Sheathing is one of the most basic and widely used application for rigid insulation in residential and commercial construction. It helps create an envelope around the structure covering wall cavities and studs to increase resistance to heat transfer and moisture penetration. Sheathing is available in many different materials. Even though rigid foam insulation was introduced into the construction market back in the 1950's, it was not used with any prevalence as sheathing until the 1970's energy crisis. Due to its versatility, ease of installation and consistent performance benefits, expanded polystyrene foam-insulating sheathing is a standard in the industry.

EPS sheathing is non-structural and is primarily used as an exterior insulator, both below and above grade, although it can be utilized throughout the structure in roof, floors and ceilings. It is available in sheets measuring 2'x8', 4'x8' and 4'x9' ranging in thickness from ½ inch to 4 inches. Different densities help provide the R-value required to meet local building energy codes.

Expanded polystyrene sheathing is used in new construction as well as remodeling. It is compatible with wood and steel framing, and masonry applications. The boards are installed vertically over the exterior sides of the studs, with the vapor retarder facing the heated side of the structure. EPS sheathing can be fastened with nails, screws, and/or staples depending upon the framing surface. Utilizing spot adhesive is the norm for masonry substrates. Be advised that some adhesives contain petroleum-based solvents that will dissolve EPS on contact. The joints of the sheathing should be close and flush. Seams are taped for added tightness and corner braces are installed to increase structural stability. Air-barrier house-wrap may not be needed if the sheathing is properly installed and seam tape is used. A variety of sidings and finishes are easily affixed through the exterior sheathing to create an aesthetically pleasing building.

EPS insulated sheathing board is manufactured with an array of facers. Aluminum foil, polyethylene and kraft paper are all used to enhance performance properties and protect it from rough handling and ultraviolet degradation. EPS manufacturers use reflective aluminum foil to increase the resistance of radiant heat absorption. When a radiant barrier is combined with dead air space, it can actually add to the insulation value of the wall assembly.

A variety of EPS sheathing products are available depending on the intended application. One of the primary functions of sheathing is to control moisture by acting as a vapor retarder. A perforated foil will increase breathability when used above-grade, thus avoiding any type of condensation build-up between the interior and exterior of the structure.

In addition to acting as a vapor retarder, polyethylene facers improve the surface adhesion of the board for taping and adhesives. The tape used to seal the foam seams sticks better to the polyethylene faced board rather than unfaced. Kraft paper is bonded between the facer and foam sheathing increasing its strength and durability to protect during transportation and handling.

The use of EPS rigid insulation has grown steadily over the last decade. While most building materials may fluctuate widely in price, the cost of EPS has remained relatively consistent. This is especially appealing to contractors when bidding on a project. They don't have to worry about sacrificing profit margin due to an unexpected price increase for EPS sheathing.

Another benefit is the versatility of EPS. Manufacturers can provide the builder with insulation that varies in density. This translates into a structure that will meet or exceed energy code standards without the added expense of increased stud width. When higher R-value is required, rather than adding layers and layers of rigid insulation to meet the standards EPS is fabricated to a higher density.

The strength of expanded polystyrene is sometimes questioned. How can this foam material carry or absorb any type of movement inherent in a structure? EPS insulation used in foundations and walls bear a minimal load compared to other applications. Type I EPS material as prescribed in ASTM 578-97 is adequate to accommodate reasonable building movement without transferring stress to the joints of the building. If greater strength is needed, EPS is available in compressive strengths as high as 60 psi.

EPS has been used successfully for many years in areas where moisture is a concern, specifically below-grade. Fungus, bacteria or rot does not affect EPS. The performance properties will not deteriorate when exposed to moisture and/or water. There had been some debate that moisture would permeate any voids that exist between the foam insulation and the wall cavity, causing damage. This theory was conclusively rejected after results were released from a study conducted by the National Research Council Canada. EPS insulation below-grade performed consistently throughout the two-year study with no signs of water damage to the structure or the EPS rigid insulation. And lab testing confirmed that the performance properties were not affected either.