Benefits and uses of EPS

Roof assemblies are one of the largest and most important expenditures building owners make. These significant investments typically include a roof deck, waterproofing membrane, insulation, installation, maintenance and repair. Contractors and roof system designers are finding that roof systems incorporating expanded polystyrene (EPS) insulation can meet demanding building requirements.

EPS is a closed cell, lightweight, resilient foamed plastic insulation. It offers measurable energy savings, long-term R-value, effective water resistance, and dimensional stability when properly installed and protected from moisture. EPS insulation is compatible with all commercial roof systems, including but not limited to built-up roof (BUR); modified bitumen; and single-ply membrane systems that are either ballasted, mechanically fastened or fully adhered. EPS insulation meets the requirements of ASTM C578-04, "Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation," and CAN/ULC-S701-01, "Standard for Thermal Insulation, Polystyrene Boards and Pipe," the material standards that cover the types, physical properties and dimensions of cellular polystyrene intended for use as thermal insulation in the United States and Canada.

Following are some considerations regarding EPS insulation use.

Benefits

Energy efficiency

Energy and resource conservation continue to be important issues when deciding what materials are used in construction. As technology improves and the industry's understanding of buildings' thermal performances increases, there are many more factors to consider than just R-value. Thermal bridging, thermal shorts, air infiltration, poor construction details and bad workmanship all can contribute to a material's thermal performance or insulation system beyond R-value. EPS insulation's efficiency and durability have been documented in field and laboratory research projects for roof systems of varying ages from a wide range of geographical regions.

In "Report on Expanded Polystyrene Insulation for Use in Built-Up and Single Ply Roofing Systems" published in 1984, researchers Rene M. Dupuis, principal of Structural Research Inc., Middleton, Wis., and Jerome G. Dees, a researcher with Structural Research, show samples of EPS insulation that had no deterioration in R-value. The test results at 70 F (21 C) for thermal resistance of EPS insulation samples taken from various roof systems indicated no deterioration in R-value over time.

Another factor that affects the performance of insulation products after installation is thermal drift. Depending on the insulation materials used, R-value slowly can be reduced over time as a material ages. This should be considered when a designer calculates the expected performance of the insulation materials recommended.

For example, some foam plastic insulation materials use blowing agents that have high resistances to heat flow, causing the insulation to have a higher R-value at the time of manufacture. It now is known these blowing agents diffuse from the cellular structure of the foam until a level of equilibrium is reached many years after manufacture. As the gases diffuse, the ability of the insulation to prevent thermal flow is reduced, losing up to 30 percent of its original insulating ability. Because EPS foam does not use these types of blowing agents, its insulation performance remains stable during its entire life.

Mold

Moisture is extremely detrimental to any roof system. Concerns about mold in the construction arena have escalated during the past several years, prompting many insulation manufacturers to claim their products are "moldproof." However, mold will grow on virtually any surface in the presence of water, a food source and the right temperature. It only can be controlled through management of water and moisture.

The EPS Molders Association recently submitted test samples for ASTM C1338-00, "Standard Test Method for Determining Fungi Resistance of Insulation Materials and Facings." SGS U.S. Testing Co. Inc., Fairfield, N.J., evaluated the samples according to the test protocol and confirmed no traces of fungal growth during a 28-day incubation period for EPS foam insulation.

EPS is nonhygroscopic and does not readily absorb moisture from the atmosphere; its closed-cell structure reduces the absorption and/or migration of moisture into the insulation material. Extensive industry testing has confirmed moisture absorption has a minimal effect on the thermal performance of EPS insulation. For example, the Energy Division of the Minnesota Department of Public Service found that seven-year-old samples of EPS used in exterior foundation insulation showed moisture levels of only 0.13 percent. It also concluded EPS insulation retained between 95 percent and 97 percent of its thermal efficiency and there was no effect on its compressive or flexural strength properties.

All insulation helps prevent cold air and warm air from mixing, which is key to controlling moisture. Proper design, construction techniques and choice of insulation reduce the opportunity for moisture to leak or be driven into the insulation cavity where a system's thermal performance may be affected. As with any good building practice, a properly constructed roof system should prevent moisture intrusion.

The environment

The hottest topic in the construction world since the mid-1990s is "green," or environmentally friendly, building practices. With the plethora of products currently in the market all claiming to be "green," it's difficult to choose the best product to complement a sustainable approach to building. Important factors for specifiers and contractors to consider when choosing the right insulation are long-term performance issues, including thermal resistance, dimensional stability and compressive strength, as well as compliance with insurance and building code requirements.

Some assume traditional products, by virtue of having been used for decades, must not be green. Not so. EPS insulation was a "green" insulation before its environmental benefits were recognized. Some of EPS insulation's more tangible environmental benefits include recycled content that meets minimum performance standards and delivery proximity to job sites.

EPS insulation never has contained ozone-depleting gases, such as CFCs (chlorofluorocarbons) or HCFCs (hydrochlorofluorocarbons). Other insulation materials have phased out production using these chemicals, reformulated the entire manufacturing process and will produce entirely new materials that possess a short performance track record.

In the excitement surrounding green buildings, some companies overstate the green performance attributes of their products. During the past several years, green-building initiatives, which typically are government-sponsored programs, have published guidelines for green building in the United States and Canada. The goals of these programs are admirable; however, many are environmental-measurement theories offering marketing language directly from manufacturers' product literature. Others evaluate products based on a single criterion, such as recycled content. This may provide a limited picture of a product's green attributes. It is important to evaluate the entire contribution a product makes to a building's performance and sustainability goals. Third-party testing and analysis of products provides an objective source for roofing professionals to reference.

EPS is identified in GreenSpec® as a preferred green-building product. GreenSpec was developed by the editors of Environmental Building News, one of the most respected voices of the green-building movement. It is a database of product information containing detailed listings of more than 1,800 products with environmental data, manufacturer information and links to additional resources. It also provides guideline specifications categorized by Construction Specification Institute divisions with suggestions and sample language to incorporate into project specifications. More information about GreenSpec can be found at www.buildinggreen.com.

Applications

Tapered roof systems

Tapered roof insulation can play a vital role in the proper drainage of roof systems—a key to maximum performance and longevity. Poor drainage of commercial roof systems can result in damage far more destructive than wind or other natural elements. Ponded water and freeze-thaw cycles can result in costly repairs and even premature roof system failure, which rarely is covered under warranty or by insurance. Tapered EPS insulation provides the required positive slope while retaining the structural and economic advantages of a low-slope roof deck.

Each tapered roof system is professionally customized. EPS insulation can be molded in thicknesses up to 50 inches (1270 mm) and cut to obtain the desired slope, ranging from 1/8-in-12 (0.6 degrees) or greater. If needed, installers can cut the insulation on-site, allowing for a precise fit. EPS insulation can be applied in a single layer, creating a continual form. This design versatility translates into considerable savings in labor and framing costs. Usually, the ridges and valleys necessary to provide correct drainage are supplied at 45-degree increments.

A roof system might include skylights; internal and external drain locations; expansion joints; and height limitations of parapet and other adjacent walls, as well as house heating, ventilating and air-conditioning units. The insulation layout must work around these components and within a specified budget. Solutions are found by choosing the correct layout option to move unwanted water off the roof.

In general, there are four ways water can be diverted from a roof:

  • Shed roof design—This is the most basic layout design. The roof slopes from a high point to a low point, causing the water to travel to the edge of the building or into a gutter.

  • Two-way slope—The roof is split into two parts with a slope on each part. It can either slope inward toward the center of the building or to the outside. The two-way slope is more cost-effective than the one-way slope because the total thickness of the insulation is less at the high point because it is split into two sections.

  • Three-way slope—This layout is used on roofs of smaller buildings that are attached to larger structures because it slopes to a point that lies on a side of the drained area. Instead of forming a valley as seen in the two-way slope, the water is forced to a small area, usually a roof drain.

  • Four-way slope—This is considered the most effective layout to ensure proper drainage. The insulation is sloped to a single point located inside the area being drained, or the slopes are inverted to create a high roof center where the water is directed to the building's edges. In some designs, the center of the slopes can create ponding. To eliminate this occurrence, crickets and saddles should be installed.

Once a layout is determined, the insulation's thickness and slope must be calculated. A specifier or contractor must take into account the insulation's cost, as well as the required R-value when determining the thickness needed to achieve adequate slope. Steep slopes may do a great job eliminating ponded water, but the problems could outweigh the benefits. Cost can escalate, and the roof drains may be overloaded. Extra-long fasteners also will increase the cost of a tapered roof.

A roof system designer must determine minimum acceptable slope; determine whether a layout is suitable for four- or two-way design; insist on a minimum valley slope of 1/8-in-12 (0.6 degrees) where possible when designing crickets and saddles; and note areas of tapered and flat insulation on plans with hatching and clear notes.

EPS insulation manufacturers are available to provide complete specification and design assistance. Field assistance becomes even more valuable when unanticipated modifications become necessary. When problems occur at a job site and the insulation's slope needs to be adjusted, contractors typically experience delays while requisitioning a reconfigured piece. Delays are avoided with EPS insulation because the material can be cut to specification on-site, achieving a precise fit and decreased labor costs.

BUR

EPS has been used in BUR system applications for more than 30 years. BUR systems constructed with concrete roof decks allow roof insulation to be applied directly to a deck because there is no requirement for a thermal barrier as a result of the noncombustible concrete deck. EPS foam may be adhered to concrete with asphalt using a technique known as "mop and flop." Hot asphalt is applied to the concrete deck and allowed to cool slightly before the foam permanently is dropped into place. Then, a cover board is "mopped and flopped" onto the EPS insulation with the joints overlapping and taped. Asphalt then is applied on top of the cover board, which provides a uniform, consistent base for the weatherproofing system.

Because of EPS insulation's thermoplastic properties, it is necessary to place a cover board above the foam in BUR systems. This provides protection from hot asphalt used to adhere and build up the waterproofing materials. In a typical BUR system, there will be three or four alternating layers of materials that will include bitumen—either asphalt or coal tar—and roofing felts, which can be surfaced with some type of exposed aggregate embedded in hot asphalt or a smooth coating or cap sheet.

EPS foam used in BUR systems can be installed as described or delivered to a job site as a complete composite panel that would include the insulating foam, cover board and, when necessary, a thermal barrier. This composite panel then can be mechanically fastened to a concrete or steel deck. The use of prefabricated composite panels for BUR applications has been growing steadily because of the labor savings involved in installation.

Single-ply roof systems

Available in thicknesses up to 40 inches (1016 mm) and in a variety of joint details, such as tongue-and-groove and shiplap edge, EPS insulation is ideal for single-ply roof systems. It provides maximum dimensional stability and high thermal efficiency and can be specified to meet numerous design conditions. Although the typical thickness for roof applications is 12 inches (305 mm), EPS insulation can be tapered to a 1/8-in-12 (0.6-degree) slope to allow for adequate drainage on structurally low-slope roof decks.

Polystyrene foam is subject to deterioration when subjected to petroleum-based solvents. Single-ply membrane adhesives contain solvents that will attack EPS. Designers and contractors should consult membrane suppliers to determine the compatibility of a membrane and installation adhesives with EPS foam.

A recent roofing project in Anchorage, Alaska, posed some unique challenges that were addressed by using EPS insulation as part of a single-ply roof system. The design for the new facility was a low-slope single-ply roof combined with an architectural metal roof designed around a parabola. The main roof area consisted of a mechanically attached membrane over a 1/2-inch- (12.7-mm-) thick cover board, a tapered Type II EPS insulation system, 5/8 of an inch (16 mm) gypsum and a 10-mil- (0.01-inch-) thick vapor retarder. In addition to the complex details of the roof system, a key challenge was installing the roof between November and February when the average temperature is 10 F (-12 C) with only six hours to eight hours of daylight. The project represents an innovative problem-solving application without making compromises in appearance, quality or performance while improving the overall value.

Codes and standards

ASTM C578-04 is the material standard that addresses the types, physical properties and dimensions of cellular polystyrene intended for use as thermal insulation in the United States. CAN/ULC-S701-01 is the national standard of Canada that specifies performance requirements for EPS insulation material. The ASTM specification lists minimum data values for four material types indicated by density. However, EPS manufacturers offer a wide variety of custom insulation materials in different thicknesses and densities.

Compressive stress/strain characteristics of EPS insulation are determined using ASTM D1621, "Standard Test Method for Compressive Properties of Rigid Cellular Plastics," or ASTM C165, "Standard Test Method for Measuring Compressive Properties of Thermal Insulations."

The most important property of EPS insulation is its resistance to compressive stresses, which increase as the density becomes higher. EPS has a compressive resistance between 10 pounds per square inch (psi) (68.95 kPa) to 60 psi (413.7 kPa) for most construction applications. Within that range, EPS can be produced to meet specific strength requirements. Optimum performance of a load-carrying insulation often is related to strength characteristics and resiliency. Resiliency is the ability of a material to recover its strength following deformation caused by stress. If greater strength and rigidity are needed, compressive resistance up to 60 psi (413.7 kPa) can be specified.

According to the EPS Molders Association, in roofing, Type I EPS material provides the dimensional stability and compressive strength necessary to withstand light rooftop traffic and equipment weight at reasonably high surface temperatures. EPS foam insulation may experience dimensional and property changes when it is exposed to extreme temperatures (greater than 167 F [75 C]); however, low-density EPS not subjected to load will show no noticeable loss of dimensional stability at temperatures up to 184 F (84 C). Duration of temperature, external load conditions and density are the variables affecting foam insulation at elevated temperatures. EPS insulation should be adequately protected from temperatures greater than 165 F (75 C) during installation and may require the use of cover boards, reflective ballast or a light-colored membrane depending on the roof covering system involved.

In March 2003, a need for a new high-density standard EPS was identified by the EPS Molders Association. The objective is to introduce a new consensus material type with a compressive resistance of 40 psi (275.8 kPa), which would be submitted to ASTM for consideration to be included in ASTM C578. The new EPS type would be used in high-load applications, such as roofing.

ASTM approved the addition of the EPS Type XIV during its October 2004 meeting in Washington, D.C. The revised standard is expected to be published later this year.

Other details

As with any product, the successful use of EPS insulation depends on its correct installation according to good building practices. Because of its high resiliency and strength characteristics, EPS insulation offers the following:

  • Absorption of substrate and facer movement caused by temperature changes and structural deflections

  • Absorption of substrate irregularities

  • Thickness recovery following excessive construction load exposures

  • Suitable subgrade reaction for effective load distribution

Depending on the type of roof system chosen, EPS insulation is available in faced and unfaced rigid foam. It may be laminated at the point of manufacture or on a job site and helps to provide added strength and durability. To protect insulation from rooftop traffic, hot asphalt, single-ply adhesives and modified bitumen applications, the lamination is adhered to the top of the insulation. It is laminated on the bottom when a thermal barrier is required.

A track record

Currently, the EPS industry uses highly sophisticated processes and technologies to manufacture cost-effective products. EPS insulation products have been the subject of extensive research and evaluation for the past 50 years, helping lend confidence to roofing professionals. For more information about EPS insulation, contact the EPS Molders Association at (800) 607-3772 or visit www.epsmolders.org.

Betsy Steiner is executive director of the EPS Molders Association.

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