Tag Archives: XPS

Much to Think About with Cavity Walls

slaton patterson sutterlinFAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE

In response to greater focus on building envelope energy performance, insulation use in the exterior wall cavity has increased. For all U.S. climate zones, the 2012 International Energy Conservation Code (IECC) requires continuous insulation (ci), which is defined by the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) as “insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings.” In cavity wall construction, this is typically accomplished with a continuous plane of rigid or semi-rigid insulation outboard the water (or weather)-resistive barrier/air-vapor barrier (WRB/AVB).

Foam plastics (e.g. extruded polystyrene [XPS]) and semi-rigid mineral wool insulation have been the most commonly used in exterior wall cavities for this purpose. Each has certain advantages and disadvantages. For example, XPS has a slightly higher R-value (nominally 5.0 per inch) as compared to mineral wool (nominally 4.2 per inch), but is considered combustible while mineral wool is not. Use of foam plastic insulation within the exterior wall cavity of Type I to IV construction triggers the need for testing per National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components.

In the 2012 International Building Code (IBC), Section 1403.5 also requires combustible WRBs in the exterior wall assembly of buildings greater than 12 m (40 ft) in height comply with NFPA 285 testing of the assembly. The 2015 IBC appears to have recognized the burden this requirement has placed on the construction industry; NFPA 285 testing is no longer required when the WRBs are the only combustible material present, and are covered with non-combustible claddings like brick, terra cotta, concrete, or metal.

High-density closed-cell foam plastic insulations can function as air barriers and Class 2 vapor retarders. However, when improperly detailed or installed, they can retard the drying of moisture that enters the wall assembly and collects against the WRB/AVB. Thus, care must be taken to detail and install the insulation to minimize the passing of bulk water inboard of its exterior face.

Mineral wool insulation, while typically free-draining, can retain moisture and wet the WRB/AVB until the moisture drains through or evaporates. Some mineral wool insulation products are manufactured with enhanced water-resistance, making them more suitable for use in an exterior wall cavity or rainscreen application. No matter which insulation is used, the wall cavity should be designed with sufficient ventilation provisions to allow materials within to dry out.

WRB/AVBs used inboard of the insulation have evolved to include fluid-applied products, which have different properties than traditional sheet barriers. Recognizing the potential for moisture or bulk water that enters the wall cavity to be held against the WRB/AVB by the insulation, the designer must understand the limitations of all products involved in the installation to avoid the failure shown in the photo below.

Moisture collecting on the horizontal surface likely contributed to the failure of this fluid-applied water-resistive barrier/air-vapor barrier (WRB/AVB). Photo courtesy Jeffrey N. Sutterlin

Moisture collecting on the horizontal surface likely contributed to the failure of this fluid-applied water-resistive barrier/air-vapor barrier (WRB/AVB). Photo courtesy Jeffrey N. Sutterlin

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of the CSI or The Construction Specifier.

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.
David S. Patterson, AIA, is an architect and senior principal with WJE’s Princeton, New Jersey, office, specializing in investigation and repair of the building envelope. He can be e-mailed at dpatterson@wje.com.
Jeffrey N. Sutterlin is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. He can be contacted via e-mail at jsutterlin@wje.com.

Concern regarding long-term insulation data

The December 2013 issue of The Construction Specifier included the article, “Out of Sight, Not Out of Mind,by Ram Mayilvahanan. The feature focused on expanded polystyrene (EPS) and included reference to a particular industry study. In response to the piece, we recently received the following e-mail from John Ferraro, executive director of the Extruded Polystyrene Foam Association (XPSA):

This article included conclusions on the long-term thermal performance of XPS in below-grade applications contrary to more broadly evaluated and accepted industry data. It references a 2009 evaluation published by the EPS Industry Alliance (IA) industry trade organization, then known as EPSMA, and since republished in many forms by EPS-IA members.

In our opinion, the results of this EPS evaluation, which in essence rely on one data point, are not well-supported and are inconsistent with previous significant research conducted in this field. This EPS evaluation also was not independently peer-reviewed within the industry. The data used was reportedly the result of tests conducted by the same test lab and at the same test site, which were apparently employed in two prior studies: Society of the Plastics Industry’s (SPI’s) 1994 report, “Expanded Polystyrene Thermal Insulation Performance in a Below-grade Application” (Twin City Testing Corp.) and AFM Corp.’s 1996 report, “Thermal Transmission and Moisture Content Analyses Conducted on Buried EPS Perform Guard Insulation” (Maxim Technologies/Twin City Testing). There are unanswered questions surrounding the data reliability from these previous analyses that may also carry forward into the EPS evaluation.

The long-term thermal performance of below-grade foundation insulation is an important building design consideration that directly impacts building comfort and energy conservation. We want to draw your attention to a more comprehensive and objective review of the long-term thermal performance of polystyrene foam insulation in below-grade applications that was conducted by the American Society of Civil Engineers (ASCE) 32 Committee during its revisions to ASCE 32-01, Design and Construction of Frost-protected Shallow Foundations.

This committee’s work was documented in the technical paper, “Below-ground Performance of Rigid Polystyrene Foam Insulation: Review of Effective Thermal Resistivity Values Used in ASCE Standard 32-01, Design and Construction of Frost-Protected Shallow Foundations,” which was published in the Journal of Cold Regions Engineering in June 2010.

Based on this critical review of frost-protected shallow foundation designs, the ASCE committee recommends for below-grade vertical orientation (i.e. exterior of walls) using effective in-service design R-value equal to:
● 90 percent of the ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation, R-value for XPS; or
● 80 percent of the ASTM C578 R-value for EPS because of the potential for water absorption.

The ASCE committee also recommends for below-grade horizontal orientation (i.e. under concrete slabs) using effective in-serve design R-values equal to:
● 80 percent of the ASTM C578 R-value for XPS; or
● 65 to 67 percent of the ASTM C578 R-value for EPS because of the potential for water absorption.

We believe it is very important to provide your readership and the industry with objective and accurate information to support and facilitate informed choices in building design. By reporting data from a single, non-peer-reviewed, narrow-scope study and ignoring the vast amount of research and experience, this article does not serve the best interest of the industry.

XPS, EPS, and Dock Flotation

After the feature, “Out of Sight, Not Out of Mind: Specifying Thermal Insulation Below-grade and Under-slab” ran in our December 2013 issue, we received a letter from retired architect, Joseph S. Bond. Mr. Bond wrote that the article in question “seems to reverse the findings” from both his personal and professional experience with expanded and extruded polystyrene (EPS and XPS):

I am a retired architect, and may not have the best current information on EPS and XPS, but when these two products were mistakenly used as ‘flotation’ for lake docks and later removed, the XPS bales were like new and had no water soakage beyond the first (1/8 in.). Continue reading

Out of Sight, Not Out of Mind: Specialty Insulations for Enhanced Moisture Protection

by Ram Mayilvahanan

Neither expanded nor extruded polystyrene (EPS nor XPS) are intended to provide the primary waterproofing or dampproofing on below-grade foundation walls or under slabs. However, rigid foam insulation can offer an additional barrier to ground water, especially those products designed with that goal in mind.

Two classes of products to consider for enhanced moisture protection are faced insulation panels and panels with pre-cut drainage grooves.

Rigid foam insulation is available with polymeric laminate facers virtually impervious to moisture. The thin factory-applied facer keeps water from entering the panel, and thereby away from concrete foundations and slabs.

In instances where a building sits on a high water table or the soil is otherwise regularly saturated, rigid foam insulation drainage boards can help reduce the hydrostatic pressure of the backfill on the foundation wall. Such boards have narrow, regularly spaced channels cut into the face of the foam. A factory-applied filtration facer installed over the grooved face keeps soil out of the channels so water continues to flow. One such widely available product can drain up to 62 l/min/meter (5 gal/min/ft).

To read the full article, click here.

Out of Sight, Not Out of Mind: Specifying thermal insulation below-grade and under-slab

All photos courtesy Insulfoam

All photos courtesy Insulfoam

by Ram Mayilvahanan

In the push to forge more energy-prudent buildings, design professionals are leaving no part of the envelope unexamined. Walls and roofs have always presented a clear target for better thermal performance. Somewhat less obvious are surfaces that are out of sight—below-grade foundation walls and floor slabs. Well-engineered insulation in these locations can provide significant energy savings.

What separates below-grade insulation types from one another? Moisture retention, R-value stability, and compressive strength are the key performance attributes to consider when evaluating and comparing different below-grade insulations.

Below-grade insulation enhances thermal performance in buildings and helps protect concrete from freeze-thaw damage.

Below-grade insulation enhances thermal performance in buildings and helps protect concrete from freeze-thaw damage.

Installing thermal insulation on below-grade foundation or perimeter walls and under slabs is important because un-insulated concrete provides a thermal and moisture bridge between the heated building interior and the relatively cooler earth surrounding the building, or through exposed slab edges to the outside air.

The U.S. Department of Energy (DOE) estimates insulating the exterior edge of slabs in slab-on-grade buildings can reduce winter heating bills from 10 to 20 percent.1 Likewise, the lack of insulation on below-grade foundations, crawlspaces, and under slabs accounts for up to 25 percent of a structure’s total energy loss, the Expanded Polystyrene (EPS) Industry Alliance reports.2

In addition to saving energy, installing thermal insulation on foundations and slabs helps:

  • improve comfort in below-grade and daylight basements;
  • reduce interior condensation on foundation walls; and
  • protect concrete from freeze-thaw cycling, thereby helping minimize cracking, spalling, and frost heave.

In below-grade and under-slab applications, rigid foam insulation reigns compared to other materials. Traditionally, specifications have called for extruded polystyrene (XPS) in these areas, but EPS can perform as well, while being less costly and offering more design flexibility.

Drip, dry, drip, dry
Moisture degrades a material’s ability to insulate. Below-grade insulation frequently contacts wetted soil, so the key is to select a material that does not retain moisture. How do XPS and EPS compare with regard to moisture retention?

EPS made in accordance with ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation (which governs both

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EPS and XPS), has very low moisture retention—it does not waterlog.

In-situ test results for below-grade insulation/

In-situ test results for below-grade insulation.

This means EPS releases moisture rapidly, quicker than XPS does. Over time, when soil is wetted and dried as the weather varies, EPS retains a lot less moisture than does XPS. A real-world evaluation by Stork Twin City Testing—an accredited independent testing laboratory—demonstrated this point. The lab examined sheets of EPS and XPS removed from a side-by-side installation after 15 years in service on a below-grade foundation in St. Paul, Minnesota. As summarized in Figure 1, the XPS was significantly wetter on extraction, with 18.9 percent moisture content by volume compared to 4.8 percent for the EPS. Further, after 30 days of ‘drying’ (to simulate practical temperature swings), the XPS still had elevated moisture of 15.7 percent, while the EPS had dried to 0.7 percent.

In cases where higher moisture shielding of foundations and slabs is crucial, EPS insulations are available with water-impervious facers and pre-cut drainage channels. (For more information, see “Specialty Insulations for Enhanced Moisture Protection”). These facers, which are factory-laminated to both sides of the EPS, make it almost impervious to moisture, and provide an enhanced level of moisture protection performance.

R-today, gone tomorrow
Beyond helping to keep water away from other building components, the degree to which exterior-applied insulations absorb moisture affects their R-value.

The aforementioned 15-year Minnesota in-situ testing also evaluated the R-value of EPS and XPS. The results showed the former retained 94 percent of its specified R-value, whereas XPS experienced a loss of almost half its R-value.3

Rigid foam insulation can be used on either or both the exterior and interior of below-grade foundation walls.

Rigid foam insulation can be used on either or both the exterior and interior of below-grade foundation walls.

A layer of foam insulation helps protect the water proofing on foundation walls during backfill.

A layer of foam insulation helps protect the water proofing on foundation walls during backfill.









In addition to the degrading effects of moisture on R-value, the aptly called ‘thermal drift’ of an insulator is another factor affecting insulating performance. EPS has long-term stable R-values, since it uses blowing agents that by design are already completely diffused at the time of manufacturing. In comparison, XPS uses blowing agents that diffuse from the foam’s cellular structure over the product’s life, thereby reducing its thermal performance with each day in the field. Thermal stability also gives EPS its ability to retain R-value through years of freeze-thaw cycling.

A simple way to check the long-term thermal performance of any insulation is to review the manufacturer’s warranty. Established EPS manufacturers typically warrant 100 percent of the published R-value for 20 years. By comparison, most XPS warranties typically cover only up to 90 percent of the published R-value in order to account for the degradation occuring in the field.

The International Energy Conservation Code (IECC) enumerates prescriptive R-value requirements for below-grade walls and slab-on-grade floors by climate zone. In the 2012 code, Table C402.2 (“Opaque Thermal Envelope Requirements”) has specific values, but it should be confirmed with the local building official.

When is strong too strong (or too expensive)?
A good below-grade insulation must be strong enough to withstand the pressure of the loads above it. For this reason, some EPS manufacturers provide a wide range of compressive strengths, from 69 to 414 kPa (10 to 60 psi)—this has made the material suitable for use as structural fill for highways and airport runways.

The compressive resistance of EPS is demonstrated in its use as geofoam in demanding structural void fill applications.

The compressive resistance of EPS is demonstrated in its use as geofoam in demanding structural void fill applications.

While the insulation strength is an important consideration, a common erroneous design assumption often leads to over-engineering for compressive resistance, which in turn adds unnecessary, and often very high, material costs. Over-engineering a building with 689 kPa (100 psi) below-grade insulation, when a 276 kPa (40 psi) board would have been adequate, can almost double the material cost.

Avoiding this error requires taking into account how the slab and sub-grade interact. Often, the assumption is made that concentrated loads applied to a slab (such as from a forklift or a vehicle) transfer directly to the sub-grade in a pyramidal prism shape. In reality, concrete slabs distribute loads evenly, which results in a lower compression strength needed for the insulation.

For example, a typical case might involve a 100-mm (4-in.) thick concrete slab under a forklift load of 3629 kg (8000 lb) applied via a tire footprint of 0.04 m2 (60 sq. in.). If one assumes the load transfers through the slab at a 45-degree angle, the tire’s force would be distributed over approximately 0.16 m2 (250 sq. in.) of insulation, for a force of 220 kPa (32 psi).

A more accurate calculation involves using a formula for the Theory of Plates on Elastic Foundations:

W = F / 8√(KD)


  • W = slab deflection;
  • F = load on slab;
  • K = subgrade reaction modulus of insulation in lb/cu. in.;
  • D = EH3 / 12(1−u2);
  • E = modulus of elasticity of concrete in lb/sq. in.;
  • h = thickness of concrete slab; and
  • u = Poisson’s ratio for concrete (0.15).

The result of such a calculation for the previously stated scenario, and with a 50-mm (2-in.) thick layer of Type II EPS, for example, is a load on the insulation of only 17.2 kPa (2.5 psi)—well below the 58.6 kPa (8.5 psi) compression rating (at one percent deformation) of commonly available Type II EPS. Therefore, the EPS has plenty of strength for the applied load. As XPS is more expensive per inch than EPS, specification of a higher strength XPS would have unnecessarily increased the insulation costs.

Don’t be bugged
Sometimes concerns arise about rigid foam insulations used below grade providing a conduit for termites or carpenter ants to burrow through to reach the wood in a structure.

Typical below-grade installation.

Typical below-grade installation.

Following building code best practices in termite-risk regions can alleviate this concern. Local codes should be consulted for specific requirements. Additionally, some rigid foam insulations are available with non-toxic, inert additives that deter wood-damaging insects throughout the insulation’s service life.

Installing rigid foam insulation
In below-grade applications, rigid foam insulation is applied over the dampproofing or waterproofing using a polystyrene-compatible adhesive or mechanical fasteners (Figure 2). Applying a bead of polystyrene-compatible caulk or mastic to the top of the insulation board minimizes water infiltration behind it. Additionally, the waterproofing or dampproofing must be properly cured before insulation is installed.

For under-slab insulation, the rigid foam is typically installed over a gravel base, with a poly vapor diffusion retarder between the gravel and insulation. Additional insulation is applied along the slab edges, as this is a primary surface for heat loss. To avoid damage to the insulation, it is necessary to ensure removal of any jagged surfaces or irregularities in the substrate before installing the rigid foam panels.

In both applications, it is important to confirm all details with the insulation manufacturer and local authority having jurisdiction (AHJ).

Bottom line
Expanded polystyrene offers similar or better performance characteristics as extruded polystyrene across key below-grade and under-slab insulation attributes: moisture retention, R-value stability, and compressive strength. While XPS provides a higher R-value per inch of thickness, EPS matches the performance at a much lower cost, thanks to the latter having the highest R-value per dollar among rigid insulations. Additionally, because EPS can be designed in various sizes and compressive strengths, it provides a greater degree of flexibility than does XPS. These factors are making EPS the go-to product for building professionals to help design the right below-grade insulation solutions at the right cost.

EPS can offer myriad benefits when used in geofoam applications.

EPS can offer myriad benefits when used in geofoam applications.

Many building projects throughout North America have used EPS successfully on foundation walls and beneath slabs. For example, the project engineers for a 2012 expansion to the Cold Climate Housing Research Center in Fairbanks, Alaska, specified 300 mm (12 in.) of EPS under a 150-mm (6-in.) floor slab. They were able to use a thicker, yet lower-compressive resistance product than they had initially planned, which improved the thermal performance, at a lower cost than originally budgeted.

In an example of a hot-region project, the concrete contractor for the Starwood Hotel Finance Headquarters in Scottsdale, Arizona, installed 6040 m2 (65,000 sf) of faced EPS panels under the floor slab.

Whether selecting EPS or XPS insulation, to ensure appropriate performance, it is critical to check that the specific product has been manufactured per ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation.

1 See U.S. Department of Energy’s Office of Building Technology, “Slab Insulation Fact Sheet” at www.ornl.gov. (back to top)
2 See the EPS Industry Alliance’s “EPS Below Grade Series 103” Technical Bulletin at www.epsindustry.org. (back to top)
3 Ibid. (back to top)

Ram Mayilvahanan is the product marketing manager for Insulfoam, a division of Carlisle Construction Materials. He specializes in commercial building insulation. Mayilvahanan can be reached at ram.mayilvahanan@insulfoam.com.

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