SIP Anatomy

SIPs Defined

Stress-Skin Panel

Structural Insulated Panels or SIPs as they are commonly referred to by construction and fabrication companies, are high performance building panels. SIPs are commonly employed in walls, floors, and roofs for residential and light commercial buildings.

SIPs consist of three components: the two outer skins, an inner core of insulating material, and the adhesive used to bond the facings to the core. When manufactured under the right factory controlled conditions, SIPs result in a building system that is energy efficient, structurally strong, and cost effective.

Building with SIPs has proven to be far superior to “stick framing” in that SIPs require less labor, time, and money.

The Three Components of SIPs


1. The Facings


OSB Small StackThe two outer skins of the SIP typically consist of either plywood or oriented strand board (OSB) with OSB being the principal facing used in SIPs since it is available in large dimensions and has had many successful structural tests performed. The later being increasingly important not only for personal safety but also where strict building codes have been implemented.


2. The Core


The core of SIPs are made from various materials,Foam Blockincluding expanded polystyrene (EPS), extruded polystyrene (XPS), and urethane foam. These various insulating cores in the SIPs provide in part the necessary R – value to protect us from the harshest of elements.


3. Adhesives


Glue on PanelThe adhesive in SIPs join the facings and the insulating core together. The most common adhesives resist moisture penetration, prevent panel delamination, and resist the forces of buckling and racking. As with all other ingredients in a SIP, they are code compliant.

Structural Insulated Panels are a Symbiotic Unit

It is interesting to note that the two facings of a SIP and the foam core are nonstructural by themselves. However, when the facing, foam core, and proper adhesives are pressure laminated under the right conditions, these materials act synergistically to form a composite product far stronger than the sum of its individual parts.

Common Misconceptions

Not all Building Panels are SIPs. There are many terms that get associated with building panels, such as: foam core panels, stress-skin panels, nail-base panels, sandwich panels, and curtain-wall panels, to name a few. However, many of these panels are nonstructural, and some have no insulation.

To add to the confusion, the term “panelized construction” can include prefabricated stud walls usually associated with the modular industry. Foam Laminates of Vermont uses the terms: foam core panels, SIPs, and stress-skin panels interchangeably, and we mean them to reflect building panels which can withstand high load stress and provide exceptional insulation.



SIPs are truely a dynamic product.OSB spline joint cross sectionAccording to the Forest Products Laboratory branch of the U.S. Department of Agriculture, a SIP panel is “a layered structural system composed of a low-density core material bonded to, and acting integrally with, relatively thin, high-strength facing materials. When used as a wall, roof, or floor system in housing, the sandwich panel provides exceptional strength for the amount of material used.”

“In a load-bearing wall, the two facings act as slender columns continuously supported by the core material to resist compression and bucking. In bending due to a live load or a wind load applied to a panel, the facings take most of the tensile and compressive forces and the core provides resistance to shear. The core and facings acting integrally provide the exceptional stiffness to the member.”

Wonderfully stated, but let’s examine these Structural Insulated Panels more closely.


OSB Stack

SIPs manufacturers use various different materials as the facing, but oriented strand board (OSB) is the main material currently in use.There are two main reasons why OSB is the facing of choice for SIPs manufactures. First, it is highly engineered wood product that has been rigorously tested and found to be structurally supportive enough to act as a load-bearing wall. Second, OSB is readily available in multiple dimensions, a criteria demanded by the SIPs industry.

Foam Laminates of Vermont uses the highest quality OSB currently available to manufacturers. In addition, our OSB panels have a fully waterproof bond with an Exposure 1 rating by the Engineered Wood Association (APA). An Exosure 1 rating means that our SIPs are designed for applications and locations where potential long construction delays could result in significant exposure to weather. Finally, our SIPs are treated with an an edge sealant in order to combat the tendency of wood to absorb moisture.

Core Materials


Most manufacturers use foam as the core material in aFoam BlockSIP. There are currently three main categories of foam core: EPS, XPS, and urethane foam. While each of these cores have unique properties, they all provide the necessary structural and fire resistance characteristics as required by the various building code authorities. In addition, foam offers exceptional thermal protection and energy efficiency.

Foam Laminates of Vermont uses EPS as the core of its panel, since it is proven and wildly accepted technology. EPS foam is essentially a closed-cell, moisture resistant structure composed of millions of tiny air-filled pockets. It is manufactured from beads that are formed by the polymerization of a styrene monomer with an expansion agent acting as the catalyst.

This expandable polystrene was first patented in 1950 by the BASF Corporation, and has since reached overwhelming popularity. In fact, it has been estimated that 85% of all SIPs use EPS as the core.

Our EPS foam core has the industry standard density of 1 pound per cubic foot (pcf), giving it an R-value of 3.85 at 30 degrees F per inch of thickness. To view our comparison of EPS foam core SIPs versus traditional fiberglass stick frames, click here. Finally, extensive testing has shown that our EPS foam panels are not susceptible to thermal drift, which is the tendency of a material to lose its insulating value over time.

Announcing Neopor

Neopor is an EPS product that has graphite integrated into the foam bead which reflects and absorbs radiant heat. It provides an increase in R value of about .8/inch over type I EPS. Neopor is a green product that does not contain any CFC’s, HCFC’s, or HFC’s. For R values see our technical section


Neopor Structural Panels



The adhesives is the final necessary keyPanel with glue exposedingredient of a SIP; it bonds the facings to the core. The strength of the glue is an important but often overlooked feature since it has to resist the forces of buckling and racking, resist moisture penetration, and prevent panel delamination.

Foam Laminates of Vermont uses the industry standard urethane glue. This adhesive can bond metal and various types of plastic skins to foam cores. In the case of the structurally insulated panel, this glue bonds EPS to our OSB facings.




To write the complete history of any product’s history especially when it has undergone numerous changes, is no easy task. For starters, the inception, creation, and subsequent development of an idea can have many contributors. This is especially true during the development phase of a product’s history, where many different sources may improve upon existing features or add new ones altogether.

To further add to the confusion, inventors may establish ideas at the same time independently, leading to a debate over priority. Further still, some inventors may develop ideas in silence and privacy. The end result is that these pioneers never receive credit and public recognition.

As any historian can attest, it is inherently difficult to provide a comprehensive history if for no other reason then some information is unknowable. For this reason, and the others mentioned previously, this article focuses more on the more popular and major contributions to the evolution of the SIP.

OSB Spline

Forest Products Laboratory

The origin of a structural insulated panel or SIP – according to many sources – can be traced back to 1935. At this time, engineers working at Forest Products Laboratory (FPL) located in Madison, Wisconsin speculated that plywood and other forms of hardboard sheathing could provide the necessary structural support in wall applications. With their concept in mind, the FPL engineers built a panel which consisted of insulation sandwiched between two skins with additional framing members placed within the panel for extra support.

After the creation of their prototype, Forest Products Laboratory entered their custom designed SIP into the marketplace where it sold for next thirty years. During this time, these SIPs underwent an extensive testing period. Indeed, these sandwich-panels were constantly disassembled and reexamined in order to find the best combination of components. FPL engineers also experimented with new designs and materials in order to maximize both strength and efficiency in their SIP.

Frank Lloyd Wright

Engineers from Forest Products Laboratory weren’t the only ones churning out structural panels. In fact, the 1930s saw sandwich-panel technology emerge from another source. Indeed, some of the earliest examples of SIPs can be found in the Usonian houses designed by none other than the famed architect Frank Lloyd Wright.

Frank Lloyd Wright was exceptionally innovative, and his SIPs were a result of his efforts to incorporate beauty and simplicity into cost-effective homes. Wright’s attempt at a panel contained no insulation; they consisted of three layers of plywood and two layers of tar paper. Due to the lack of insulation, this prototype failed to achieve widespread popularity and they were never produced on a large scale.

Alden B. Dow

Alden B. Dow – an architecture student of Frank Lloyd Wright – experimented further with the concept of structural panels. Dismayed over the lack of proper insulation in Wright’s Usonian homes, concerned about energy efficiency, and fearful over depleting natural recourses, Dow sought to create a structural panel with an insulated core. In 1950, Dow did just that and as a result he is generally credited with creating the first structural insulated panel.

Alden B. DowDow’s earliest SIPs were employed in homes located in Midland Michigan. The panels were composed of 1 5/8 in. styrofoam cores for the necessary insulation and 5/16 plywood facings to add structural support for the load bearing walls. These same SIPs were installed over roof framing on 42 inch centers. Many of these original SIPs can still be found in homes today.

Alden B. Dow is considered by many to be the first practitioner of SIPs.

Koppers Company

After the success of Dow’s SIP, it wasn’t long before entrepreneurs and manufacturers teamed up to offer their own SIPs as a new building material alternative. The most significant manufacturer to offer SIPs to the masses was Koppers Company. In 1959 they converted a Detroit auto production plant into a SIP production facility.

The Koppers Company SIP consisted of blowing preexpanded styrofoam beads between two sheets of plywood. The components were then bonded using steam and glued to a solid framework. This process resulted in a strong insulated panel.

However, there were some serious drawbacks to these new SIPs that prevented them from achieving widespread use. First, the Koppers’ manufacturing process was slow, resulting in longer then desirable construction delays. Second, Koppers met fierce and unexpected resistance from carpenters’ unions in the northern states who feared that SIPs could be built so quickly, they would result in lost wages.

From fear of loosing their jobs, these unions deliberately slowed the erection process from the typical two days to four. This sabotage and coupled with the fact that this was a period of inexpensive energy and labor, the panels couldn’t remain competitive. Koppers left the residential construction business and turned to building refrigerators.

Alside Home Program

In the 1960s, Alside Home Program (AHP) decided to make its own entry into the SIPs marketplace. The Alside Home Program SIP had a large advantage over its predecessors in that it could reduce production time of SIPs from several hours to 20 minutes. However, after years of production and less than 100 SIP homes built, the company went out of business due to a lack of demand. It wasn’t until the 1980s that SIP manufacturers returned who could produce SIPs with the capacity and demand that consumers expected.

The SIPs Resurgence

SIPs are gaining much popularity nowadays. Why the sudden change? Inevitably, there is always a lag between the invention of a building technology, its subsequent development and improvement process, and its acceptance into the marketplace. As in the case of SIPs, this delay has expanded across many decades.

The lack of performance data can also hinder a products acceptance by consumers which can leave doubt as to a product’s safety and reliability. Moreover, consumers may not be able to easily access this information. If that wasn’t enough, there is always resistance to change and a fear of using a non-mainstream technology.

Today, extensive testing shows that SIPs are a viable alternative building option. Indeed, SIPs are universally accepted by all building codes. To top it off, increased energy costs, heightened environmental concerns, negative anti-oil industry sentiment, and more globally conscious communities, have made SIPs a powerful alternative to conventional stick framing. Simply put: SIPs are the right material at the right time.

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