The use of stressed skin panels was popularized with the revival of timber framing - as infill panels between the timbers.

Stressed-skin panels can be used in walls (including foundation walls), floors, and roofs - either independently or in combination.  SIPs can also be used with conventional framing.  For instance, using roof and floor trusses with stressed-skin panel walls.  The greatest savings in time, energy use, and materials, are achieved when panels are used for the entire house.

Stressed-skin panels offer excellent structural performance.  Because they are sheathed on both faces and the sheathing is fully adhered across their entire surfaces, they behave as an integral system and resist wind forces and building dead and live loads better than conventional stick framing does.

A stressed-skin panel is a manufactured "sandwich" assembly with a rigid insulating core whose exterior and interior surfaces are bonded into panels.  The most common type of panel has an expanded polystyrene foam core, faced with oriented strand board (OSB).  The core may also be isocyanurate or urethane foam.  Iocyanurate and urethane insulated panels are made by injecting liquid foam in place between two skins, a process that requires presses and control of both temperature and chemical mix.

The high initial R-value per inch of urethane claimed by manufacturers may decline slightly over time, unlike the R-value per inch of EPS, which remains constant.  However, the urethane systems maintain an R-value of about six per inch thickness, compared with EPS at four per inch.  Waferboard, plywood, sheet metal, and drywall are also used.

The skins distribute and carry the structural loading while the bonded foam core provides insulation and keeps the two skins aligned.  Since there are fewer framing members, there is little thermal bridging and the R-value remains higher.

Stressed-skin panels are typically manufactured in four feet by eight feet sizes, although some manufacturers can produce panels up to 8 x 40 ft.  They are available in a variety of thicknesses, roughly corresponding to conventional framed sections.  Wall panels are typically placed on a single bottom plate and have a single top plate.

Panels can be joined in several ways, depending on the manufacturer.  Generally, connection details involve a tongue and groove joint with splines.  Adding an expanding foam sealant at joints as recommended by the manufacturer will provide a tight air seal.

Panel products differ from one another in several ways, including water vapour permeability, foam and panel strength, temperature sensitivity, combustibility, and others.  These may affect the appropriateness of a specific product for a given application.

Stressed-skin panels go up quickly and give a weather tight shell early in the construction process.  Because they incorporate both the structural frame and thermal insulation in a single step, supervision and co-ordination of trades are simplified.

Some panel suppliers offer full design and engineering services.  Some can custom design panels by pre-cutting rough openings, edges, angles, and other complex shapes.  These are then delivered on the site pre-cut and ready for installation.

Structural insulated panels are resource-efficient as they use less wood than conventional framing.  The sheathings provide an efficient use of wood fibre while the foam core material has high insulation values.  Although the foam core comes from a non-renewable resource, it is an efficient use of this resource.  One quart of an oil-refining by-product is expanded to create forty quarts of EPS foam.

An average SIP wall section contains about 25% less wood than a similar wall framed with 2x4s at 16" on-centre.  Panels can structurally cover large spans, requiring very little supplemental framing (such as sill plates and top plates).

Despite the higher material costs, the stressed-skin panel house can result in net envelope cost savings.  A SIP demonstration house was built at the University of Oregon.  They calculated that using SIPs they saved 2,720 board feet of wood, or nearly 50% of the framing lumber for conventional construction, and the house required 161 fewer man-hours to build.

Energy Efficiency
Stressed-skin panel homes are very energy-efficient.  When panels are used throughout, the whole building envelope is insulated.  The insulation is virtually continuous and panels allow little air leakage, so a stressed-skin panel house is generally easier to make airtight than a conventional stick frame house.

A house built with SIPs could produce heating energy savings of 12% - 17% compared to a stud-frame house of equal size and R-value.  This is largely due to the more efficient insulation approach that eliminates any potential for themosiphoning or wind washing which can be a problem with improperly installed batt insulation.

The panels themselves provide the required air and vapour barrier.  However, to ensure a continuous air barrier, the joints at the panel to floor, panel to panel, panel to roof, and at penetrations must be sealed with long life sealants.  It is important to be aware that some adhesives and caulks are incompatible with foam insulation, so care must be taken when selecting sealants.

Design & Engineering
Stressed-skin panels offer design flexibility.  For example, if they replace conventional roof trusses, they can accommodate a living area under the pitched roof or where it would otherwise be necessary to use very large rafters.  The BC Advanced House is one example of this.  The roof structure encloses a large volume while creating interesting living spaces.

Limitations
Because the panels do not have open cavities, they require different approaches to plumbing and electrical systems than conventional framing.  Ideally, a designer should incorporate as much of the plumbing and electrical work as possible on interior (non-panel) walls.  Where that is not possible, walls can  be furred out for plumbing vent stacks and electrical runs.

Pre-cut electrical chases are standard in some systems, but must be specially requested in others.  Often, wiring on outside walls is fed from the top or bottom, so a single saw cut is made to insert the wiring, and then refilled with foam, but each manufacturer will supply information on these kinds of details.

Most stressed-skin panels require 1/2" interior gypsum drywall.

Building officials, while not necessarily familiar with stressed-skin panels, have generally been receptive to their use, but will likely ask for a professional engineer to verify the structural performance.  Suppliers may provide this.

Larger stressed-skin panels, often used in a floor and roofs, can be so heavy that they require cranes or other machinery for handling.

If stressed-skin panels become wet, they take a long time to dry out and may harbour mould growth.  MDI resin offers greater water resistance than phenolic resins in OSB skins.

Carpenter ants have been found to nest in stressed-skin panels, as they do in conventional framing.  Measures for protection against termites and rodents are the same as they are for conventional construction.  Some manufacturers have developed relatively non-toxic borate foam treatments that seem effective at preventing infestations.

Environmental & Performance Factors
There are tradeoffs for each type of stressed-skin panel.  None of these products is without environmental impact.  Overall, stressed-skin panels are a resource-efficient building technology with significant energy-efficiency and forest-conservation benefits, and hold distinct advantages over stick framing.

The allowable roof or floor panel span will depend on the type of foam used in the core.  Each manufacturer will provide that design information.  Thicker panels reduce long-term deflection or deformation due to creep.

Reprinted with permission from Solplan Review, the independent journal of energy conservation, building science and construction practice (issue No. 99, July 2001). For subscription information: P.O. Box 86627, North Vancouver, BC V7L 4L2 or solplan@direct.ca

At Northern Timberhouse/Timberhouse Supply, we do not push for any one particular brand of panel and we like to inform our clients on the options, suggest a course of action and reach a joint decision.

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