XPS Insulation: R-Value, Moisture Resistance & Buyer's Guide

XPS (extruded polystyrene) is the rigid foam you grab when moisture is the primary concern. At <0.3% moisture absorption by volume, it outperforms every other rigid foam in wet environments — foundations, below-grade walls, under-slab, and anywhere else water or soil contact is involved. The pink boards (Owens Corning Foamular) and blue boards (formerly Dow Styrofoam) at every building supply store are XPS. R-4.5 to R-5.0 per inch, stable to slightly improving in cold temperatures, with compressive strength grades ranging from 15 psi for walls to 100 psi for highway applications. Two honest caveats: R-value declines 5–10% over 10–15 years as blowing agents dissipate, and XPS carries a higher environmental footprint than EPS.

Quick Answer: XPS delivers R-4.5–5.0 per inch with the lowest moisture absorption of any rigid foam (<0.3% by volume). It's the standard for below-grade foundations, under-slab applications, and high-moisture environments. R-value is stable in cold temperatures (unlike polyiso, which derates 38–50%). Two caveats: R-value drops 5–10% over 10–15 years as blowing agents escape, and its HFC blowing agent has high environmental impact (GWP ~1,430). Cost: $0.30–$0.50/board foot, roughly 2× EPS.

Table of Contents

• R-Value & Temperature Performance
• Moisture Resistance
• Product Grades by Compressive Strength
• Best Applications
• Environmental Impact
• Cost
• XPS vs EPS
• XPS vs Polyiso
• Common Mistakes
• Key Takeaways
• FAQ

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R-Value & Temperature Performance

XPS delivers R-4.5 to R-5.0 per inch. Fresh-from-the-factory boards test at R-5.0; the number settles to R-4.5–4.7 as the product ages.

Long-Term R-Value Aging

Why it declines: XPS is manufactured with a blowing agent (HFC-134a, transitioning to HFO and CO₂) trapped in its closed cells. Over time, the blowing agent slowly diffuses out and is replaced by air, which has lower insulating value. ASTM C578 includes Long-Term Thermal Resistance (LTTR) testing to account for this aging.

The practical impact is modest. A 5–10% decline over 10–15 years means 2 inches of XPS drops from R-10 to R-9.0–9.4 over a decade. For most assemblies, this is within design margins. Compare to polyiso, which loses 38–50% of its R-value in cold conditions — a much more significant performance gap.

Temperature stability: XPS performs well in cold. R-value holds steady or slightly improves as temperature drops — the opposite of polyiso's behavior. This makes XPS the preferred rigid foam for exterior applications in cold climates (zones 5–8) where polyiso derates significantly. Building Science Corporation's research on rigid foam performance confirms this temperature-dependent behavior across all foam types. The R-value per inch chart shows the comparison.

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Moisture Resistance

This is XPS's defining advantage. At <0.3% moisture absorption by volume, XPS absorbs less water than any other common insulation material.

Why this matters below grade: Foundation walls and under-slab applications involve direct contact with soil moisture, groundwater pressure, and condensation. An insulation material that absorbs water loses R-value, promotes mold, and deteriorates structurally. XPS's <0.3% absorption means it maintains its thermal performance and structural integrity in these conditions for decades.

Vapor permeability: ~1.0–1.1 perms per inch, making 1 inch of XPS a Class II vapor retarder. At 2 inches (~0.5 perms), it approaches Class I territory. This means XPS controls vapor diffusion in addition to resisting liquid moisture — a dual benefit for basement walls and crawl space applications. The vapor barrier guide explains the Class I/II/III system.

Pro Tip: For below-grade foundation walls, XPS is the only rigid foam we use without hesitation. EPS works below-grade too (thousands of ICF foundations prove it), but it needs a drainage plane to manage its higher moisture absorption. XPS doesn't — it shrugs off soil moisture with no performance loss. If your project involves direct earth contact without drainage, XPS is the safer choice.

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Product Grades by Compressive Strength

XPS is manufactured in multiple grades for different structural loads. Owens Corning's Foamular line is the most widely available:

How to choose: For most residential projects, Foamular 150 (15 psi) handles walls and above-grade applications. Below-grade foundation walls: Foamular 250 (25 psi) for soil pressure resistance. Under-slab: Foamular 400 (40 psi) minimum — the slab and everything on it bears down on this foam permanently.

All grades deliver the same R-value (R-5.0/inch new, aging to R-4.5–4.7). The difference is compressive strength and price — higher grades cost more per board foot. Common thicknesses: 1 inch, 1.5 inch, 2 inch, and 3 inch. The insulation thickness chart shows how thickness maps to R-value targets.

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Best Applications

XPS wins anywhere moisture or cold temperatures are factors. For warm-side applications where moisture isn't a concern, polyiso delivers more R-value per inch and per dollar.

Foundation Installation Tips

Below-grade XPS installation requires a few specific practices for long-term performance:

• Apply to the exterior of the foundation wall before backfilling (new construction) or after excavation (retrofit). The foam goes on the outside of the waterproofing membrane.
• Use construction adhesive compatible with polystyrene — solvent-based adhesives dissolve XPS. Look for adhesives specifically labeled for use with foam board.
• Protect the above-grade portion with a UV-resistant coating, stucco finish, or metal flashing. XPS degrades in direct sunlight — the surface turns chalky and the R-value of the exposed layer declines.
• Extend the foam at least 2 feet below grade (code minimum varies by zone). In climate zones 5+, extending to the footing provides the best thermal performance.
• Tape all seams with compatible tape (3M 8067, Siga Wigluv, or manufacturer-recommended) to create a continuous air and moisture barrier.

For existing homes, exterior foundation insulation requires excavation — a $5,000–$15,000+ project. Interior application of XPS against the basement wall is a more cost-effective retrofit approach. Our basement insulation guide details both methods.

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Environmental Impact

We present this honestly because it matters to a growing number of builders and homeowners.

The issue: XPS is traditionally blown with HFC-134a, which has a Global Warming Potential (GWP) of approximately 1,430 — meaning one pound of HFC-134a traps as much heat as 1,430 pounds of CO₂ over 100 years. This makes XPS the highest-environmental-impact rigid foam by a significant margin.

The transition: The industry is actively moving to HFO (hydrofluoroolefin) and CO₂ blowing agents with GWP of 1–3. Owens Corning has transitioned much of its production. By 2025–2026, most North American XPS production uses low-GWP agents. The DOE's insulation materials overview covers rigid foam types and their general characteristics.

For environmentally conscious projects: EPS has the lowest environmental impact of all rigid foams — air-blown, no persistent blowing agents, lowest embodied energy. Graphite-enhanced EPS (GPS) approaches XPS R-values at lower environmental cost. The full comparison is at XPS vs EPS.

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Cost

XPS costs roughly double EPS per board foot. The premium buys superior moisture resistance (<0.3% vs 2–5%), higher initial R-value (R-5.0 vs R-3.6–4.4), and built-in vapor retarder properties. Whether the premium is worth it depends on the application — below-grade, it usually is. Above-grade, EPS often delivers adequate performance at substantially lower cost.

Cost per R-value is where the comparison gets interesting. XPS at R-5.0/inch and $0.40/bf averages ~$0.08 per R per square foot. EPS Type II at R-3.8/inch and $0.20/bf averages ~$0.05 per R per square foot. So EPS delivers about 40% more R-value per dollar. However, when moisture resistance is the primary concern (foundations, slabs, below-grade), XPS's near-zero absorption justifies the premium — you're paying for moisture performance, not just R-value. In above-grade wall sheathing or roof applications where moisture exposure is manageable, EPS or graphite-enhanced EPS (GPS at R-4.7–5.0/inch) is often the significantly smarter overall value play.

For project-specific pricing, the insulation cost calculator and cost per square foot pages have current data.

Pro Tip: For below-grade foundation walls where XPS is the clear material choice, buy in bulk. A typical basement (1,000 sq ft of wall at 2 inches thick) needs roughly 2,000 board feet of XPS — $600–$1,000 in material. Bulk pricing from building supply stores or direct from distributors can save 15–20% versus buying by the sheet from a home center.

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XPS vs EPS

The most common rigid foam comparison. Brief summary here — the full analysis is at XPS vs EPS.

When XPS wins: Below-grade without drainage, under-slab, applications where <0.3% moisture absorption is critical.

When EPS wins: Budget projects, environmentally conscious builds, applications where long-term R-value stability matters, SIPs and ICFs.

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XPS vs Polyiso

When XPS wins: Cold-climate exteriors, below-grade, moisture-dominant environments.

When polyiso wins: Warm-climate applications, roofing, interior continuous insulation, any location where the foam stays above 40°F.

The complete rigid foam comparison is at the rigid foam insulation guide.

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Common Mistakes

1. Using the wrong compressive strength grade. Foamular 150 (15 psi) under a concrete slab will compress permanently, reducing both R-value and structural support. Under-slab applications require Foamular 400 (40 psi) minimum. Match the product grade to the load.

2. Not accounting for long-term R-value aging. Designing an assembly to exactly meet code using fresh-XPS R-values (R-5.0/inch) means the assembly will be below code in 10–15 years when R-value drops to R-4.5–4.7. Use R-4.7/inch for conservative design or add 10% extra thickness.

3. Choosing XPS over EPS purely on R-value without considering cost. XPS at R-5.0/inch for $0.40/bf vs EPS at R-4.0/inch for $0.20/bf means EPS delivers more R-value per dollar. For above-grade applications where XPS's superior moisture resistance isn't critical, EPS is often the better investment. Run the numbers at the R-value calculator.

4. Leaving joints untaped on exterior continuous insulation. XPS boards installed as exterior wall sheathing need taped seams to function as a continuous air barrier and vapor retarder. Untaped joints create air bypass paths and vapor migration points that compromise the entire assembly. Use manufacturer-recommended tape on every joint.

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Key Takeaways

• XPS delivers R-4.5–5.0 per inch with <0.3% moisture absorption — the best moisture resistance of any rigid foam.
• R-value is stable in cold temperatures, making XPS ideal for exterior applications in zones 5–8 where polyiso derates significantly.
• R-value declines 5–10% over 10–15 years as blowing agents dissipate — a modest but real reduction.
• Product grades range from 15 psi (walls) to 100 psi (highway). Match grade to load: 150 for walls, 250 for below-grade, 400+ for under-slab.
• Environmental impact is the highest of rigid foams (HFC blowing agent, GWP ~1,430), though the industry is transitioning to low-GWP agents.
• Cost: $0.30–$0.50/board foot — roughly 2× EPS but with superior moisture and initial R-value performance.
• Best for: below-grade foundations, under-slab, crawl space walls, exterior wall ci in cold climates, and any high-moisture application.

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FAQ

Does XPS lose R-value over time?

Yes — approximately 5–10% over 10–15 years. New XPS tests at R-5.0/inch; after 10–15 years it settles to R-4.5–4.7/inch as the blowing agent slowly diffuses out and is replaced by air. This is a much smaller decline than polyiso's cold-weather derating (38–50%), and the ASTM C578 standard accounts for it through Long-Term Thermal Resistance testing. For design purposes, using R-4.7/inch is a safe conservative number.

Can I use XPS below grade?

Absolutely — below-grade is where XPS excels. Its <0.3% moisture absorption means it maintains R-value and structural integrity in direct soil contact for decades. Use Foamular 250 (25 psi) or higher for foundation walls to resist lateral soil pressure. For under-slab, use Foamular 400 (40 psi) or higher for the compressive strength needed under a concrete slab. XPS is the standard recommendation for all below-grade rigid foam applications.

XPS vs EPS — which should I choose?

XPS for below-grade without drainage, under-slab, and any application where <0.3% moisture absorption is critical. EPS for budget projects, above-grade applications, environmentally conscious builds, SIPs/ICFs, and where long-term R-value stability matters (EPS doesn't age-derate). Full comparison at XPS vs EPS.

Is XPS environmentally friendly?

XPS has historically been the highest-environmental-impact rigid foam due to its HFC-134a blowing agent (GWP ~1,430). The industry is actively transitioning to HFO and CO₂ agents with GWP of 1–3 — a dramatic improvement. EPS remains the lowest-impact rigid foam (air-blown, no persistent agents). If environmental impact is a priority, consider EPS or graphite-enhanced EPS (GPS), which approaches XPS R-values with lower environmental cost.