XPS vs EPS Insulation: Which Rigid Foam Board Is Right?

XPS and EPS are both polystyrene foam boards — same base polymer, different manufacturing processes, very different performance characteristics. XPS (extruded) gives you higher R-value and better moisture resistance. EPS (expanded) gives you lower cost, more stable long-term R-value, and a smaller environmental footprint. We get this question constantly, and the right choice depends on moisture exposure, budget, and whether you're thinking in 5-year or 30-year terms.

Quick Answer: XPS (R-4.5–5.0/inch, $0.30–$0.50/board foot) beats EPS (R-3.6–4.4/inch, $0.15–$0.25/board foot) on R-value per inch and moisture resistance (<0.3% vs 2–5% absorption). But EPS costs roughly half as much, has more stable long-term R-value (no blowing agent loss), and carries lower environmental impact. Use XPS for below-grade and high-moisture applications where moisture resistance is critical. Use EPS for budget projects, SIPs, and when environmental impact or long-term R-value stability matters.

Table of Contents

• Master Comparison Table
• R-Value Comparison
• Moisture Resistance
• Cost Comparison
• Temperature Stability
• Long-Term R-Value: The Aging Factor
• Environmental Impact
• Compressive Strength
• Applications: Which Foam Where
• Choose XPS When / Choose EPS When
• Key Takeaways
• FAQ

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Master Comparison Table

For how XPS and EPS compare against polyiso (the third rigid foam type), check the full rigid foam insulation guide.

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R-Value Comparison

XPS delivers approximately 15–25% more R-value per inch than EPS:

At 2 inches, XPS gives R-10 where EPS gives approximately R-8. For continuous exterior insulation in climate zones 5–8 where code requires R-5 ci (per 2021 IECC Table R402.1.3), 1 inch of XPS comfortably delivers R-5.0 — but 1 inch of EPS at R-3.6–4.4 may fall short. You'd need 1.5 inches of EPS to reliably hit R-5.

However — this comparison changes over time. XPS R-value declines as blowing agents dissipate (more on this below). After 10–15 years, XPS settles to R-4.5–4.7/inch while EPS stays at its original R-value. The long-term gap narrows to approximately 5–15%.

Winner: XPS — on day-one R-value. EPS closes the gap over 10–15 years as XPS ages.

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Moisture Resistance

This is XPS's biggest advantage and the primary reason to choose it over EPS. In our experience, moisture performance is the deciding factor in about 80% of XPS-vs-EPS decisions.

XPS absorbs less than 0.3% moisture by volume. Its closed-cell, extruded structure creates a nearly impervious surface. Below grade, where soil moisture and hydrostatic pressure push water against the board for years, XPS maintains its R-value and structural integrity.

EPS absorbs 2.0–5.0% moisture by volume. Its expanded bead structure has small air gaps between beads that can wick moisture over time. Below grade, moisture absorption gradually reduces R-value and can compromise long-term performance.

Practical context: The 2–5% absorption figure for EPS sounds alarming, but in practice EPS performs well below-grade when proper drainage (dimple mat, gravel, perimeter drain) is provided. Thousands of ICF (insulated concrete form) foundations use EPS as the core material, and they've demonstrated decades of solid performance. The key: don't rely on EPS as a moisture barrier — it's insulation that tolerates some moisture, not waterproofing.

For below-grade applications where drainage is less reliable or soil moisture is high, XPS's <0.3% absorption provides a meaningful safety margin. When in doubt below grade, XPS is the safer call.

Winner: XPS — <0.3% vs 2–5% absorption. This is the deciding factor for below-grade and high-moisture applications.

Pro Tip: If you're using EPS below grade, add a dimple mat or drainage board between the soil and the foam. This directs bulk water away from the EPS surface, reducing moisture absorption to negligible levels. With proper drainage, EPS works fine in most residential foundation applications — and saves you roughly 50% on material versus XPS.

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Cost Comparison

EPS costs approximately half what XPS does — and for large projects, the savings are substantial.

Example: 1,000 sq ft of foundation wall with 2 inches of rigid foam:

• XPS: $1,000–$2,400 material
• EPS: $700–$1,800 material
• Savings: $300–$600 (material only — installation cost is the same)

For exterior continuous wall insulation on a 1,500 sq ft home (~1,000 sq ft of wall area), 1 inch of EPS saves $150–$300 versus XPS. Across an entire below-grade foundation, the savings compound further.

Installation costs are identical for both materials — same cutting, same fastening, same taping. The difference is purely in material cost.

Winner: EPS — roughly half the price per board foot.

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Temperature Stability

Both XPS and EPS perform well across the temperature range found in building applications — and both outperform polyiso (which loses up to 46% of its R-value at 0°F).

EPS is the most temperature-stable rigid foam. Its R-value barely changes from 0°F to 100°F. What you specify is what you get, regardless of climate.

XPS actually improves slightly in cold temperatures — R-value at cold temps is R-5.0 or higher, compared to R-4.5–5.0 at room temperature. The blowing agent trapped in XPS cells becomes denser in cold, marginally improving insulating performance.

For climate zones 5–8 where exterior insulation faces sustained cold, both XPS and EPS are reliable choices. Polyiso is not — its R-value drops to R-3.0–4.0 per inch at 0°F mean temperature. Building Science Corporation recommends XPS or EPS over polyiso for cold-climate exterior applications.

Winner: EPS (most consistent) — though XPS performs well too. Both beat polyiso in cold climates.

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Long-Term R-Value: The Aging Factor

This is EPS's strongest technical argument.

XPS R-value declines over time:

The decline occurs because the blowing agent (HFC or HFO) trapped in XPS's closed cells slowly diffuses out through the cell walls, replaced by air. Air insulates less effectively than the blowing agent. ASTM C578 accounts for this with LTTR (Long-Term Thermal Resistance) testing.

EPS R-value stays constant. EPS is blown with pentane during manufacturing, but the pentane fully dissipates during and shortly after production — replaced by air. Since the insulating gas is already air, there's nothing left to dissipate. The R-value you measure on day one is the R-value 30 years later.

After 15 years, the effective R-value gap between XPS and EPS narrows from ~25% to ~10%. If you're designing for 30-year performance, EPS's stability is a genuine advantage.

Winner: EPS — no R-value decline over time. XPS loses 5–10% over 10–15 years.

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Environmental Impact

EPS has the lowest environmental footprint of any rigid foam insulation.

EPS manufacturing uses pentane as a blowing agent, which fully dissipates during production — the final product contains only air. No fluorocarbon blowing agents remain in the finished board, giving EPS near-zero Global Warming Potential from the material itself.

XPS uses HFC-134a (GWP ~1,430) or newer HFO blowing agents (GWP <5). The blowing agent remains trapped in the cells for years, slowly leaking into the atmosphere. Major manufacturers like Owens Corning are transitioning to lower-GWP formulations, but the transition isn't complete across all product lines.

EPS is also recyclable through specialized facilities (limited but growing) and is used in some circular construction programs. XPS has more limited recycling options.

Winner: EPS — lowest GWP, no retained blowing agents, most recyclable.

Pro Tip: If a project prioritizes environmental impact — green building certification, LEED points, or personal values — EPS is the rigid foam of choice. It delivers solid R-value (R-3.6–4.4/inch), costs the least, and has the smallest carbon footprint. The R-per-inch gap versus XPS is real, but you can compensate with slightly thicker boards. An extra half-inch of EPS is cheaper than the environmental cost of XPS blowing agents.

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Compressive Strength

XPS offers a wider range of compressive strength ratings, which matters for under-slab and load-bearing applications.

For standard residential slabs and foundations, both materials offer adequate compressive strength. EPS at 10–25 psi handles typical residential loads without issue. XPS's advantage shows up in heavy-duty applications: garage slabs, commercial floors, or any situation requiring compressive strength above 25 psi.

Winner: XPS — broader range of compressive strength ratings for heavy-load applications.

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Applications: Which Foam Where

For how XPS and EPS fit into full wall and foundation assemblies, the wall insulation guide and basement insulation guide cover the complete picture.

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Choose XPS When / Choose EPS When

Here's how we advise clients on this decision:

Choose XPS When:

• Below-grade without reliable drainage — XPS's <0.3% moisture absorption provides the widest safety margin
• High compressive loads — XPS offers products up to 100 psi for commercial and heavy-duty applications
• Maximum R-value in minimum thickness — R-5.0/inch vs R-3.6–4.4 means fewer inches needed
• Direct soil contact for extended periods — the moisture resistance advantage compounds over decades

Choose EPS When:

• Budget is a priority — approximately half the material cost of XPS
• Long-term R-value stability matters — no blowing agent loss = consistent performance for 30+ years
• Environmental impact matters — lowest GWP, no retained blowing agents, most recyclable
• SIPs, ICFs, or EIFS applications — EPS is the industry standard for all three
• Above-grade continuous insulation — moisture absorption is less of a concern above grade; EPS delivers solid value
• Below-grade with proper drainage — EPS performs well with dimple mat and perimeter drains at significant cost savings

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Key Takeaways

• XPS delivers R-4.5–5.0/inch; EPS delivers R-3.6–4.4/inch. XPS is 15–25% higher on day-one R-value.
• XPS R-value declines 5–10% over 10–15 years as blowing agents dissipate. EPS R-value stays constant. The long-term gap narrows to ~10%.
• XPS has far superior moisture resistance (<0.3% vs 2–5% absorption) — the primary reason to choose it for below-grade and high-moisture applications.
• EPS costs roughly half as much ($0.15–$0.25 vs $0.30–$0.50/board foot). For large projects, savings are substantial.
• EPS has the lowest environmental impact of any rigid foam: air-blown, no retained blowing agents, near-zero GWP.
• Both are temperature-stable in cold climates — both outperform polyiso when temperatures drop.
• For below-grade with good drainage, EPS is the better value. For below-grade without reliable drainage, XPS provides a critical moisture margin.
• Neither can be left exposed in occupied spaces — both require a thermal barrier (½" drywall) per code.

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FAQ

Does XPS lose R-value over time?

Yes. XPS starts at approximately R-5.0 per inch and declines to R-4.5–4.7 over 10–15 years — a 5–10% reduction. This happens because the blowing agent (HFC or HFO) trapped in the closed cells slowly diffuses through cell walls and is replaced by air, which insulates less effectively. ASTM C578 LTTR testing accounts for this. The decline is gradual and modest, but it means XPS won't deliver R-5.0/inch for the life of the building. EPS doesn't experience this aging because its insulating gas is already air.

Can I use EPS below grade?

Yes — with proper drainage. EPS absorbs 2–5% moisture by volume, which is higher than XPS (<0.3%), but in practice EPS performs well below grade when a dimple mat or drainage board directs bulk water away from the foam surface. Thousands of ICF foundations and basement retrofits use EPS successfully. The key: pair EPS with adequate drainage and don't rely on it as a moisture barrier. For high water tables or clay soils with poor drainage, XPS provides a wider safety margin.

Which is better for the environment — XPS or EPS?

EPS is significantly better. It's manufactured using pentane that fully dissipates during production — the finished product contains only air (near-zero GWP). XPS retains fluorocarbon blowing agents (HFC or HFO) that slowly leak into the atmosphere over the product's lifetime. Even with the industry's transition from high-GWP HFC to lower-GWP HFO, XPS still carries a larger carbon footprint than EPS. EPS is also more widely recyclable. For green building certifications and carbon-conscious projects, EPS is the clear choice among rigid foams.

Is XPS worth twice the price of EPS?

It depends entirely on the application. For below-grade foundation walls in wet soil without reliable drainage — yes, XPS's moisture resistance justifies the premium. For above-grade continuous wall insulation or any application where moisture absorption is minimal — EPS delivers excellent value at roughly half the cost. The R-per-inch advantage of XPS (15–25%) can be offset by using slightly thicker EPS boards. For many residential projects, EPS with proper installation practices delivers equivalent real-world performance at substantially lower cost.