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Structural Insulated Panels

A "foam-core panel," stress-skin panel, sandwich panel, or structural foam panel is the same thing as a Structural Insulated Panel (SIP) which is the modern term for this type of construction. A SIP consists of carefully engineered laminate with a foam core 4 to 8 inches thick (10.2 to 20.3 cm) with a structural facing on each side. The most common types of facings are drywall and/or structural wood sheathing such as plywood and oriented strand board (OSB).

SIP construction can replace stud (a.k.a. "stick") framing in almost any construction setting, but are most common in residential construction. The greatest advantage of these panels is that they provide superior and uniform insulation when compared to more common methods of house construction. When installed properly, SIP's also provide a more airtight dwelling. This makes the building more comfortable, improves energy use in both the winter and summer, and makes the house quieter.

The speed of construction when using SIP's is much faster than other types of residential construction, especially if the builder is familiar with them. Shells can be erected quickly, saving time and money, without compromising quality. Testing has shown that a wall panel with two, half-inch (1.3 cm) thick OSB skins is nearly three times stronger than a conventional 2´ 4 inch (5.1´ 10.2 cm) stud wall, even though the SIPs were assembled many times faster than a "stick" framed wall of similar size.

Many SIP manufacturers also offer "panelized housing kits." The builder needs only to assemble the pre-cut pieces. Additional openings for doors and windows can be cut with standard tools at the construction site. Even though SIPs cost more than other construction systems, they require considerably less skilled labor too.


The Florida Solar Energy Center (FSEC) found a 12% to 17% energy savings from using SIP construction. The FSEC also monitored side-by-side SIP and conventional wood-framed structures for several winter months. The airtightness of the foam-core house (measured at 0.21 air changes per hour [ach]) and was better than the conventional wood-framed house (measured at 0.27 ach).

Types of Panels

SIPs use a rigid-insulation core made of one of three plastics: 1) expanded polystyrene (EPS); 2) polyurethane; or 3) polyisocyanurate, a polyurethane derivative. Some manufacturers are also examining ways of using cementitious or fibrous core insulating materials. A company based in Texas has started limited production of a SIP product made with a core of compressed straw laminated to OSB.

EPS and XPS Panels

The majority of SIPs are manufactured with expanded polystyrene (EPS.) This foam is commonly known as beadboard. This type of SIP has a nominal R-value of about 4 per inch (2.5 cm) of thickness. Unlike other types of foam insulation, beadboard uses pentane as the expanding agent. Extruded Polystyrene (XPS), with R values of 5 per inch (2.5 cm) of thickness is also sometimes found. It uses a hydrochlorofluorocarbon (HCFC) gas as the expanding agent.

Standard thickness' for either type range from 3.5 to 7.5 inches (89-190 mm) for wall panels and 5.5 to 11.5 inches (140-292 mm) for ceiling panels. They are available in almost any size, however, common wall panel are 41´ 81 inches (1.04´ 1.06 meters) and weighs 110 pounds (50 kilograms [kg]). Most manufacturers can also make panels as large as 81´ 281 inches (1.06´ 7.14 m), which require a crane to erect.

Polyurethane/Isocyanurate SIPs

Some manufacturers choose to use polyurethane and isocyanurate as the insulating material. The foam is injected between the two wood skins under considerable pressure and, when hardened, produces a strong bond between the foam and the skins.

Aged polyurethane and isocyanurate SIPs have a nominal R-value of around R-6 to R-7 per inch (2.5 cm) of thickness. Both contain a blowing agent (an HCFC gas), some of which escapes over time, reducing the initial R-value of the SIP from about R-9 to R-7.

Wall panels made of polyurethane or isocyanurate are typically 3.5 (89 mm) thick. Ceiling panels are up to 7.5 inches (190 mm) thick. Polyurethane/isocyanurate panels, although more expensive, are more fire and water vapor-diffusion resistant than EPS, and insulates some 30% to 40% better than EPS or XPS, per given thickness.

There are also non-structural panels made of with any of the above mentioned foams. These are far weaker structurally than true SIPs and are only intended for applications such as curtain walls with no loads imposed on them and roofs where there is no attic space for additional insulation.


SIP walls are superior to conventional walls in a number of ways. SIPs combine a high level of insulation with speed and ease of construction. The solid foam core eliminates air movement within the walls and minimizes thermal bridges through wood studs. Together, all these reduce air infiltration and with proper installation, make a tightly sealed/ easily controlled house.

When installed according to manufacturers' recommendations, SIPs meet all building codes and pass the American Society for Testing and Materials (ASTM) standards of safety. Fire investigators have found that in buildings constructed of SIPs the panels held up well. For example, in one case where the structure exceeded 1,000°F (538°C) in the ceiling areas and 200°F (93°C) near the floors, most wall panels and much of the ceiling remained intact. An examination of the wall panels revealed that the foam-core had neither melted nor delaminated from the skins. In similar cases, a lack of oxygen seemingly caused the fire to extinguish itself. The air supply in a structural insulated panel home can be quickly consumed in a fire.


Fire safety and insect problems are the two main issues associated with SIPs. However, experts agree that since the walls are covered with a fire-rated material such as drywall the toxic gases released by burning carpets and other furnishings are far more dangerous. The fire resistance of the drywall covering also protects the foam long enough to give the occupants a good measure of escape time.

Insects and rodents (like with any house) may become a problem for SIPs too. Any foam can provide a good environment for them to dwell. A few cases have been noted where insects and rodents have tunneled throughout the SIPs. Some manufacturers issue guidelines for preventing these problems. Such guidelines often include: applying insecticides to the panels, treating the ground with insecticides both before and after initial construction, backfilling, maintaining indoor humidity levels below 50%, locating outdoor plantings at least two feet (0.6 meters) away from the walls, and trimming any over hanging tree limbs. Boric acid-treated insulation panels are also available. This is an insecticide used in other insulation materials that is relatively harmless to humans and pets.

The airtightness of a well-built SIP structure requires controlled fresh-air ventilation for safety, health, and performance, and by many building codes as well. This is the way well-built modern houses should be anyway. The air in a building cannot be conditioned and controlled efficiently unless it can be contained. SIP's do a very good job of this, as long as the builder pays strict attention to the manufacturer's installation and construction and guidelines. Failure to follows these guidelines could negate the benefits of a SIP structure. A well-designed and installed and properly operated mechanical ventilation system is also very important to achieve the energy savings benefits of a SIP structure, and to avoid indoor moisture problems, especially in humid climates.


An increasing number of houses are being built with SIPs. They are attractive because of their relatively high-uniform R-values, square, flat and plumb walls, and ease and speed of construction. Problems with natural pests can be minimized with adequate prevention measures. Buildings made of SIPs appear to be safer than some other types, even in fire.

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Environmental Advantages

Greenix Panels are an eco-friendly method of construction:

Environmentally Friendly Materials

The standard outer skins of GP/W Greenix Panels are manufactured using Oriented Strand Board (OSB). This is made from young fast growing trees which are deliberately grown in plantations accredited by the Forest Stewardship Council (FSC). Young trees produce oxygen and remove more carbon dioxide from our atmosphere than mature trees and are renewable, recyclable, bio-degradable and non-toxic. The core insulation is Polyurethane foam (PUR) which is CFC and HCFC free and has an ozone depletion potential of zero.

Reduced Timber Usage

To make a product with the same strength, timber uses 20% of the energy needed to make concrete. Timber is naturally low in thermal conductivity and an excellent insulator. It is 15 times better as an insulator than concrete and 1770 times better than aluminum. A 1" timber board has better thermal resistance than a 5" brick wall. When compared with traditional timber frame buildings, Greenix Panel construction uses approximately 50% less wood, which all adds to the positive environmental impact of this form of construction.

High Insulation Value

Greenix Panels provide an extremely high thermal performance and R-values can be as high as R-40. There is minimal thermal bridging compared to conventional construction and the insulation will not sag or slump over time. The SIP envelope is designed to be draft free and is warm in the winter and cool in the summer. This drastically reduces energy consumption which leads to a corresponding reduction of up to 60% in carbon dioxide emissions from the burning of fossil fuels for the lifetime of the building.

Minimal Waste

Building with Greenix Panels produces less waste than other forms of construction. The panels are engineered in a controlled environment, reducing the amount of waste produced on the building site and allowing unused materials to be recycled.

Green Building Programs

Building with Greenix Panels substantially cut home energy consumption, making it easy to reach Energy Star qualifications.  Greenix Panels contribute towards certification under both the LEED and NAHB standards. An Energy Star labeled home qualifies for LEED credits in multiple categories.

Physical Advantages

The composite structure of Greenix Panels offer many physical advantages:


Once completed, a Greenix Panel structure forms a monolithic shell is up to seven times stronger than a traditional frame structure and is a popular building method in areas subject to earthquakes, tornadoes and hurricanes. This provides a building with a very robust and solid feel which can be indistinguishable from a traditionally constructed building once complete.

Airtight and Quiet

Buildings constructed with Greenix Panels are quieter and more comfortable to live in. The panels provide significant resistance to airborne sound. The inherent airtight construction lends itself to modern ventilation systems which reduce energy costs and improve air quality.

Resistant to Rodents, Mold and Microbes

The PUR (Polyurethane) core and the OSB facings used in the manufacture of Greenix Panels do not provide any food value to vermin and are resistant to attack by mold and microbes.

Integral Insulation

The solid core of PUR foam insulation in the panels cannot slump over time and the minimal thermal bridging ensures that the building is heated evenly, free from cold spots and condensation. The building will benefit from reduced heating costs for its entire lifetime. This is an important consideration at a time of spiralling energy costs.

Rigid Surface for Installations

The standard OSB skin of a SIPs structure provides a continuous surface for easy installing of services, lights, hangings and kitchen units without having to build in additional support in the structure.

Choice of Finish

A Greenix Panel structure will form the hidden shell of the building. The external walls can then be finished in many ways. These can include traditional cavity and facing brickwork, stucco and siding. Internal walls are normally finished with drywall. Roofs are finished as normal, although the inherent strength of a Greenix Panel roof does not require the same weight of finish as a conventional vented roof void. This can allow the use of a lightweight roof finish if desired. Although an alternate roof design may be required.

Financial Advantages

Cost comparisons have shown that building with Greenix Panels is no more expensive than traditional building techniques when the following factors are taken into consideration:

Reduced Cost of Land

A Greenix Panel SIP Plan can be designed to utilize attic space, thus creating additional living area which results in a smaller footprint.  Example: A three bedroom house can be turned into a five bedroom house on the same size lot.

Reduced Cost of Foundation

A Greenix Panel structure created a lighter building envelope than brick or block and therefore could allow for a reduced foundation design.

Reduced Cost of Loss through Damage or Theft

The elements which make up a SIPs structure are generally very large and robust and are therefore inherently unlikely to be damaged or stolen from the jobsite. A significant percentage of traditional building materials are either lost, damaged or stolen from jobsites. The speed of build and the nature of Greenix Panels virtually eliminates this problem.

Reduced Cost of Labor

A Greenix Panel structure simplifies the process of construction and can reduce construction time by up to 60%. Once the foundations have been completed, a Greenix Panels pre-engineered structure arrives at the jobsite and within weeks can become a weather-tight structural shell ready for windows, doors, services and internal and external finishes. The nature of Greenix Panels provides a completed unit that also reduces installation steps and so reduces the eventual cost of multiple inspections.

Reduced Cost of Waste Disposal

A Greenix Panel structure is pre-engineered off-site to avoid both the additional labor and the ever increasing cost of the safe disposal of waste materials.

Reduced Jobsite Costs

A faster build program will reduce the overall cost of project management, and in turn will also reduce the cost of hiring scaffolding, plant, safety and security equipment and temporary accommodations.

Reduced Heating Costs

The unbeatable insulating qualities of Greenix Panels allow a much smaller heating system, with combined with a mechanical ventilation system to be installed. This will permanently reduce heating bills by up to 60% over the lifetime of the building when compared with traditional methods of construction.

Quicker Return on Investment

Building a Greenix Panel structure can radically increase the speed of construction, providing a quicker return on investment. This allows the capital to be used on the next project when it would otherwise still be tied up on a traditional build. A significant gain can be made over a period of time maximizing the return on the initial investment and maintaining a positive cash flow.

Better Resale Value

Greenix Panel houses are attractive to future buyers and occupants, due to the reduced utility costs and the many other benefits that a Greenix Panel house provides.

SIP Design Advantages

Building with Greenix Panels has many advantages for you, your architect and your builder:

Design Flexibility

Greenix Panels create new possibilities of design without the constraints of conventional building materials.

Additional Floor Area

Greenix Panels can create additional warm living space in the roof by eliminating conventional trusses and creating loft or living attic areas.  Be sure to have your designer plan for SIPs when drawing your plans, as many conventional plans do not allow for truss-less construction.  When properly designed for SIPs,  Greenix Panels can provide up to an additional 25% of floor space at no additional cost and makes use of all of the potential space in the building. 

Thermal Performance

Greenix Panels can offer a thinner wall thickness than a typical EPS wall of lesser R-Values, and still deliver improved insulation values, gaining greater internal space for the same size overall footprint. Unbeatable insulation value means energy costs can be reduced by up to 60% over traditional construction.

Structural Advantages

Structural superiority has been proven by independent testing. Point loads can be dispersed through the monolithic structure.

Construction Benefits

Greenix Panels are better for your builder. Pre-cut and fabricated panels can reduce installation time by 60%. The building process requires less skilled labor and less supervision. Specialized tools are not required and fixed pricing allows budgets to remain accurate.