October 1, 2011
By W. James Whalen, P.Eng., MSCE, CSC
Moulded expanded polystyrene (EPS) is an air-filled, closed-cell, rigid foam plastic that does not contain any hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) as blowing agents. The closed-cell structure of EPS insulation provides constant thermal resistance, is dimensionally stable and non-corrosive, provides excellent mechanical properties, and can be recycled where facilities exist.
The generic term ‘geofoam’ is used to describe any synthetic geotechnical material created in an expansion process using a gas (i.e. blowing agent) and resulting in a texture of numerous closed cells. (See J.S. Horvath’s Geofoam Geosynthetic [Horvath Engineering, 1995]). Some typical EPS geofoam applications for buildings include:
Non-building uses include:
The National Standard of Canada that addresses EPS insulation in building applications is Underwriters Laboratories of Canada (CAN/ULC) S701-11, Standard Specification for Thermal Insulation, Polystyrene, Boards, and Pipe Covering. Figure 1 provides the material properties for the three EPS insulation types included in CAN/ULC S701.
EPS geofoam applications require material performance properties beyond those specified in CAN/ULC S701. Figure 2 provides additional material properties for the three insulation types that may be used for design in EPS geofoam applications.
An additional industry standard often used for the material is ASTM D 6817, Standard Specification for Rigid Cellular Polystyrene Geofoam. Figure 3 summarizes the associated material properties for EPS geofoam types.
Some EPS geofoam performance properties that may be considered based on application requirements include:
Thermal resistance/thickness relationship
Thermal resistance value of an insulation material is stated as R-value or RSI; thermal resistivity is stated by R-value or RSI per unit of thickness. The R-value/RSI of insulation is a relative measure of the material’s ability to resist heat flow, with a higher R-value/RSI indicating a greater resistance to heat flow.
EPS insulation’s thermal resistance is closely related to the density of the finished product. Within the normal range of EPS densities, as the density increases, so too do the thermal resistance values.
The major mechanism of heat transfer in foam plastic insulations is by conduction, which occurs through both the gas and solid portions of the foam. Since gases occupy approximately 90 to 98 per cent by volume of cellular plastics, conduction through the foam’s gas portion is by far the most significant.
Some cellular plastics depend on blowing agents (e.g. HFCs or HCFCs) inside their cellular structure to increase the thermal resistance value. However, since foam plastic insulations are not enclosed within gas-impermeable barriers, some of the blowing agent in the cellular structure diffuses out over time and is replaced by air, which has a lower thermal resistance value. This phenomenon is known as ‘thermal aging.’ Designers must request design thermal resistance values based on long-term thermal resistance (LTTR) test values to predict the insulation material’s performance for the life of the structure.
The Canadian thermal insulation industry has adopted a test method for predicting the design R-value of foam plastic insulations with a blowing agent within the cellular structure, and reporting of LTTR values is a requirement in current National Standards of Canada for foam plastic insulation. CAN/ULC S770, Determination of Long-term Thermal Resistance of Closed-cell Thermal Insulating Foams, is recognized as the test method for predicting the LTTR of closed-cell foam plastic insulation containing a gas other than air. It provides a means for predicting LTTR for foam plastic insulation with a captive blowing agent based on an accelerated laboratory test.
Since the closed cellular structure of EPS insulation contains only stabilized air, LTTR requirements are not applicable and the thermal resistance of EPS does not decrease with age. In other words, an EPS insulation’s published thermal resistance values are design values and do not require any adjustments for aging over the life of the structure.
Compressive resistance of foam plastics is determined using ASTM C 165, Standard Test Method for Measuring Compressive Properties of Thermal Insulations, or ASTM D 1621, Standard Test Method for Compressive Properties of Rigid Cellular Plastics. The value included in foam plastic insulation standards, including EPS, is the compressive stress at 10 per cent strain (deformation from original thickness) or yield, whichever comes first. This 10 per cent strain value is not a ‘failure strength,’ but rather is intended for product evaluations and quality control, as well as for comparing relative compressibility of different foam plastic insulations. The compressive resistance of EPS is closely related to product density.
The compressive resistance at 10 per cent deformation should not be used for design purposes when EPS is subjected to short- or long-term compressive loads. If such compressive loads are anticipated, the compressive resistance at one per cent deformation is typically required for design purposes.
As EPS is a closed-cell foam plastic insulation, it resists the absorption of moisture into the cellular structure. Current foam plastic insulation standards specify maximum water absorption (percentage by volume) obtained from a laboratory test method. Maximum water absorption, specified in CAN/ULC-S701, is determined using ASTM D 2842, Test Method for Water Absorption of Rigid Cellular Plastics, which states the following under Section 2, “Significance and Use.”
The purpose of this test is to provide a means for comparing relative water absorption tendencies between different cellular plastics. It is intended for use in specifications, product evaluations, and quality control. It is applicable to specific end-use design requirements only to the extent that the end-use conditions are similar to the immersion period (normally 96 hours) and 5.1 cm (2 in.) head requirements of the test method.
There are now numerous published reports demonstrating that moisture resistance of EPS exposed in actual applications over extended periods is much lower than water absorption obtained from laboratory test methods. For example, a Finnish study comparing results from laboratory water absorption test methods to values from field applications found that actual water absorption in below-grade applications was less than half that predicted by laboratory test values. (See Oajnen et al’s “Moisture Performance Analysis of EPS Frost Insulation” in vol. 3 of the 1997 ASTM STP 1320, Insulation Materials–Testing and Applications). Similarly, a Norwegian report about EPS lightweight fill samples retrieved from applications after up to 40 years in service in a drained position found that moisture contents were less than one per cent by volume. (For more information, see Roald Aabøe’s “The Norwegian Public Roads Administration: 40 Years of Experience with the Use of EPS Geofoam Blocks in Road Construction,” which was part of the Proceedings of the 4th International Conference on Geofoam Blocks in Construction Applications, June 2011).
In August 2008, the EPS Molders Association (EPSMA) requested independent third-party testing to evaluate the field performance of ASTM C 578 Type I EPS insulation and Type X extruded polystyrene (XPS) insulation installed side-by-side on a below-grade foundation application. (The EPSMA technical bulletin is no. 103, “15-Year In-Situ Research Shows EPS Outperforms XPS in R-value Retention.” To read the document, visit www.epsmolders.org/PDF_FILES/EPS%20Below%20Grade103.pdf). EPSMA published a technical bulletin highlighting results from the test program for each insulation type removed after 15 years of service on the exterior of a commercial building in St. Paul, Minn., at a depth of approximately 1.9 m (6 ft) below grade. In this side-by side installation, the EPS insulation moisture content (MC) was about 25 per cent of the XPS insulation moisture content at the time samples were removed. Consequently, the EPS insulation retained a higher proportion of its R-value at 94 per cent of the specified R-value.
Another method of comparing moisture performance of different foam plastic insulation exposed to high humidity or moist environments is equilibrium moisture content (EMC). A material’s EMC is the moisture content at which the material is neither gaining nor losing moisture when exposed to a specific relative humidity (RH) and temperature. Although it is a dynamic equilibrium, after the moisture content of EPS has reached its equilibrium value under given conditions, EMC changes that take place as conditions change would not exceed 0.15 per cent of the material’s mass. This EPS material property is another reason why actual moisture content measured from field applications differs greatly from results obtained in laboratory tests that expose test specimens to conditions not experienced in actual product applications.
One laboratory test method historically used to characterize freeze-thaw durability of foam plastic insulation is a version of ASTM C 666, Test Method for Resistance of Concrete to Rapid Freezing and Thawing. The procedure was modified to subject insulation samples to up to 600 cycles of full-thickness freezing in air, and thawing by complete submersion in water.
This test procedure has not been correlated to conditions encountered with typical application for an insulation product. It is used to establish how many freeze-thaw cycles are required to create a ‘failure’ of a product, rather than the insulation’s durability in an actual in-situ application.
A review of the performance of foam plastic insulations in below-grade applications by the University of Minnesota Underground Space Centre concluded freeze-thaw testing—involving hundreds of full-thickness freeze-thaw cycles of a fully or partially submerged insulation—is poorly related to the expected performance of insulation for below-grade applications over a building’s reasonable economic life. (The October 1986 report is entitled, “Moisture Absorption and its Effect on the Thermal Properties of EPS Insulation for Foundation Applications: A Review Analysis of Published Laboratory and Field Tests.”) The report also showed the impact of freeze-thaw cycling in a drained, below-grade building foundation application should not be large since the annual number of freeze-thaw cycles is small below grade, and little of the insulation thickness experiences sub-freezing temperatures.
To address the lack of correlation between laboratory test methods and known product performance, the Canadian EPS industry conducted a research project in co-operation with the National Research Council of Canada/Institute for Research in Construction (NRC-IRC) to evaluate the durability of EPS insulation installed in an exterior below-grade application as part of the Exterior Insulation Basement Systems (EIBS) project. (See N. Normandin et al’s “In-situ Performance Evaluation of Exterior Insulation Basement Systems (EIBS)–EPS Specimens” (National Research Council of Canada [NRC] Report No. 3132.1, 1999) and Swinton et al’s “Performance of Thermal Insulation on the Exterior of Basement Walls” [NRC Construction Technology Update No. 36]). EPS insulation installed as exterior insulation on a basement wall was monitored over a 30-month exposure period. In-situ thermal performance of the EPS insulation, site weather conditions, and soil moisture content were instrumented and monitored throughout the project.
The in-situ thermal performance of the EPS exposed to soil backfill was monitored using thermal couples attached to the insulation and concrete basement wall. The monitoring indicated the EPS’ thermal performance remained stable and was not adversely affected by water movement in the soil in contact with the insulation.
Material properties were also determined for EPS insulation removed after the 30-month exposure. Testing confirmed all types of EPS in the research program retained their specified thermal and mechanical properties even after being subjected to in-situ freeze-thaw cycling. The moisture content of EPS insulation removed after the 30-month exposure was in the range of 0.01 to 0.96 per cent by volume.
The NRC-IRC research project identified the following key EPS insulation performance attributes:
A second part of the research project included development of a laboratory test protocol that subjected test materials to extreme thermal gradient and environmental cycling—including freeze-thaw cycling—to assess durability. (See N. Normandin et al’s “Development of a Draft Test Protocol for Evaluating Durability Under Environmental Cycling of Insulation Products for Exterior Basement Applications” [NRC Report No. 3132.2, 1999]). Laboratory testing performed by NRC-IRC on samples from the same manufacturing lot of material subjected to the 30-month field exposure confirmed all types of EPS insulation retained their specified material properties even after being subjected to the laboratory durability test protocol.
The draft test protocol from this project has been subsequently developed into ASTM C 1512-10, Standard Test Method for Characterizing the Effect of Exposure to Environmental Cycling on Thermal Performance of Insulation Products, to provide a means of assessing durability performance of all insulation types. The test method was originally published in 2001; ASTM Interlaboratory Study 44 was later conducted to establish a precision statement for C 1512 and the test method was re-published in 2010. (See the 2009 ASTM Research Report No. C16-1036, “Research Report to C 1512 Standard Test Method for Characterizing the Effect of Exposure to Environmental Cycling on Thermal Performance of Insulation Products.”)
The monitoring of in-situ thermal performance during the NRC-IRC research project discussed earlier detected the presence of water at the outer surface of the EPS insulation during periods of heavy rain and major thaws; however, the surface of the concrete basement wall showed no evidence of water penetration through most of its height. This demonstrated that when used as below-grade foundation insulation, the EPS surface acts as a capillary-breaking layer. The insulation’s surface resists water penetration and provides a drainage plane. If adequate provision for drainage is provided at the base of the wall, water not removed at the ground surface drains to this location.
EPS geofoam is supplied in the form of either blocks or boards. Construction is rapid—depending on the application, products are usually installed on delivery without requiring onsite stockpiling or storage. Individual EPS geofoam blocks or boards can be lifted by construction equipment or placed manually by a work crew.
As indicated, EPS geofoam has been used successfully for a wide range of geotechnical applications. Figure 4 identifies numerous applications and highlights some of the typical design considerations.
EPS insulation applied to the exterior of foundation walls ensures the concrete is not subject to expansion and contraction caused by temperature differences between the interior heated space and the exterior air/soil temperature. The EPS R-value on the exterior of the foundation reduces the likelihood condensation will form on the concrete wall’s inner surface, which would be at room temperature.
EPS insulation applied over dampproofing or waterproofing coatings on foundation walls protects them from damage. Select foundation insulation products can provide a monolithic insulation layer to eliminate thermal shorts and reduce thermal stresses that can cause cracking of concrete walls. Additionally, the EPS’ smooth surface, in contact with the soil, acts as a capillary-breaking layer to direct water to the drainage tile. At the same time, the grooved surface, in contact with the foundation wall, also provides an additional drainage plane to relieve any hydrostatic pressure that may develop adjacent to the foundation wall and also direct water to the drainage tile.
Concrete slab insulation
Heat loss through an uninsulated concrete slab can be a significant source of energy loss. When applied below the concrete slab as part of new construction, EPS offers a monolithic insulation layer that reduces heat loss and results in a uniform floor surface temperature that can be maintained at the interior room temperature.
Another application for concrete slab insulation becoming more common in energy-efficient residential and commercial construction is use as a component in radiant floor heating systems. In this application, hydronic tubing is cast into the concrete slab and EPS insulation is installed beneath the concrete slab to ensure heat is distributed uniformly throughout the entire floor area.
EPS insulation is also commonly used as a load-bearing component in residential, commercial, and industrial floor systems. Once design loads have been determined, the most cost-effective type of EPS insulation for use as sub-slab insulation can be selected.
Utility lines are usually placed below the level where frost penetration into the soil is expected to prevent water freezing in the lines. However, high water tables, rocky outcrops, or soil conditions can make it impossible to bury lines deep enough. Under these conditions, the utility line can be insulated with EPS at the time of installation to increase the duration the pipe can store still water before freezing.
It is also feasible to reduce stresses on large flexible culverts by placing EPS geofoam as a lightweight fill material above a culvert. In this application, a portion of the fill material above the culvert is replaced with geofoam. The product can significantly reduce loads on buried utility lines because it is up to 50 times lighter than other traditional fills with similar compressive resistance properties.
Highway pavement designs may be governed by subgrade stress and deformation criteria or frost heave protection requirements. Where the pavement thickness is controlled by frost heave conditions, considerable savings can be realized by installing EPS geofoam to insulate the pavement structure. In this application, high-density material is used to provide the required R-value, compressive resistance to carry high design load, and long-term moisture resistance.
When geofoam is used as a subgrade insulation element, care must be exercised to address differential icing between insulated and uninsulated portions of the roadway. One method of controlling differential icing is by varying the thickness of the pavement structure above the insulation.
Road embankments or bridge approaches
Another common application for EPS geofoam is as a structural lightweight fill material to construct road embankments or bridge approaches. When roads must be constructed over subsoils with low load-bearing capacity or soils with unacceptably high settlement characteristics, EPS can be used to replace traditional aggregate fill materials; this reduces the settlement and the load on the subsoil.
Other poor soil conditions exist at rivers or muskegs where the bridge structure may be supported on a pile foundation. In this type of construction, bridge approaches are constructed using EPS geofoam.
When side-slope failure occurs in road applications, it is often the result of subsurface soil with low load-bearing capabilities. EPS is used to improve the balance of driving and resisting forces along a potential failure surface. Improvement in slope safety factor against failure can be achieved by soil excavation and replacement with geofoam in the driving block.
The back slope of the excavation is sloped to be self-supporting. EPS lightweight fill material has a density of about one to two per cent of soil or rock, but it has sufficient strength to support typical loads encountered in highway construction applications.
Retaining wall or abutment
EPS geofoam lightweight fill material can be used as part of the design solution for retaining wall or abutment applications. The basic concept is to place geofoam block in a wedge-shaped configuration with the natural or built-up soil behind the retaining wall shaped at a sufficient angle of repose so it is self-supporting.
Since EPS density is low relative to standard aggregate backfill materials, vertical stresses that develop behind a retaining wall or abutment are much lower. When vertical (axial) load is applied to materials, they usually expand laterally (transversely) in the other two directions perpendicular to the axial compression. Poisson’s ratio is the ratio of the percent change (strain) in the horizontal dimension, divided by percent change in the vertical dimension for a material being compressed.
Lateral forces exerted by EPS geofoam are minimal because its Poisson’s ratio within the elastic stress range is approximately 0.1, which is significantly less than the Poisson’s ratio for soil. The light weight of EPS geofoam also results in reduced vertical stresses. As well, the tendency of the interlocked block to settle with a slight tilt away from the wall face reduces the potential lateral load on a wall. The material’s low mass and relatively high compressibility may also offer an additional advantage by limiting horizontal forces against retaining structures. The transition zone between the geofoam and soil should be free draining. Adequate subsoil drainage must be provided to prevent development of hydrostatic pressure and buoyancy.
Additionally, the insulating properties of EPS geofoam reduce the effects of lateral earth pressure resulting from freezing air temperatures. The material’s thermal insulation properties prevent freezing and expansion of the soil behind the retaining wall, as well as protect the retaining wall drainage system.
Where swelling or shrinking soils are known to be present, the 2010 National Building Code of Canada (NBC) requires structures to be designed to prevent damage by soil movement. Figure 5 provides the applicable code requirements.
EPS geofoam can be designed as a compressible inclusion between swelling soils and structural elements. Where long-term soil movement can be predicted, compressible EPS inclusions are designed to reduce the forces that would be induced on structures by soil movement, reducing the likelihood of damage to the structure above. The compressible fill material properties are neither affected by the presence of water in the short term nor dependent on the presence of moisture in the long term.
Expanded polystyrene’s versatile performance properties make it suitable for use in various geotechnical applications beyond its long-term service as an insulation material. In addition to its function as an insulation material that provides constant long-term thermal resistance, the range of products available also allow EPS geofoam to offer design properties for building applications from use as compressible inclusions for protecting structural elements to use as an insulation material for supporting structural elements. In other geotechnical applications, EPS geofoam can be employed as lightweight fill material to reduce loads on subsoils or structures or as a structural material to support road construction.
W. James Whalen, P.Eng., MSCE, CSC, is the technical marketing manager for Plasti-Fab Ltd. He is a registered professional engineer and a member of the Canadian Society for Civil Engineering (CSCE). Whalen is also a long-time member of Construction Specifications Canada (CSC) and the Construction Specifications Institute (CSI). With almost 20 years of experience in the expanded polystyrene (EPS) industry, he chairs the Technical Committee at the EPS Molders Association (EPSMA) and is an active member on various industry standards organization committees, including Underwriters’ Laboratories of Canada (ULC) and ASTM International. He can be contacted at email@example.com.
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