Rigid Foam Board Insulation: EPS vs XPS vs Polyiso Compared

Quick Answer: Rigid foam board insulation comes in three types: EPS (R-3.6–4.4 per inch, cheapest and most temperature-stable), XPS (R-4.5–5.0 per inch, best moisture resistance, blue or pink boards), and polyiso (R-5.6–6.5 per inch, highest R-value but drops significantly in cold weather). Rigid foam is used for continuous exterior insulation over wall sheathing, basement and foundation walls, under-slab applications, and cathedral ceilings. Choosing the right type depends on your climate zone, moisture exposure, and budget.

Table of Contents

• EPS vs XPS vs Polyiso — Master Comparison
• EPS (Expanded Polystyrene)
• XPS (Extruded Polystyrene)
• Polyisocyanurate (Polyiso)
• Applications: Where to Use Rigid Foam
• Cold-Climate Strategy
• Cost Comparison
• Environmental Considerations
• Common Mistakes
• Key Takeaways
• FAQ

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EPS vs XPS vs Polyiso — Master Comparison

This table is the core reference for choosing between the three rigid foam types. Every property that matters is here.

A board foot = 1 sq ft at 1 inch thick. See our R-value per inch chart for how rigid foams compare against all insulation types.

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EPS (Expanded Polystyrene)

EPS is the white foam board you've seen in everything from coffee cups to insulated concrete forms. It delivers R-3.6 to R-4.4 per inch — the lowest of the three rigid foams — but compensates with the lowest cost, best temperature stability, and lowest environmental impact.

Why we like EPS: Its R-value doesn't change with temperature. When the mean temperature drops to 0°F, EPS still delivers its full rated R-per-inch. Polyiso loses 38–46% of its label R-value under the same conditions. This makes EPS the most predictable performer in cold climates — what you specify is what you get, year-round.

EPS is manufactured by expanding polystyrene beads with steam (air-blown — no fluorocarbon blowing agents). This gives it the smallest carbon footprint of any rigid foam. It's also recyclable in many markets, though actual recycling infrastructure remains limited.

The trade-off: EPS absorbs more moisture than XPS or polyiso — 2.0–5.0% by volume. For below-grade applications (foundation walls, under-slab), this matters. Moisture absorption reduces R-value and can promote deterioration over decades. We recommend EPS below-grade only with proper drainage and waterproofing, or when encapsulated (as in ICFs).

Best applications: SIPs (structural insulated panels — EPS is the standard core), ICFs (insulated concrete forms), budget-conscious continuous exterior insulation, applications where temperature stability matters more than moisture resistance. Detailed specs at our EPS insulation guide.

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XPS (Extruded Polystyrene)

XPS is the colored foam board — pink (Owens Corning Foamular), blue (Dow Styrofoam), or green (Kingspan GreenGuard). It delivers R-4.5 to R-5.0 per inch with the best moisture resistance of any rigid foam (<0.3% absorption by volume).

We reach for XPS in any application where the foam will be in contact with moisture — foundation walls, below-grade exterior, under-slab, and crawl space applications. Its closed-cell structure and extruded manufacturing process create a nearly impervious moisture barrier. A 2-inch XPS board below grade maintains its R-value for decades where EPS would gradually absorb moisture and degrade.

The aging issue: Fresh XPS starts at approximately R-5.0 per inch. Over 10–15 years, the blowing agents slowly diffuse out and are replaced by air, reducing R-value to approximately R-4.5–4.7 per inch. ASTM C578 Type X specifications account for this with LTTR (Long-Term Thermal Resistance) ratings. In practice, the 5–10% decline over a decade is modest — but it means you shouldn't assume R-5.0/inch for long-term calculations.

Environmental concern: XPS historically used high-GWP HFC blowing agents. Major manufacturers have been transitioning to lower-GWP HFO formulations, but XPS still carries the highest environmental impact of the three rigid foam types. If environmental impact is a priority, EPS or polyiso are better choices.

Best applications: Below-grade foundation walls, under-slab insulation, crawl space walls, any high-moisture application where long-term moisture resistance matters. Detailed data at our XPS insulation guide. For a direct comparison, our XPS vs. EPS page covers the tradeoffs.

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Polyisocyanurate (Polyiso)

Polyiso delivers the highest R-value per inch of any rigid foam at R-5.6 to R-6.5 (tested at 75°F mean temperature). It's the standard for commercial roofing insulation and increasingly common as residential continuous wall insulation. It typically comes with foil facing on both sides, which creates a near-zero vapor permeance (<0.05 perms).

But polyiso has a critical weakness that most insulation guides bury in a footnote — and it's the single most important thing to understand about this material.

The Cold-Weather Derating Problem

Building Science Corporation's research on polyiso temperature performance documents a dramatic R-value drop as temperatures fall:

At 0°F mean temperature, polyiso delivers R-3.0–R-4.0 per inch — approximately half of its labeled R-value. The National Roofing Contractors Association (NRCA) recommends using R-5.0 per inch (not R-5.6–6.5) for polyiso in cold-climate calculations.

Why does this happen? The blowing agent gas trapped in polyiso's closed cells condenses at low temperatures, reducing the insulating gas effect. EPS and XPS use different blowing agents (or air) that don't exhibit this condensation behavior.

Practical implications: If you're in climate zones 5–8 and using polyiso on the exterior of your walls, the exterior face of the board spends significant winter hours at temperatures well below 25°F. The R-6.5/inch you specified may only deliver R-4.0–4.5 when you need it most. Derating polyiso by 15–25% in cold climate calculations is standard practice among building scientists.

This doesn't mean polyiso is bad — it's excellent in warm and mild climates (zones 1–4) where mean temperatures through the assembly rarely drop below 40°F. It's also fine on the interior side of wall assemblies in cold climates (where it stays warm). The problem is specific to cold-side exterior applications in cold climates.

For the full specs and application guidance, check our polyiso insulation guide.

Pro Tip: The foil facing on polyiso creates a near-zero perm vapor barrier. In cold climates, this can trap moisture in the wall assembly if the polyiso is on the exterior and the wall doesn't have a drying path to the interior. Building Science Corporation recommends that foil-faced polyiso on the exterior must provide sufficient R-value to keep the sheathing above the dew point — otherwise condensation forms on the cold side of the foil and rots the sheathing. The R-value ratio rules in IRC Table R702.7.1 specify the minimum exterior ci R-value required for each climate zone.

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Applications: Where to Use Rigid Foam

Continuous Exterior Wall Insulation

The highest-impact application for rigid foam. Adding 1–2 inches over wall sheathing breaks thermal bridging through framing — improving whole-wall R-value by 15–30%. A 2×6 wall with R-19 cavity insulation + R-5 ci delivers ~R-19 whole-wall, versus ~R-14.5 without ci.

Which type: Polyiso (zones 1–4 where cold derating isn't an issue), EPS or XPS (zones 5–8 for temperature stability), or Rockwool Comfortboard 80 (non-combustible, vapor-permeable — premium but excellent). Our exterior wall insulation guide covers material selection and installation details.

Basement and Foundation Walls

Rigid foam applied to interior or exterior of concrete foundation walls. XPS is the default for below-grade exterior (best moisture resistance). EPS works below-grade with proper drainage. Polyiso is acceptable for interior-side foundation insulation (stays warm). Typical: 2 inches of XPS (R-10) on foundation walls in zones 4–8.

Our basement insulation guide covers the full approach.

Under-Slab Insulation

XPS is preferred for under-slab (highest compressive strength at 15–100 psi and excellent moisture resistance). EPS works at lower compressive loads. Polyiso is not recommended under slabs (moisture and compression concerns). Typical: 2 inches of XPS (R-10) under basement or garage slabs in zones 4+.

Cathedral Ceiling Above Roof Deck

Rigid foam installed above the roof sheathing during a new roof or re-roof. This creates an unvented roof assembly (per IRC R806.5) without eating into rafter depth. Two to four inches of polyiso (R-11 to R-26) above the deck, combined with cavity insulation between rafters, meets code for most zones. Read more at cathedral ceiling insulation.

SIPs (Structural Insulated Panels)

EPS is the standard core for SIPs — typically 4–8 inches of EPS between two layers of OSB. A 6-inch SIP delivers approximately R-24 with minimal thermal bridging (no studs). SIPs provide superior air tightness compared to stick-framed walls.

Re-Siding Projects

If you're replacing siding on an existing home, adding 1–2 inches of rigid foam under the new siding is one of the most cost-effective energy upgrades available. The incremental cost is modest (the siding crew is already there), and you gain R-5 to R-10 of continuous insulation that breaks thermal bridges. Wall insulation covers this approach.

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Cold-Climate Strategy

Choosing rigid foam in zones 5–8 requires accounting for polyiso's cold-weather derating. Three approaches work:

1. Use EPS or XPS instead of polyiso. XPS delivers R-4.5–5.0/inch at all temperatures (improves slightly in cold). EPS delivers R-3.6–4.4 stable across all temperatures. Neither has cold-weather derating. Two inches of XPS (R-10) is a reliable, predictable choice for cold-climate exterior ci.

2. The polyiso sandwich. EPS on the exterior (cold side) and polyiso on the interior (warm side). The EPS handles cold exposure at full R-value; the polyiso stays warmer behind the EPS and maintains closer to its label R-value. This approach can deliver 10–15% more real-world R-value than an all-polyiso assembly in cold climates.

3. Derate polyiso in your calculations. If using polyiso on the exterior, calculate with R-5.0/inch instead of R-6.5. This means specifying thicker boards — where you'd need 1 inch of polyiso in a warm climate, you need 1.5 inches in a cold climate to deliver the same real-world R-value.

We've shifted toward recommending EPS or XPS for exterior continuous insulation in zones 5+ over the last several years. The cost difference is modest, and the peace of mind of temperature-stable R-value is worth it. Polyiso remains our first choice in zones 1–4 and for commercial roofing in all climates.

Pro Tip: Whatever rigid foam you use on the exterior, tape all seams with manufacturer-recommended tape (3M All Weather, Siga Wigluv, or similar). Untaped joints allow air to bypass the insulation — negating much of the air-barrier benefit. Think of it this way: a continuous insulation layer with untaped seams is neither continuous nor an air barrier. Taping adds 30 minutes and $50 in tape to a wall project — one of the cheapest quality upgrades available.

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Cost Comparison

Material Cost per Square Foot (2025–2026)

Project Cost Examples

For project-specific estimates, the insulation cost calculator generates personalized pricing. Material costs vary significantly by region and quantity.

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Environmental Considerations

EPS has the smallest environmental footprint of the three rigid foams. It's blown with air (no fluorocarbon agents) and has near-zero GWP from manufacturing. EPS is recyclable through specialized facilities (limited but growing availability). It's the clear choice for environmentally conscious projects.

XPS carries the highest environmental concern. Traditional XPS used HFC-134a with a GWP of approximately 1,430. Major manufacturers (Owens Corning, DuPont) have been transitioning to lower-GWP HFO blowing agents, but the transition is ongoing and not all products have switched. Ask for HFO-blown XPS if environmental impact matters.

Polyiso uses pentane as a blowing agent (GWP of ~5 — much lower than traditional XPS). Its environmental profile is between EPS and XPS. Foil facing adds a small recyclability challenge but isn't a major concern.

None of the three rigid foams are biodegradable. All persist in landfills for centuries. If end-of-life environmental impact is a high priority, cellulose insulation (compostable, 80% recycled) and mineral wool (inert, recyclable) have better profiles — but they can't replace rigid foam in most of its applications.

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Common Mistakes

1. Using polyiso in cold climates without derating. Specifying R-6.5/inch on the exterior of a home in Chicago or Minneapolis means you're designing to an R-value you won't actually get in winter. Derate to R-5.0/inch, or switch to XPS or EPS. This is the most common rigid foam specification error in residential construction.

2. Using EPS where high moisture resistance is needed. EPS absorbs 2–5% moisture by volume — significantly more than XPS (<0.3%). For below-grade exterior applications, foundation walls in wet soil, or any location with sustained moisture contact, XPS is the better choice. EPS works below-grade with proper drainage, but XPS is more forgiving.

3. Not accounting for XPS long-term R-value decline. XPS starts at R-5.0/inch but settles to R-4.5–4.7 over 10–15 years. If your code compliance depends on achieving exactly R-5.0/inch, you may fall short after aging. Use LTTR values or add a small safety margin.

4. Leaving rigid foam exposed. All rigid foam boards are combustible and must be covered by a 15-minute thermal barrier (typically ½" drywall) in occupied spaces per building code. In basements, garages, and living areas, exposed rigid foam fails code inspection and creates a fire hazard. Budget for drywall or an equivalent covering.

5. Not taping or sealing joints. Rigid foam used as continuous insulation and/or an air barrier must have taped and sealed joints. Unsealed seams allow air bypass, which eliminates the air-barrier function and reduces thermal performance. Use manufacturer-recommended tape rated for the specific foam type and temperature range.

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Key Takeaways

• Three types of rigid foam: EPS (cheapest, most stable R-value), XPS (best moisture resistance), and polyiso (highest R-per-inch but derated in cold).
• Polyiso loses up to 46% of its labeled R-value at 0°F mean temperature. In zones 5–8, derate polyiso to R-5.0/inch or use EPS/XPS instead.
• XPS R-value declines 5–10% over 10–15 years as blowing agents dissipate. Fresh R-5.0 → long-term R-4.5–4.7.
• EPS is the most temperature-stable and environmentally friendly rigid foam (air-blown, no fluorocarbon agents).
• Primary applications: continuous exterior wall insulation (breaks thermal bridging), basement/foundation walls, under-slab, cathedral ceilings, and SIPs.
• Below-grade: XPS or EPS (not polyiso). Under-slab: XPS (compressive strength + moisture resistance).
• Cold-climate exterior ci: EPS or XPS for reliable R-value, or the "polyiso sandwich" (EPS outside, polyiso inside).
• All rigid foam must be covered by a thermal barrier (½" drywall) in occupied spaces — it's combustible.
• Tape all seams for air-barrier continuity — untaped joints negate much of the benefit.
• Material costs: EPS $0.35–$0.90/sq ft, XPS $0.50–$1.20/sq ft, polyiso $0.70–$1.50/sq ft (all per 1" board).

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FAQ

Which rigid foam is best?

There's no single best — it depends on the application and climate. For below-grade and high-moisture: XPS wins (lowest moisture absorption at <0.3%). For cold climates (zones 5–8) exterior ci: EPS or XPS (temperature-stable R-value). For warm/mild climate exterior ci: polyiso (highest R-per-inch at R-5.6–6.5). For budget projects: EPS (cheapest at $0.35–$0.90/sq ft for 1" board). For environmental priority: EPS (air-blown, lowest GWP). Match the material to your specific conditions — our R-value chart and types of insulation guides help with the full comparison.

Can I use polyiso in cold climates?

You can, but you must derate the R-value. At 25°F mean temperature, polyiso delivers only R-3.5–4.5 per inch — roughly 62% of its label value. The NRCA recommends calculating at R-5.0/inch in cold climates rather than the labeled R-5.6–6.5. Practical strategies: use thicker polyiso to compensate, switch to EPS/XPS for the exterior layer, or use a "polyiso sandwich" with EPS on the cold side. Building Science Corporation has published extensive temperature-dependent R-value data.

How thick should rigid foam be for continuous insulation?

It depends on your climate zone and what your cavity insulation provides. The 2021 IECC (zones 5–8) requires either R-5 ci (with R-20 cavity) or R-10 ci (with R-13 cavity). One inch of polyiso gives R-5.6 (zones 3–4). One inch of XPS gives R-5.0 (any zone). For R-10 ci in cold climates, 2 inches of XPS (R-10) or 2.5 inches of EPS (R-9–11) works. For a complete wall assembly calculation, use our R-value calculator.

Does XPS lose R-value over time?

Yes, but modestly. Fresh XPS measures approximately R-5.0 per inch. Over 10–15 years, the blowing agents slowly diffuse out and are replaced by air, dropping R-value to approximately R-4.5–4.7 per inch — a 5–10% decline. ASTM C578 LTTR (Long-Term Thermal Resistance) specifications account for this. In practice, the decline is gradual and minor. If your design depends on exactly R-5.0/inch, add a small safety margin or use thicker boards. EPS does not experience this aging because it's air-blown from the start.

Is rigid foam a vapor barrier?

It depends on the type and thickness. Foil-faced polyiso is essentially a vapor barrier (<0.05 perms). XPS at 1 inch is approximately 1.0 perm (Class II vapor retarder); at 2 inches, roughly 0.5 perms. EPS at 1 inch is 2.0–5.0 perms (vapor-permeable). This matters enormously for wall assembly design — a vapor-impermeable exterior layer (like foil-faced polyiso) prevents outward drying, which requires the assembly to dry inward only. Our vapor barrier guide covers the implications for each climate zone.