Think tapered: Expanded polystyrene insulation hard at work on the roof

December 13, 2019

by David Stassi

Photos courtesy Insulfoam[1]
Photos courtesy Insulfoam

According to the 2015 International Building Code[2] (IBC), roofs are required to have a design slope. Today, the design-build sector is increasingly specifying tapered expanded polystyrene (EPS) insulation solutions to comply with codes—many existing buildings that are being retrofitted require the slope to be altered to meet regulations—as well as save time and money on the roof.

While the busy summer construction season has wrapped up, fall is still ideal for roofing projects. The season’s crisp weather means construction crews do not have to toil under extreme heat. Spring and summer storms have cleared away, decreasing the likelihood of roofers encountering lightning, hail, and high winds. There is also time to prepare for the freeze-thaw, ice dams, and dampness winter brings. For specifiers, fall is the right time to ensure proper selection of insulation for low-slope and roofing recover projects, setting up crews for success on the building top. Certainly, the design-build sector is familiar with insulation’s energy-saving return on investment for commercial building owners and facility managers. The ideal commercial roof insulation will also result in material, labor, and time savings during construction—all benefits given the roofing industry’s current climate. As Roofing Contractor’s State of the Industry Report[3] and Survey 2019 outlines, 68 percent of roofing contractors polled perceive “lack of qualified workers” as a challenge. The second biggest hurdle to the pace of overall construction is “increased building material costs” at 57 percent, given the tariffs imposed on steel and aluminum.

These industry-wide workforce and resource concerns are especially felt when building up slope. The 2015 IBC[4], section 1507.10.1, “Slope,” requires “Built-up roofs shall have a design slope of not less than one-fourth unit vertical in 12 units horizontal (two percent slope) for drainage, except for coal-tar built-up roofs that shall have a design slope of not less than one-eighth unit vertical in 12 units horizontal (one percent slope).” To create code-compliant positive slopes on flat or low-sloped roof assemblies for drainage purposes, building and design professionals often turn to insulation.

Typically, teams have to move and install multiple layers of insulation to create said slope, requiring considerable time and resources to achieve the desired build-up in the roof assembly. These insulations are limited to 100 mm (4 in.) in thickness. Conversely, EPS is manufactured in thicknesses of up to 1016 mm (40 in.), which allows EPS to cut various tapered configurations. EPS also offers custom crickets saddles and sumps to assure positive drainage. These customizable tapered EPS insulation solutions can deliver on material, labor, time, and overall cost savings for low-slope commercial roofing assemblies, while still affording long-term thermal performance.

Cost-savings in low-sloped and flat roof systems

Specifically, tapered options are cut from EPS and available with customizable thickness from 3 mm (1/8 in.) to 1016 mm. Manufacturers can fabricate the rigid foam insulation in order to meet a project’s exact dimensional requirements, including density. Since it is composed of 98 percent air, the tapered solution is also ultra-lightweight.

For more data on how tapered EPS’ physical and mechanical properties impact projects’ bottom lines, it is important to consider material, labor, and time savings, respectively.

Material savings

Compared with extruded polystyrene (XPS) and polyisocyanurate (ISO), EPS requires fewer layers of rigid foam insulation to get the job done. Since XPS and ISO are limited in thickness, many layers are needed to achieve higher R-value assemblies. Each layer costs additional labor and materials (such as adhesive) to install.

Using EPS assemblies, crews typically install a single layer or two of tapered rigid foam insulation versus having to install numerous sheets of relatively thin insulation to achieve the same slope and R-values as XPS and ISO assemblies.

Additionally, since EPS manufacturers create a custom design to meet job-specific needs, there is less product waste. Such resource savings is critical when one considers the rising costs of construction materials[5].

Expanded polystyrene (EPS) is made up of millions of spherical polystyrene beads. Packed together, they form sheets and blocks of rigid foam insulation.[6]
Expanded polystyrene (EPS) is made up of millions of spherical polystyrene beads. Packed together, they form sheets and blocks of rigid foam insulation.

Labor savings

Along with the obstacle of more expensive building materials, roofers still have to contend with a shortage of skilled laborers, from “asking workers to do more work, struggling to meet deadlines, increasing costs for new work, and having to reject new projects,” the National Roofing Contractors Association[7] (NRCA) reports. Fortunately, during the aforementioned tapered EPS installation process, workers do not have to navigate complicated filler panel systems. While XPS and ISO require complex stack and fill techniques, teams utilizing EPS have pre-cut blocks that already account for roof crickets, saddles, valleys, and ridges, along with all types of drainage systems and layouts. In application, crews can easily and precisely handle and install these customized, lightweight tapered EPS pieces into roofing assemblies.

Time savings

The tapered EPS approach to building up a slope can significantly reduce the number of person-hours onsite. In total, factory-cut, custom-built blocks call for less material handling, do not need to be cut prior to installation, and can be installed faster than their XPS and ISO counterparts due to their ultra-lightweight nature. All of these factors help accelerate tapered roofing jobs’ timelines.

As detailed in Figure 1, specifying tapered EPS insulation can result in material and labor savings compared to other tapered insulation options. Some manufacturers estimate contractors can see up to 30 percent overall cost reduction. By utilizing more cost-effective approaches to rigid foam insulation, professionals can address common industry concerns and stay in the black.

Understanding EPS rigid foam insulation and thermal performance

Figure 1: Polyiso & Expanded polystyrene Cost Comparison: Tapered Roof Insulation[8]
Figure 1: Polyiso & Expanded polystyrene Cost Comparison: Tapered Roof Insulation

To fully understand how EPS compares to XPS and ISO, it is important to know how these common types of rigid foam insulation are made.

EPS rigid foam insulation consists of a uniform, closed cellular structure that can meet or exceed the requirements of ASTM C578[9], Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. As ASTM C578 says, “This specification covers the types, physical properties, and dimensions of cellular polystyrene boards with or without facings or coatings made by molding (EPS) or extrusion (XPS) of expandable polystyrene.”

Block molding plants make EPS by producing polystyrene beads that are the size of sugar granules. Manufacturers bring the beads into contact with steam, and the beads’ inner pre-foaming agent begins to boil, expanding them to 40 to 50 times their original size. After a maturing period, the beads settle into a mold, where they are exposed to steam again. During this process, the beads fully expand and fuse together[10] to fill the specified mold cavity.

By comparison, the manufacturing process for XPS and ISO varies greatly from EPS. With these rigid foam insulations, manufacturers create a foam using a gas expansion/blowing agent. First, they introduce the gas into a liquid plastic, which forms a foam via numerous bubbles. The gas then gets trapped in the bubbles as the plastic hardens.

Since the gas’ R-values are greater than air’s R-value, XPS and ISO have higher initial resistance to heat flow than EPS. Like ISO, XPS uses blowing agents that are also fugitive. However, during the rigid foam insulations’ service life, air diffuses in, while blowing agents diffuse out of the cells. Known as thermal drift, this phenomenon can cause R-values of XPS and ISO to degrade over time.

Both ISO and XPS are subject to using a long-term thermal resistance (LTTR) value, the time-weighted average for five years. However, most roofs last longer than five years. The expected long-term R-value for XPS is 4.3 per inch, which is a similar R-value to Type IX EPS. For ISO, the value has been under scrutiny by the NRCA, with values expected less than the posted LTTR value.

Since EPS is composed of 98 percent air, the rigid foam insulation is safe from thermal drift. As the EPS Industry Alliance (EPS-IA) explains, “EPS is stable and does not experience any thermal drift and does not lose R-value over its life. In the long run, the thermal performance of EPS insulation is constant.” Third-party research backs this claim, showing EPS does not experience deterioration in R-value. The researchers tested EPS insulation samples from roofs of varying ages at 21 C (70 F) for thermal resistance, and found no change in R-values[11]. This consistent thermal performance has led some EPS manufacturers to provide a long-term warranty of the rigid foam insulation’s full R-value.

Tapered EPS insulation’s compressive strength, moisture resistance, and environmental benefits proved an ideal solution for the Washington State Convention Center’s (WSCC’s) roof replacement and future green roof system.[12]
Tapered EPS insulation’s compressive strength, moisture resistance, and environmental benefits proved an ideal solution for the Washington State Convention Center’s (WSCC’s) roof replacement and future green roof system.


Familiar with tapered EPS insulation’s long-term R-values, compressive strength, and customizable thickness, a California-based manufacturer chose to incorporate the rigid foam insulation in its roofing and waterproofing membrane for the Washington State Convention Center (WSCC). The Seattle landmark building’s roofing system had started to leak and fail due to weathering and age, and the manufacturer was brought in to replace the outdated assembly with one that would withstand the cold, wet Pacific Northwest climate.

The manufacturer required the convention center’s insulation have a compressive strength of 276 kPa (40 psi), which would be strong enough to be able to support a garden following the roof replacement. The manufacturer also specified 1 x 2-m (4 x 8-ft) tapered EPS insulation panels with thicknesses from 25 to 150 mm (1 to 6 in.) for ease of manageability. This allowed for a quick installation, and cut down on the amount of labor and construction time in Seattle’s tight labor market.

To address the leaking, the manufacturer chose tapered EPS panels that were custom-designed at a 3-mm (1/8-in.) slope to allow maximum drainage and prevent ponded water. In addition to promoting positive drainage, the manufacturer sought an insulation with resistance to moisture retention.

Water resistance

In a city where rain occurs for an average of 152 days per year, the manufacturing company needed to utilize construction techniques and product solutions adept at overcoming Seattle’s rainy weather[13]. They addressed this problem by turning to EPS for its non-hygroscopic properties—the insulation has little or no tendency to absorb moisture due to its closed cell structure.

EPS does have some small interstitial spaces, but it expels water well. All insulations take up moisture, so the question becomes, ‘How well do they dry?’ While it takes XPS a long time to dry, EPS dries quickly and returns to posted R-values.

As illustrated in a study by third-party agency Energy Materials Testing Laboratories (EMTL), EPS does not absorb appreciable moisture in well-constructed roofing assemblies. EMTL simulated winter exposure conditions[14] and found, “the same amount of moisture absorbed (an average of 0.2 percent by weight) has little or no effect on its compressive or flexural strength, and the EPS insulation retains between 95 and 97 percent of its thermal efficiency.”

EPS’ ability to block heat flow despite frequent rainfall made the product a natural fit for the Seattle roofing membrane.

Environmental benefits

Like moisture retention, energy efficiency and environmental concerns were intrinsic to the WSCC project. Naturally, the choice of insulation would have to align with the convention center’s environmental building practices.

EPS supports energy efficiency by creating a tighter building envelope. Less air leakage keeps the convention center cooler in the summer and warmer in the winter, lowering heating and cooling bills, and reducing the facility’s impact on the environment.

Moreover, this rigid foam insulation does not contain any ozone-depleting gases, may have recycled content, and is 100 percent recyclable if it is ever removed or replaced. By combining sustainable form with environmentally friendly functionality, “EPS plays a key role[15] in a building’s ability to comply with ASHRAE 90.1–2001, Energy Standard for Buildings Except Low-Rise Residential Buildings, a benchmark requirement for most green building programs,” says EPS-IA.

With its environmental attributes contributing to long-term thermal performance, coupled with its ability to withstand moisture, tapered EPS is built to last on the rooftop of Seattle’s premier convention center.

Lasting the test of time

Autumn’s mild temperatures make it the ideal season to get to work on the roof, and positive slope and insulation are key components of many such projects. While considered a roofing construction best practice, a positive slope can be costly and material/labor intensive to create with stacks of thick rigid foam insulation. To face these challenges, which echo industry-wide commodity price and workforce issues, tried-and-tested products like customizable tapered EPS[16] continue to hold the answer.

David Stassi is field technical support manager at Insulfoam, a manufacturer of expanded polystyrene (EPS) insulation products. A graduate of Colorado State University, prior to joining Insulfoam, Stassi worked as a field sales representative for OMG Roofing Products. Stassi can be reached via e-mail at[17].

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  2. International Building Code:
  3. Roofing Contractor’s State of the Industry Report:
  4. 2015 IBC:
  5. costs of construction materials:
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  7. National Roofing Contractors Association:
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  9. ASTM C578:
  10. expand and fuse together:
  11. no change in R-values:
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  13. Seattle’s rainy weather:
  14. winter exposure conditions:
  15. EPS plays a key role:
  16. customizable tapered EPS:

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