Insulating below-grade with high-performing expanded polystyrene

November 24, 2020

Photos courtesy Insulfoam[1]
Photos courtesy Insulfoam

by Kevin Keith

The distant sound of hammers pounding and the low murmur of a crew’s radio announce a day of hectic construction. When the temperature warms up, excavators can easily turn up the soil and crews can pour concrete without risk of frost. Teams can also accomplish more work below-grade with extended hours of daylight.

During peak construction seasons, there is a renewed focus on materials that can ensure efficient, cost-effective structures starting at the foundation. Expanded polystyrene (EPS) rigid foam is among these solutions, and the customizable insulation material is a suitable choice for an array of residential and commercial applications.

Given EPS’ ability to repel water, withstand loads, and to maintain R-values throughout its time in service, the insulation lends itself well to today’s construction industry—one hungry for energy-efficient, cost-effective materials suitable for below-grade building endeavors.

The importance of below-grade insulation

Before pouring a foundation, energy-prudent builders need to consider adding insulation below-grade and under slab. If constructed without adequate insulation, bare slab concrete operates as a thermal bridge between a building’s heated interior and the cooler earth soil, a costly misstep for residential and commercial projects alike. For example, according to the EPS Industry Alliance (EPS-IA), lack of insulation on foundations, crawl spaces, and under slabs can account for up to 25 percent of a structure’s total energy loss. This unnecessary loss of energy can be damaging to an owner’s utility bill.

Expanded polystyrene (EPS) is often incorporated below-grade due to its moisture-resistant characteristics.[2]
Expanded polystyrene (EPS) is often incorporated below-grade due to its moisture-resistant characteristics.

Fortunately, there are many below-grade insulation solutions on the market to aid in the energy efficiency of a structure, such as EPS. As one field study conducted by the Institute of Research in Construction (IRC) found, EPS is a durable basement insulation. After withstanding freeze-thaw cycles, the insulation showed no degradation[3] of its physical and thermal properties, even when surrounded by moisture-laden heavy clay. The material was not only able to resist the presence of water, but also can effectively help conserve energy with its consistent R-values.

EPS’ physical properties

As noted in the above IRC study, EPS’ long-term, moisture-resistant physical properties can combat common below-grade challenges and are worth exploring in greater detail.

Moisture mitigation

For below-grade foundations, moisture management is critical. In these applications, insulation is frequently exposed to moist soil and runs the risk of becoming wet itself. Since moisture can negatively impact R-values and render the material ineffective, insulation used below-grade must demonstrate minimal retention of moisture in the long-term.

Real-world studies have proven EPS offers moisture-resistance below-grade (read “Technical Bulletin[4]: 15-Year In-Situ Research Shows EPS Outperforms XPS in R-Value Retention”). Further, EPS exhibits drying potential when put under strenuous freeze-thaw cycles, which is very critical to maintaining consistent, long-term R-values. In climates where higher moisture shielding is vital for foundations and concrete slabs, EPS is available with water-impervious facers and pre-cut drainage channels.

Mold resistance

The presence of seeping moisture can also be detrimental to foundation walls and below-grade projects on the inside, serving as a breeding ground for interior mold and mildew. Since EPS ‘breathes,’[5] and, consequently, does not require expensive venting to mitigate moisture, moisture is less likely to be trapped in below-grade areas incorporating this material. By insulating with EPS, builders can reduce interior condensation and lower the risk of fungi impacting human health and potentially damaging foundational integrity.

Due to its versatility, building professionals can cut rigid foam into sheets, slabs, or any desired shape to meet specific below-grade needs, whether commercial or residential.[6]
Due to its versatility, building professionals can cut rigid foam into sheets, slabs, or any desired shape to meet specific below-grade needs, whether commercial or residential.

EPS’ thermal properties

Aside from an increased likelihood for higher moisture levels than above-grade areas, below-grade applications are also susceptible to energy loss, requiring proper insulation to optimize efficiency. Generally speaking, the higher an R-value, the greater the insulating effectiveness, so below-grade insulation solutions must post high thermal resistance over the long-term to ensure energy savings.

EPS’ thermal properties help lay the foundation for high and stable R-values, making it an attractive option for the below-grade builder. The rigid foam is composed of 98 percent air, as air does not leech out of the cellular structure throughout the product’s time in service. This results in stable, reliable insulation performance, and it is not uncommon for EPS manufacturers to warranty R-values for 20 years, guaranteeing lasting thermal performance[7].

EPS’ mechanical properties

As for mechanical properties, EPS’ resistance to compressive stresses can align with foundation load requirements. Contrary to popular belief, concentrated loads applied to a concrete slab do not transfer directly to the sub-grade below. Instead, concrete slabs disperse loads evenly, meaning the insulation can have a lower compressive strength and still comply with below-grade building codes.

With a compressive resistance resting between 69 to 414 kPa (10 to 60 psi), manufacturers can produce EPS to meet strength requirements of multiple building codes. For example, for foundation “applications in which the foam insulation bears a minimal load, ASTM C578 [Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation] Type I (nominal density of 14 kg/m3 [0.9 lb per cf]) material is adequate. EPS board produced to meet the requirement of Type I EPS has been tested and found to fall within 69 to 96 kPa (14 psi). The resilience of EPS insulation board provides reasonable absorption of building movement without transferring stress to the interior or exterior finish at the joints,” says EPS-IA.

Bearing this in mind, builders and contractors can satisfy below-grade insulation requirements with EPS without over-engineering, leading to material savings.


These attributes of EPS insulation—mitigation of moisture, enduring R-values, and compressive strength—are evident in numerous real-world below-grade applications. From increasing energy efficiency in an Albuquerque perishable food warehouse, New Mexico, to insulating foundations to ensure military personnel are sheltered in warm, comfortable housing near the Arctic Circle, construction teams seek out EPS commercially and residentially.

EPS keeps perishables fresh in Albuquerque warehouse

Energy-conserving insulation is crucial in places such as sun-drenched, arid Albuquerque, New Mexico. With daytime temperatures often exceeding 38 C (100 F) in the summer months, Albuquerque builders are in a constant battle to keep buildings cool without sending energy costs through the roof. This struggle extends to all cold storage applications, including a food and nutrition services kitchen and storage warehouse designed to preserve produce and perishables used for student breakfasts, lunches, and snacks.

At 10,034 m2 (108,000 sf), this cold storage facility was central to the Albuquerque Public Schools’ (APS’) educational rebuild and remodel. To keep moisture at bay, provide consistent thermal performance to lower energy costs, and offer compressive strength, the project’s general contractor and commercial concrete contractor installed EPS rigid foam insulation below the facility’s concrete slab floor.

Cold storage warehouses located in climates like New Mexico run the risk of warm, outside air seeping into the space and coming into contact with colder building materials, resulting in added cooling cost and condensation. The presence of condensation in these environments can cause electronics to fail, goods to deteriorate, and harmful mold and bacteria to grow, in addition to compromising the building’s structural integrity. Building teams were able to sidestep these challenges thanks to EPS with premium, factory-applied laminate polymeric facers. The insulation and facer combination helps reduce the formation of condensation on foundation walls and keeps the droplets from entering into the rigid foam material, shielding the concrete foundation and slabs from moisture penetration.

Given EPS’ ability to withstand loads, and maintain R-values throughout its time in service, the insulation is suitable to meet the goals of energy-efficient construction and cost-effective materials.[8]
Given EPS’ ability to withstand loads, and maintain R-values throughout its time in service, the insulation is suitable to meet the goals of energy-efficient construction and cost-effective materials.

With less heat-conducting moisture present, the kitchen and storage center’s thermal performance also improves. EPS’ high, continuous R-values further benefit the facility’s overall thermal performance. With the rigid foam solution’s ability to maintain insulating effectiveness for up to 20 years in some cases, EPS can contribute to lower energy costs, in line with project goals. As one can imagine, higher energy costs are a significant expense in operating a cold storage facility and can negatively impact bottom lines. However, by decreasing the amount spent on monthly cooling costs, the APS district can allocate dollars elsewhere to implement progressive student programming, better support classroom teachers, and more.

Understanding the compressive strengths of various rigid foam insulations is also crucial to saving project dollars. For example, when insulation is over-engineered, it can lead to expensive, unnecessary material costs. In contrast, cost-effective, properly engineered compressive strength insulation can meet sub-slab insulation requirements and better accommodate budgets. In the case of the APS project, the commercial concrete contractor installed two layers of 76-mm (3-in.) thick EPS with the strength of 276 kPa (40 psi) rather than three layers of 50-mm (2-in.) thick extruded polystyrene (XPS) rigid foam insulation. This not only met the cold storage’s compressive strength mandates, but also saved the contractor approximately $20,000 in material costs.

This economical compressive strength, together with EPS’ water-resistant proprieties and ability to resist the flow of hot or cold over the project’s lifetime made the rigid foam insulation a fitting choice for the facility’s sub-slab freezer floor. Further, the material lowered the district’s utility bills and maximized energy efficiency, a resource savings helping the project ultimately achieve Leadership in Energy and Environmental Design (LEED) certification.

EPS affords military personnel warm and comfortable housing

In stark contrast to Albuquerque, applications in subarctic climates that are subject to long, cold winters and short, warm summers also call upon EPS. Take Fort Greeley, for example. Located two hours southeast of America’s coldest city—Fairbanks, Alaska—the army base’s proximity to the Arctic Circle posed a unique set of challenges for the team that was tasked with constructing military housing for more than 1700 service members and Department of Defense personnel. With EPS installed below-grade, the construction crew was able to achieve its mission of creating a structurally sound, warm, and energy-efficient home environment.

Specifically, the building team installed 300 mm (12 in.) of EPS with advanced polymeric laminate facers vertically and horizontally on the housing units’ foundation perimeters and on the bottom of the 150-mm (6-in.) concrete slab to prevent seasonal frost heave. Heaving can occur when –26 to –32 C (–15 to –25 F) frigid temperatures penetrate the ground and cause  ice to form and displace the soil. With this phenomenon, abrupt bumps form on the surface, potentially causing the foundation to crack and fail. However, the presence of EPS insulation thwarts this process due to its moisture-resistance and ability to withstand freeze-thaw, as mentioned above. Another example is an airport control tower in Galena, Alaska, where builders used the rigid foam to safeguard the slab-on-grade foundation. Despite the temperatures dropping to –51 C (–60 F) for weeks on end, there has been no frost heaving[9] or differential settlement.

The use of EPS below-grade was also key for delivering a warm interior, while meeting project energy goals. Studies such as a joint research project by the Expanded Polystyrene Association of Canada (EPAC) and the National Research Council Canada/Institute for Research in Construction (NRC/IRC) have confirmed the rigid foam retains its R-value even after exposure in an exterior below-grade application that is subjected to in-situ freeze-thaw cycling (for more information, read N. Normandin, W. Maref, M.T. Bomberg, and M.C Swinton’s “In-Situ Performance Evaluation of Exterior Insulation Basement Systems [EBIS] – EPS Specimens,” a National Research Council Canada [NRC] report). In effect, the material provides resistance to the escape of heat over the long-term to maintain a comfortable indoor environment. This consistency also generates reduced heating costs, helping the military housing project meet the stringent LEED for Homes Silver Level energy-efficiency requirements.


A proven solution for below-grade insulation, EPS offers moisture mitigation, long-term thermal performance, and compressive strength—physical, thermal, and mechanical properties crucial to foundation-level projects. Due to its versatility, building professionals can cut the rigid foam into sheets, slabs, or any desired shape to meet specific below-grade needs, whether in a commercial cold storage facility in New Mexico or on the bottom of military residential housing in Alaska. EPS further provides builders with the ability to meet energy-efficiency requirements, without sacrificing durability and strength for enduring foundations supporting today’s structures well into the future.

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  5. EPS ‘breathes,’:
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  9. no frost heaving:
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