EPS Insulation: R-Value, Cost & Why It Outperforms Long-Term

EPS (expanded polystyrene) doesn't win the spec-sheet competition on any single line item. XPS has better moisture resistance. Polyiso has higher R-per-inch. But EPS wins two categories that matter more than any individual spec: long-term R-value stability and cost per R-value delivered. In our experience, EPS is the most underrated rigid foam on the market. It's air-blown — no blowing agent to dissipate over time, no cold-weather derating, no R-value degradation decade after decade. It costs half what XPS does per board foot. And for projects where the gap in R-per-inch matters, graphite-enhanced EPS (GPS) closes that gap to less than R-0.5/inch compared to XPS — at a lower price.

Quick Answer: EPS delivers R-3.6–4.4 per inch depending on density, at $0.15–$0.25/board foot — the cheapest rigid foam. Its R-value is the most stable of all rigid foams (no blowing agent loss over time), and it has the lowest environmental impact (air-blown, no persistent chemicals). Moisture absorption (2–5%) is higher than XPS but manageable with proper drainage. For higher R-value, graphite-enhanced EPS (GPS) delivers R-4.7–5.0/inch. EPS is the standard core material for SIPs, ICFs, and EIFS.

Table of Contents

• R-Value & Density Grades
• The Long-Term R-Value Advantage
• Temperature Stability
• Graphite-Enhanced EPS (GPS)
• Moisture Performance
• Best Applications
• Environmental Advantage
• Cost
• Common Mistakes
• Key Takeaways
• FAQ

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R-Value & Density Grades

EPS R-value is density-dependent. Higher density = more polystyrene per cubic inch = higher R-value and compressive strength.

For most residential projects: Type II (1.5 lb/ft³, R-3.8/inch, 15 psi) is the standard grade — it's what you'll find in SIPs, ICFs, and most builder supply stores. We specify Type IX (1.8 lb/ft³, R-4.0) for higher-performance applications. Type I (1.0 lb/ft³) is the economy grade, common in packaging and basic applications where thermal performance isn't critical.

The R-value per inch chart compares EPS density grades against all other insulation materials. The insulation thickness chart shows how EPS thickness maps to R-value targets by application.

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The Long-Term R-Value Advantage

This is EPS's most underrated property — and arguably its most important.

EPS is manufactured by expanding polystyrene beads with steam. The pentane blowing agent used during manufacturing dissipates completely during the production process. The finished product contains only air in its closed cells. Air doesn't migrate, dissipate, or change insulating properties over time.

Result: EPS R-value on day one is EPS R-value on day 10,000. It doesn't age-derate. Ever.

Compare this to the competition:

After 15 years in a zone 6 foundation wall, the effective R-value gap between XPS (R-4.5 aged) and Type II EPS (R-3.8 stable) narrows to just R-0.7/inch. That's a $0.15/bf price premium for less than R-1 of difference — a premium that's hard to justify for many projects.

Building Science Corporation's rigid foam comparison acknowledges this stability advantage. For 30+ year applications like foundation walls and SIPs, long-term stability matters more than initial peak performance.

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Temperature Stability

EPS is the most temperature-stable rigid foam available. Its R-value shows minimal variation regardless of operating temperature — no cold-weather derating like polyiso, no aging effects amplified by temperature.

This makes EPS an excellent choice for:

• Cold-climate exterior continuous insulation (zones 5–8)
• Below-grade applications where temperatures stay at 45–55°F year-round
• Crawl space and basement walls where temperature fluctuates seasonally

In cold-climate exterior applications, EPS Type II (R-3.8/inch) delivers more R-value per dollar than polyiso (which derates to R-3.5–4.5/inch in winter) at half the cost. The math favors EPS.

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Graphite-Enhanced EPS (GPS)

GPS (Graphite Polystyrene) is the premium tier of EPS — same manufacturing process, but with infrared-absorbing graphite particles mixed into the polystyrene beads before expansion.

GPS closes the gap with XPS. At R-4.7–5.0/inch, GPS matches or approaches XPS (R-4.5–5.0 aged) at a lower cost and lower environmental impact. For above-grade applications where XPS's moisture resistance isn't critical, GPS is increasingly the smarter choice.

Brand: Most GPS products are based on BASF Neopor raw material, manufactured by various regional EPS producers. Look for the characteristic gray color.

Pro Tip: If standard EPS doesn't quite hit your R-value target but XPS is overkill for the application, GPS is the answer. Two inches of GPS delivers R-9.4–10.0 — comparable to 2 inches of XPS (R-9.0–9.4 aged) at 20–30% lower cost and a fraction of the environmental footprint. For exterior wall continuous insulation in zones 4–6, GPS is the sweet spot of performance, cost, and sustainability.

Where to Find GPS Products

GPS isn't stocked at every big-box store, but availability is growing rapidly. Check specialty building supply dealers, insulation distributors, and some Lowe's/Home Depot locations. Many SIP and ICF manufacturers now offer GPS core options as standard or premium upgrades. When sourcing GPS, confirm you're getting the graphite-enhanced product (gray color, R-4.7+/inch on the data sheet) rather than standard EPS that happens to be gray. The BASF Neopor certification is the reliable indicator of genuine graphite-enhanced performance.

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Moisture Performance

EPS absorbs more moisture than XPS — 2–5% by volume vs XPS's <0.3%. This is the most common objection to EPS in below-grade applications, and it deserves an honest assessment.

The reality: EPS works fine below-grade when paired with proper drainage. Thousands of ICF (Insulated Concrete Form) foundations demonstrate this — ICF systems use EPS as the permanent formwork for concrete foundation walls, with direct soil contact on the exterior. These systems perform well for decades.

The key: Don't rely on EPS alone as a moisture barrier. Below-grade EPS needs:

• Foundation waterproofing membrane (applied to concrete before EPS)
• Drainage board or drainage plane behind the EPS
• Proper exterior grading to direct water away from the foundation

Vapor permeability: 2.0–5.0 perms per inch, making EPS a Class III (semi-permeable) material. This is actually an advantage in some assemblies — it allows drying in both directions, which can prevent moisture trapping. The vapor barrier guide explains when permeability is beneficial vs. problematic.

When to choose XPS instead: Applications with direct soil contact and no drainage (XPS's <0.3% absorption handles this without drainage), under-slab without a moisture barrier, or any application where the foam will be submerged or continuously wet. The full comparison lives at XPS vs EPS.

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Best Applications

EPS is the standard where proven performance at low cost matters most. SIPs, ICFs, and EIFS have collectively installed billions of square feet of EPS — a track record no other rigid foam matches in sheer volume.

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Environmental Advantage

EPS is the most environmentally responsible rigid foam, and it's not close.

For green building projects, LEED certification, Passive House, or environmentally conscious homeowners, EPS is the default rigid foam choice. We increasingly recommend GPS for projects where the performance boost matters — it adds capability without changing the environmental profile.

The DOE's insulation guidance notes insulation's net positive environmental impact (energy savings vastly exceed manufacturing footprint), but for builders choosing between foam types, EPS has the smallest manufacturing footprint.

EPS recycling: While "recyclable in some markets" understates it somewhat, the infrastructure is growing. EPS can be ground into beads and reprocessed into new foam products, or densified into polystyrene blocks for manufacturing. Some manufacturers accept post-construction EPS scrap. Check with your local waste authority — recycling availability varies significantly by region, but it's more accessible than XPS or polyiso recycling.

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Cost

EPS is the cheapest rigid foam — roughly half the cost of XPS per board foot.

Cost per R-value comparison (1-inch board):

EPS delivers the lowest cost per R-value for most applications. GPS is essentially tied with XPS on cost per R-value while offering better long-term stability and lower environmental impact. The insulation cost calculator runs project-specific comparisons.

Pro Tip: EPS is a commodity product manufactured by regional producers — unlike XPS and polyiso, which are produced by a handful of national brands. This means pricing varies significantly by region and supplier. Get quotes from local EPS manufacturers (search "EPS insulation manufacturer [your state]"), not just from home centers. Direct-from-manufacturer pricing can be 20–40% less than retail, especially for large quantities.

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Common Mistakes

1. Dismissing EPS as "just Styrofoam." EPS construction-grade foam is engineered to specific density, R-value, and compressive strength standards per ASTM C578. It's not the same as packaging foam (which is typically Type I at 1.0 lb/ft³). Type II and higher grades deliver genuine thermal performance that competitors respect.

2. Using EPS below-grade without drainage. EPS absorbs 2–5% moisture by volume — manageable with drainage, problematic without it. For below-grade applications, always include a drainage plane (drainage board, gravel, or drainage mat) between the soil and the EPS. If drainage isn't practical, use XPS instead (<0.3% absorption).

3. Not considering GPS when EPS R-value falls short. If Type II EPS at R-3.8/inch doesn't meet your R-value target, the reflex is to upgrade to XPS at R-5.0. But GPS at R-4.7–5.0/inch is often the smarter step — it's cheaper than XPS, more stable long-term, and environmentally superior.

4. Ignoring fire safety requirements. Like all rigid foams, EPS is combustible and requires a 15-minute thermal barrier (typically ½-inch drywall) in occupied spaces per IBC/IRC. EPS melts and burns at ~300°F+. Never leave EPS exposed in a living space. The fire safety guide covers thermal barrier requirements for all foam types.

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Key Takeaways

• EPS delivers R-3.6–4.4/inch at $0.15–$0.25/board foot — the cheapest rigid foam by a significant margin.
• Most stable long-term R-value of any rigid foam. Air-blown cells don't lose their insulating gas over time. R-value on day one = R-value on day 10,000.
• Temperature-stable in cold weather — no derating like polyiso.
• Graphite-enhanced EPS (GPS) delivers R-4.7–5.0/inch — approaching XPS performance at lower cost and environmental impact.
• Moisture absorption (2–5%) is higher than XPS. Below-grade applications need drainage. Above-grade, it's not a concern.
• Lowest environmental impact of all rigid foams: air-blown, no persistent blowing agents, lowest GWP, recyclable.
• Industry standard for SIPs, ICFs, and EIFS — proven in billions of square feet of installation.
• Cost per R-value: EPS wins or ties in virtually every above-grade comparison. For below-grade without drainage, XPS is worth the premium.

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FAQ

Is EPS the same as Styrofoam?

Technically, "Styrofoam" is a Dow Chemical (now DuPont) trademark for XPS — not EPS. Colloquially, people use "Styrofoam" for any polystyrene foam, including EPS. Construction-grade EPS (Type II and above) is manufactured to specific ASTM C578 standards with controlled density, R-value, and compressive strength. It's a legitimate structural insulation material used in foundation systems, SIPs, and commercial buildings.

Can I use EPS below grade?

Yes — with proper drainage. Thousands of ICF foundations use EPS in direct soil contact with excellent long-term performance. The key: EPS absorbs more moisture than XPS (2–5% vs <0.3%), so pair it with a drainage plane (drainage board, gravel backfill, or drainage mat) and a foundation waterproofing membrane. If drainage isn't available, XPS is the safer choice. The basement insulation guide covers below-grade options.

What is GPS (graphite EPS)?

Graphite Polystyrene (GPS) is EPS with infrared-absorbing graphite particles mixed into the polystyrene beads. The graphite reduces radiant heat transfer within the cells, boosting R-value from R-3.8/inch (standard Type II) to R-4.7–5.0/inch — approaching XPS territory. GPS retains EPS's advantages: stable long-term R-value, low cost, and lowest environmental impact. It costs 20–30% more than standard EPS but delivers 24–32% more R-value per inch.

EPS vs XPS — which is better?

Neither is universally "better" — they serve different sweet spots. XPS for below-grade without drainage, under-slab, and maximum moisture resistance. EPS for budget-conscious projects, above-grade ci, SIPs/ICFs, long-term R-value stability, and environmental priority. GPS narrows the performance gap while maintaining EPS's cost and environmental advantages. The full head-to-head comparison is at XPS vs EPS.