Structural Insulated Panel (SIP) Construction

SIPs are a "new" building material that has actually been in use since 1952. The first known SIP buildings were erected by Alden Dow, son of the owner of the Dow Chemical company. These buildings have been in continuous occupation ever since and are still structurally sound. The concept of opposing faced panels grew out of WWII research on stressed skin panels used to build lightweight aircraft components. After the war, Dow and others sought peacetime uses for this revolutionary material — and developed, among other things, the ubiquitous Styrofoam® coffee cup.

SIPs consist of two outer skins and an inner core of insulating material to form a monolithic unit. Most structural panels use Oriented Strand Board (OSB) for their skins or "facings" in part because the material is available in large sizes (up to 12’ x 36’ sheets) and OSB performance has been rigorously tested for building code approvals. The core of a SIP is made from Expanded Polystyrene (EPS), Extruded Polystyrene (XPS), or Urethane Foam. All of these are high-R-value insulation materials.

The insulating core and the two skins of a SIP are insubstantial components in themselves, but when pressure-laminated together under strictly controlled conditions, these materials act synergistically to form a composite that is stronger than the sum of its parts. Panel fabricators supply splines, connectors, adhesives, and fasteners to erect these systems. When engineered and assembled properly, a structure built with SIPs needs no frame or skeleton to support it.

SIPs outperform conventional wall, roof and floor building methods in virtually every category: more energy efficient and draft free, stronger and quieter than traditional stud framing with fiberglass batt insulation. A 4" SIP wall surpasses a 6" frame wall in thermal efficiency. Fiberglass is sometimes used for furnace filters because air moves through so freely. Rigid insulation is used as a solid component insulation in almost every industry for its inherent efficiency and lack of air movement. These attributes are built right in to the SIPs building. Less air movement or leakage translates into fewer drafts, fewer penetrations for noise, lower energy bills and a significantly more comfortable and controllable indoor environment.

Thermal Insulation
The superiority of SIP construction to conventional framing with fiberglass batt insulation was clearly shown in a recent test conducted by the Oak Ridge National Laboratory (ORNL). The detailed study of various methods for constructing a house envelope (i.e. walls, roof, floor) found that SIP framing provides a much higher whole-wall R-value than a comparable conventionally framed house.

The study measured "whole wall" thermal transfer performance of SIP and conventional framed walls. Whole-wall measurements take into consideration heat loss due to seams and thermal bridging through wall studs, and are therefore more accurate than testing only the insulation material when measuring the R-values of buildings A 4-inch SIP wall scored R-14 on the whole wall tests, compared to R-9.8 for a 2" x 4" stud frame wall. The results of whole-wall tests of 6-inch SIPs compared to 2" x 6" wood stud walls were similar. The SIP wall scored R-21.6, while the wood stud wall scored a whole-wall R-value of 13.7. The most notable result was that a 4" SIP wall out-performed a 6" conventional wall.

These results are not that surprising, since SIP-built houses have fewer seams and therefore tend to be more airtight than stick-built houses — and the seams they do have are usually caulked and/or sealed with a special tape. Also, since the insulation exists between two load-bearing panels, there is less framing needed in SIP building and therefore less thermal bridging through wall studs.

SIPs perform at about 97% of their stated R-value overall, losing only 3% to nail holes, seams and splines. Because of the variety of thermal breaks in conventional wall construction, the whole wall performance is 30% less than the stated R-value of the wall. Stud walls lose thermal performance to studs, nails, screws, wiring, switch boxes, and other breaks in the thermal barrier.

Even this relatively poor performance by conventional walls depended on fiberglass batt insulation being carefully installed with no air voids. If there are even small defects in installation, R-value plumets. The ORNL study's authors, Jeffrey Christian and Jan Kosny, reported:

"[T]the whole-wall R-value of a 2 x 6 wood frame wall with R-19 fiberglass batts installed with rounded shoulders, 2% cavity voids, and the paper faces fastened to the inside surface of each stud was only R-11. This whole-wall R-value represents a 42% reduction from the R-19 value printed on the fiberglass batt's label. The seemingly insignificant insulation installation errors and thermal shorts resulting from interface details accumulate to significant impacts". ("Calculating Whole Wall R-Values on the Net" from Home Energy Magazine Online, November-December 1999).

Structural Strength
Structurally, a SIP can be compared to an I-beam: the foam core acts as the web, while the facings are analogous to the I-beam's flanges. All of the elements of a SIP are stressed, the skins are in tension and compression, while the core resists shear and buckling. Under load, the facings of a SIP act as slender columns, and the core stabilizes the facings and resists forces trying to deflect the columns. The thicker the core, the better the panel resists buckling, so larger-core SIPs offer more insulation and are stronger as well.

Compression Strength
ORNL Logo The ORNL Whole Wall Insulation Study
For its study of the effectiveness of house insulation, the Oak Ridge National Laboratory has developed a more accurate rating of insulation effectiveness that it calls the "Whole Wall" rating.

According to the study, older measures of thermal resistance are misleading because they do not take into account all of the possible "thermal shorts" through the insulation. A short is simply a place in the wall where the insulation is missing or interrupted by other materials. A stud in a conventional wall is a short, as is the gap left for an electrical box.

Oak Ridge proposes an R-value rating for the entire opaque wall (not including windows and doors) to measure the thermal performance of not only the insulation and structural elements, but also typical envelope interface details such as intersection with other walls, floor, foundation and windows. The standard also considers such previously ignored factors such as moisture resistance (the insulation value of some materials when wet degrades considerably), thermal mass, and air infiltration resistance (heat moves with air).

Using its new rating system to study the effectiveness of various insulation materials in typical house walls, the Laboratory found large differences between the nominal ratings of insulation and its actual thermal performance in a wall.

The best performer was insulated concrete forms due to the excellent thermal resistance of the expanded foam exterior combined with the thermal mass of concrete interior of these structures. The next best was structural insulated panels. The conclusion of the study was that a 4" SIP wall was found to be more effective at blocking heat transfer than a 6" conventional stud-framed wall and with 15 times less air infiltration.

For a summary of the ONRL study report, read Thermal Performance and Wall Ratings by Jeffrey E. Christian and Jan Kosny, Oak Ridge National Laboratory Building Envelope Research. You can calculate the R-value of the insulation in your own home, using the ORNL Whole Wall Thermal Performance Calculator. The results will probably surprise you. Repeated engineering tests show that SIP construction is stronger than conventional wall and roof framing. A typical SIP wall panel will withstand vertical compression of over 2000 lbs. per linear foot before structural damage occurs. This means that an 8' section of SIP wall would support about 4 Cadillacs or one Mack truck. No conventionally framed wall could survive this much weight.

Shear Resistance
Resistance to horizontal loads is called shear resistance — the ability of an object to withstand horizontal forces without damage. Racking resistance is the ability to stand this force without deforming.

Using the standard ICBO and BOCA approved test (ASTM E-72-80, “Conducting Strength Tests of Panels for Building Construction, Section 14), testers found that a standard 4'x8'x4-1/2" SIP panel wall had over three times as much resistance shear stress as a traditional wall assembly. At a load level that would destroy a conventional wall a SIP wall will deflect about 1/8". This difference is clearly evident in a SIP structure that is exposed to high winds. The absence of creaks and groans is very noticeable. This is also why a SIP building has fewer or no drywall callbacks due to cracking or fastener back-out.

Such high shear strength translates to real safety when mother nature starts to act up. The Kobe, Japan earthquake in 1993 devastated much of the city, but SIP houses in the destruction zone escaped virtually unscathed. During a 1998 tornado in Clermont, Georgia, a SIP house in the path of the storm lost its 25 mature trees and half of its roof shingles, but the house suffered no structural damage while twenty-seven other, conventionally framed houses in the town were destroyed. SIP panels are increasingly the material of choice for walls and roofs in hurricane and earthquake-prone areas. SIP houses in recent Gulf Coast hurricanes lost singles and windows, but the structures survived while conventional houses all but disappeared. A 2002 tornado in Sumner County, TN crumbled the concrete foundation under a SIP home and completely obliterated its porch, but the SIP shell survived without significant damage, continuing to protect the home's contents from wind and rain.

Combustability and Toxicity
How a material performs in a fire is a primary concern to fire code authorities. Fire has three requirements: fuel, ignition, and oxygen. SIPs have no "air" within their solid cores of insulation, so fire "running up the wall" inside a wall cavity is impossible, unlike conventionally-frame houses where it is all too common. SIPs have passed every standard fire test required of wood-based or Type V construction. A key element of fire safety is protection of the SIPs and any other underlying structure with 15-minute thermal barriers, such as gypsum wallboard.

Burning materials give off smoke and gases that in some cases can kill you rather quickly. In fact, inhalation of thermal decomposition products (smoke, gases and vapors) is responsible for the majority of fire deaths. Building codes in the U.S. have all but eliminated requirements for combustion toxicity because there is as of yet no acceptable testing protocol for simulating actual fire conditions. But, in Canada, the National Research Council has determined health risks associated with various combustible materials by essentially summing all of the risks of a material into a single index of toxicity. The higher the toxicity score, the more dangerous the material. Most building codes prohibit any material more toxic than wood smoke, which scores 50. Even wood smoke can kill you if you inhale enough of it, but any score 50 or below at least gives you a fighting chance to get out of the fire. Over 50, your chances are not so good, and any material scoring above 100 should probably not be in your home in any form, including clothing, curtains, carpet and furniture. Nylon carpet at 960, for instance, is a real killer in a fire.
Relative Toxicity of Common Household Materials National Research Council Canada

Material Toxicity Score Typical Home Uses
Polystyrene 20 SIP cores, insulation panels
Polyester 20 Clothing, curtains
Phenolic resin 30 Insulation, mastics, bonding agent in some OSB, MDF, particleboard, laminated countertops (like Formica® products, silestone, and solid surfacing materials)
Wood (White Pine) 50 Conventional roof and wall framing, plywood, oriented strand board
Cotton 60 Clothing, curtains, linens
PVC (Vinyl) 360 Appliances, siding, doors, windows, shower curtains, toilet seats, kitchenware
Wool 390 Carpets, clothing
Nylon-6 950 Clothing, furniture, curtains, carpets, linens
Bold type indicates materials common in SIP panels.


The materials used in typical SIPs: wood, resin and polystyrene, are low in toxicity risk compared to other materials commonly found in the home. Polystyrene, for example, a common core material in SIP panels, scores 20.

Cost Comparison
A SIP panel structure costs slightly more to build than a conventional framed structure. The higher cost of material in a SIP wall or roof is largely offset by savings in labor. And, when other savings are factored in, the cost of SIP construction may be substantially less than that of conventional framing.
Direct Cost Comparison:
SIP vs. Conventional Wall Framing
Description Conventional
Wall Cost 		SIP Panel
Wall Cost
SIP Panels (and
associated materials) 		— 4,120
Studs 420 —
Plates 210 210
Vapor Barrier 52 —
Sheathing 420 —
Insulation 466 51
House Wrap 190 —
Labor 3,000 600
Total 4,758 4,981
Data courtesy Sticks & Structures, L.L.C.


Direct Construction Costs
The initial cost of SIP panel construction compares favorably to conventional wall and roof framing. The SIP panels are typically more expensive than normal framing materials, but the savings in labor erases most if not all of any difference.

Keep in mind, however, that almost all of the published cost comparison studies have been either produced or sponsored by SIP manufacturers and should, perhaps, be taken with a small grain of salt. However, our experience is that they are largely accurate. SIP construction does seem to cost no more than conventional framing, if the panels are manufactured correctly. If they're not, then the cost climbs while waiting for new panels to be made and shipped. Fortunately, this is a rare occurrence.

Indirect Construction Savings
Aside from the direct constructions costs, there are other immediate cost considerations that are more difficult to quantify, but that are nonetheless quite real.

•  Smaller Furnace: A SIP home tends to be 50% to 75% more energy efficient than conventional counterparts. The heating and air conditioning system for a SIP home may also be correspondingly smaller and, therefore, less expensive.

•  Shorter Ductwork: The air duct system can be considerably shorter. The old duct rule of running all registers to an exterior wall of each room was intended to counter the inevitable air infiltration along the rim of the house common with conventional walls. The rule does not apply to a SIP building. Registers may be run to the closest wall of each room, at considerable savings. In fact, houses designed to optimize savings from SIP construction usually use a central utility pod design that further minimizes utility runs of all kinds.

•  Less Loan Interest: Using factory precut wall and roof panels, a SIP built house can be installed in days instead of weeks. If you are building under a construction loan, expect to save between 2 and 4 weeks worth of interest.

•  Cheaper Interior Finishing: Directly under the interior drywall of a SIP wall is a solid layer of OSB. Drywall, trim and cabinet installation time are greatly reduced, callbacks due to popped drywall nails and screws are virtually eliminated and the drywall is absolutely straight and flat. There is less drywall waste and fewer seams because drywall does not have to be trimmed to terminate exactly on a stud.

•  Less Job-Site Waste: Factory precut SIPs eliminate most job waste from wall framing and therefore reduce waste management cost and landfill fees.

•  Cheaper Electrical Installation: Electrical installations take less time. The pre-installed wire chases in SIP panels eliminate the need to drill studs for electrical wiring.

•  Fewer Callbacks: No bowed studs. Not only straighter, truer walls, but no return trips to straighten bowed studs before the drywall goes on.

Long Term Savings and Resale Value
In the long term, SIP structures are less expensive to operate due to the superior insulation value of SIP panels. SIP panel construction is typically Energy Star rated. An energy savings of over 50% can be expected. What this translates to in dollars depends on the cost of energy at the structure's location, but in most of Nebraska it amounts to $1,000 per year or more. Additionally, better thermal efficiency translates into higher resale value. A study by ICF Consulting funded by the Environmental Protection Agency revealed that energy efficiency increases the resale value of homes by $20 for every $1 in annual energy cost savings, or about $20,000 for the average SIP house. (Nevin, Rick, "Evidence of Rational Market Valuations for Home Energy Efficiency", The Appraisal Journal, October, 1998).