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What Is a SIP?
by Brandon Helms
Structural Insulated Panel (SIP) is a general term for a structural building component composed of load-bearing skins (outer layers) continuously bonded to a rigid insulating core. The panels are also referred to as ‘sandwich panels', which describes the way they look, and ‘stressed skin panels', which describes the way they work.
The most common panel skins are 7/16-inch thick sheets of OSB (oriented strand board), though cement skins and metal (steel or aluminum) are also available. The SIP core is most commonly expanded polystyrene (EPS), although polyurethane, polyisocyanurate, extruded polystyrene (XPS), or wheat straw cores are also available.
Introduction to SIPs
Several years ago when our engineering firm was based near Sacramento, California, we were approached by an architect we routinely worked with about doing the engineering on a new large custom home. The catch was that the homeowner insisted on building the house with structural insulated panels (SIPs). Not familiar with SIPs like many others in the design community, I reluctantly provided a proposal to the architect. To my dismay, he accepted it.
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In the back of my mind, serious questions arose about the mental stability of anyone who would build a structure out of Styrofoam. So, I set out to find what was wrong with the system and dissuade the homeowners from their decision to work with SIPs. Much to my surprise, the more I learned about SIPs, the more I realized why they were a good idea.
Applications of SIPs
SIPs can be used for a variety of applications including floors, walls, and roofs. The use of panels in floors is generally limited to the first floor over a crawlspace. The floor panels are ordered with a 3/4-inch top skin. With installed panels, the floor is completely insulated and finished. With conventional framing and batt insulation, it takes many more steps to arrive at having an insulated floor. Also, it is common to have the insulation sag, which gets worse with time, leaving the floor uninsulated. But with SIPs, the insulation is continuously bonded to the OSB and a gap between insulation and sheathing will not develop over time -- meaning that the floor will remain at a comfortable temperature.
Wall panels are typically used in exterior house and garage walls. Interior walls don't need the insulation or the sheathing that the panels are made of, so stick framing and drywall (without insulation) costs less without compromising the home's performance. Roof panels are most effectively used with a gable, monoslope, or flat roof. The panels can also be used on hipped roofs, but the installation can become more labor intensive.
How & Why SIPs Work
The first thought that crossed my mind when introduced to SIPs was, "What is wrong with people that want to build out of foam?" In reality, the foam has little to do with the structural capacity of the panels. There are several loading conditions for the SIP panels:
Transverse Loading (Floor or Roof Panels): Under this loading condition, the panels are installed horizontal and the loading is applied to the top skin of the panels. The SIP in this application replaces both the joists or rafters and the sheathing. In conventional framing, the OSB bends between supports.
With SIPs, we change the way loads are distributed to the OSB: as the SIP panel is loaded, the top skin is in compression and the bottom skin is in tension. The panels in this type of loading act like a steel 'I' beam or an engineered 'I' joist where the strong material at the edges of the member are connected by a relatively weak web, making them significantly stronger than if not connected.
Axial or Vertical Loading (Walls): A typical wall uses studs to support vertical loads from roof or floors above, whereas SIP panels rely on the skins of the panel to support all of the vertical loads. The question then becomes, how much load can a 7/16-inch OSB skin carry? As with many wood members, the main constraint on the vertical load carrying capacity of a sheet of OSB is buckling. A sheet of OSB will bow under its own weight when stood on end, meaning that a sheet of OSB on its own has virtually no load carrying capacity.
However, in the case of the SIP panels, the two layers of OSB create a composite structural member that behaves differently than the components do separately. The two skins work to prevent each other from buckling and the SIP panel can carry substantial vertical loads without any additional framing members. Distributed loads of nearly 5,000 pounds per foot of wall can be supported, which means very large buildings can use SIPs as bearing walls. We have designed multi-story buildings in Tahoe with snow loads of 250 pounds per square foot (most of Oregon is 25 pounds per square foot). We also designed 4-story buildings that used SIPs as the bearing walls on the first floor.
Axial or Vertical Loading (Headers): We try to use the SIP panel as the header over all door and window openings. There is a continuous top plate in the wall and at the top of the opening providing for the installation of 2x lumber. The result is a box beam with lumber top and bottom and sheathing on each side. The panels can also be spliced (joining two panels) over a window opening, which reduces the capacity, but in many cases the resulting header has adequate capacity to support the loads.
Lateral Loading (Out of Plane): This loading condition is usually caused by wind blowing on the outside of a wall. The panels perform in the same manner as the roof or floor panels (described above in transverse loading).
Lateral Loading (Shear): In conventional framing, the studs in a wall have very little to do with the shear capacity. The main component that resists lateral loads is the sheathing installed on the outside of the building, which creates shear walls. The nailing pattern at the perimeter of the sheathing is the main factor in the shear capacity of a wall. By virtue of what SIPs are, the building is constructed with a two-sided shear wall at the entire perimeter of the building. Because SIP panels have a limited general code acceptance, using a manufacturer's published test data for shear walls provides the greatest flexibility of design.
Code & Testing
Since SIPs are not generally accepted by the code, each panel manufacturer is required to test their panels and submit the testing to the code body for acceptance. Many manufacturers have minimal testing that can require substantial additional lumber in the project to meet the shear (seismic and wind) loading requirements of the West Coast. Selecting a manufacturer with a depth and breadth of testing and technical data accepted by the code can greatly impact the overall cost and performance of a project. More lumber in a project increases the cost and decreases the energy performance of a structure due to thermal bridging (heat moving through the solid lumber) and the potential for air leakage at the lumber connection.
Sizes & Thicknesses
Most panels found on the market are made of OSB skins with EPS cores. The OSB skins allow the buildings to remain as wood framed buildings, and the EPS cores are well insulating and easy to work with. The OSB is a consistent, engineered product that is rated the same as CDX plywood. The OSB sheets are available in sizes up to 8 by 28 feet, although the standard size is 8 by 24 feet. The large sizes allow for panels to be installed in large areas, minimizing the number of joints as well as the amount of lumber that is needed to support the SIPs. The EPS can be melted to allow for installation of lumber, electrical chases, etc., which makes it easier to work with than many of the other types of cores.
The cores of the panels come in thicknesses that match the measurements of dimensional lumber. Nominally, the standard panel sizes are 4-, 6-, 8-, 10-, and 12-inches thick. The panels can be special ordered in thicknesses to match engineered lumber since all panels are pressed specifically for each project. Curved panels of the same composition can also be incorporated into designs, though they are more expensive to manufacture.
Although the panels come in 4- and 8-foot widths, they can be adapted to fit any design. In the shop drawing process, we maximize the use of the panels in the building by using cutoffs from some panels to fill other areas. We generally recommend that the floor plan be worked out to fit the client's requirements and then we can adapt the SIP panels to fit their needs. Although any project can be converted to SIP construction, if a structure is designed with SIPs in mind from the beginning, minor adjustments simplify the construction process.
Electrical: Electrical chases are provided in all wall panels at plug and switch height, as well as vertical chases at approximately 4 foot on center. Any additional chases needed should be planned for in the architectural design, such as for wall sconces, surround sound speakers, and data cables. The preformed chases allow the electrician to have a grid in which wires can be pulled and installed. Remodel boxes are installed for all electrical boxes. Recessed lighting cannot be used in the SIP panels because the panels do not allow for the heat generated by these lights to dissipate.
Plumbing: Good construction practice is to keep all plumbing inside the building, not in the exterior walls. This is especially true with SIP structures. Plumbing is at best difficult to install in the SIPs and any maintenance or repair would require wall disassembly to access the plumbing. Planning interior plumbing walls and incorporating furred out walls in select areas greatly simplifies the construction for a building.
SIPs have a higher R-Value than conventional fiberglass batt insulation (R-24 for a 5 1/2-inch core SIP as opposed to R-19 for a 2 by 6 conventional wall). Although we have all been conditioned to think in terms of R-Value, there is much more to the energy efficiency of a building envelope than the R-Value alone. The idea being that all the R-Value in the world doesn't do any good if the front door is left wide open. By building with SIPs, we are in effect "closing the front door." A properly installed SIP system eliminates air infiltration and exfiltration from a building, meaning a 6-inch SIP wall will perform up to 60% more efficiently than a 2 by 6 conventionally framed wall.
SIPs fit well into a green building approach for both the renewable nature of the materials and the energy conservation of completed structures. The OSB is made of farmed lumber and is free from Urea Formaldehyde (no off-gassing hazard). The EPS used for the cores of the panels is an extremely stable product and does not off-gas. It's also completely recyclable and waste generated in the factory is re-ground and cast into more EPS. A panel house uses substantially less lumber in the shell, and a prefabricated panel package greatly reduces the amount of onsite waste generated during construction when compared to stick framing. From cradle to grave, panels are one of the most environmentally friendly ways to build.
In many ways, SIPs are comparable to conventional stick framing construction, which is good news to general and framing contractors who are trained and experienced in framing. The SIPs are a wood panel and are installed using all of the "normal" installation tools most framing contractors are used to working with -- circular and reciprocating saws, hammers, and nail guns. The only specialized tool that is required is a foam scoop, which generally is provided with a panel package. Additional specialized tools that can make the installation easier or faster are chainsaws and ratchet straps.
Because the skins of the SIP are carrying vertical loads, the skins of the panels have to be supported continuously by the foundation, which is the main reason for any differences in construction details.
The learning curve for building with SIPs is fairly short and any good framing contractor can build with SIPs. For the project owner, it is important to select a high quality builder since the framer is also the insulator and lays the groundwork for the electrician. All of the joints in the SIP are sealed with both an elastomeric caulk (panel mastic) as well as an elastomeric self-adhesive membrane (SIP tape) to ensure the panel is properly sealed against air leakage. Any lumber, such as a post under a ridge beam, has to be drilled to match the preformed electrical chases in the panels to ensure a continuous chase for the electrician. As with many other issues, the post can be drilled after the SIPs are installed -- it is just more labor intensive.
HVAC & Water Management
A properly sized heating and air conditioning system is vitally important to the success of a SIP structure. In addition to being a larger upfront investment, an oversized air conditioning unit will run in short bursts, which is less efficient. This "short cycling" cools the interior air quickly, but the air conditioning system does not run long enough to effectively de-humidify the air. The result is a cool clammy environment, which is ideal to promote mold growth. To avoid this situation, it is important HACs (heating and air conditioning contractors) are not allowed to work on SIP buildings unless they are willing and able to properly size equipment.
The ventilation of the structure has to be carefully planned and executed, which can be accomplished as simply as using bath and kitchen fans to exhaust air, with a duct to introduce fresh air into the air handler. Other more sophisticated methods, such as heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs), can be employed to manage proper ventilation.
One of the main reasons behind the meticulous care taken to ensure no air leakage occurs in SIPs is to ensure no water vapor is allowed to enter the interior of the panels. Managing the humidity in the building is the first step, but the proper sealing of all of the joints, particularly in the roof system, will make certain any water vapor contained in the interior air is not allowed to leak into the roof system where the water may condense. Because the SIP system is a closed system where no water vapor is allowed in, the roof no longer needs to be ventilated. This also means ceilings follow roof lines, creating vaulted ceilings inside.
Years of experience dealing with SIP projects ranging from simple single-story residential additions to large commercial projects has taught me two important lessons I encourage everyone to consider when building with SIPs. First, one of my earliest conclusions when evaluating SIP panels has held true without exception: proper installation is the key to a successful project. Second, use a high quality panel from a reputable manufacturer.
In response to the need in the industry for competent support for design professionals, contractors, and homeowners, Brandon and Mindy Helms began working with SIPs as distributors for Premier Building Systems. As the principal engineer for Maple Brook Engineering, Inc. [Newport, Oregon, maple-brook.com] and president of Panel Source, Inc., Brandon can provide education to architects, contractors, and homeowners necessary to make certain every project they are involved with will be successful from start to finish and perform well for generations to come.
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