.

Dimensions

Structural insulated panels (SIPs) panels vary in size from about four feet to twenty four feet in width by eight to nine feet in height. Designers strive to use larger panels when making SIP based structures, because less panels overall results in less seams overall and less potential for air infiltration. Panels used are generally four and a half to six and a half inches thick when used in building a wall, and up to one foot thick when building a roof deck. The most common type of foam insulation used in SIPs is Expanded Polystyrene Foam (EPS). Generally there are 3.5 to 4.5 inches of foam in the panel, resulting in an R-value around 14.

Strengths of SIPs vs Traditional Stick Frames

Using SIPs instead of a normal building frame has both benefits and drawbacks. A well engineered and constructed home with SIPs has better insulation, and reduced energy costs for building occupants. SIP panels can be used in floors, walls, and even the ceiling, and they provide excellent insulation. Due to simplified construction methods--because SIPs are standardized and can be used in multiple parts of a structure; SIPs cut down on the time needed complete a structure. SIPs also provide more axial load strength than traditional building frames, and in some ways SIPs can be more flexible in adapting to custom architecture.

Below: A three dimensional cut away of an SIP panel.

Weaknesses of SIPs vs Traditional Stick Frames

One of the biggest potential issues with SIPs is that the wood may creep which could cause structural issues. An experienced designer can minimize this potential risk by accounting for this potential issue in the design of the structure. Another problem with SIPs is that they may need to be transported a great distance, and proper installation may require an outside specialist. The last major problem with SIPs is that some roofing materials manufacturers will not warranty their product on an SIP roof. For example, asphalt shingles have been proven to have a shorter life span than normal on an SIP application. As a result of this, most manufacturers will not warranty the application of their roofing materials on an SIP substrate.

Conclusion

Structural insulated panels (SIPs) have become a viable building option, although they have some weaknesses. Perhaps the greatest benefit, other than insulation and improved quality of living is that using these panels preserves our forests by decreasing the demand for wood. Using SIPs is unconventional, so getting a proper result may cost more than expect due to the need for skilled installers and designers. Substantial costs may be incurred due to the need to transport SIPs a great distance as well. However, these panels can greatly improve the insulation of a home, and hence, improve the quality of living for the occupants.

SIPs are really fascinating... we can drastically cut down on our use of lumber and improve insulation while giving up very little. Based on the fact that contractors generally refuse to guarantee roofs on an SIP substrate, perhaps it's best not to use this material on a roof until they can start making SIPs for roofs that can address this issue.

As technology improves, and as our environmental consciousness rises, perhaps one day SIPs will become more standard than they are today. Regardless, SIPs provide a viable alternative to designers building custom architecture, or to increase building speeds and improve insulation.