Polyiso Insulation: R-Value, Cold-Weather Derating & Complete Guide

Polyiso has the highest labeled R-value of any common rigid foam — R-5.6 to R-6.5 per inch. That label tells only half the story. Polyiso's R-value drops significantly as temperature decreases, falling to R-3.5–4.5 per inch at 25°F mean temperature — a 38% decline from its 75°F label. In extreme cold (0°F), it drops to R-3.0–4.0, performing barely better than EPS at half the cost. This temperature sensitivity is the single most important factor in choosing polyiso — and one that most insulation guides either ignore or undersell. We don't.

Quick Answer: Polyiso delivers R-5.6–6.5 per inch at 75°F — the highest of any common rigid foam. But its R-value drops 38–50% in cold weather (R-3.5–4.5 at 25°F, R-3.0–4.0 at 0°F per Building Science Corporation testing). Excellent for roofing, warm-climate wall sheathing, and interior applications where it stays warm. In cold climates (zones 5–8), use XPS or EPS on the exterior instead — or derate polyiso to R-5.0/inch minimum in your calculations.

Table of Contents

• What Is Polyiso?
• R-Value: The Label vs Reality
• Cold-Climate Design Strategies
• Foil Facing & Vapor Properties
• Best Applications
• Product Options & Brands
• Cost
• Polyiso vs XPS vs EPS
• Common Mistakes
• Key Takeaways
• FAQ

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What Is Polyiso?

Polyisocyanurate (polyiso) is a thermoset plastic rigid foam board, typically manufactured with aluminum foil facing on both sides. It's denser than XPS or EPS (2.0–2.5 lb/ft³ vs 1.0–2.2 lb/ft³) and has the highest R-per-inch of the three rigid foam types — on paper.

Polyiso is manufactured as continuous boardstock in thicknesses from 1 inch to 4 inches. The foil facers give it a distinctive silver appearance and add a critical property: they make the board an effective Class I vapor barrier (<0.05 perms). This is both an advantage and a complication, depending on the assembly.

The rigid foam insulation guide covers all three rigid foam types in a single comparison.

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R-Value: The Label vs Reality

This is the section that matters most — and the reason this page exists.

Polyiso R-value is tested at 75°F mean temperature per ASTM C518. That's the number on the label: R-5.6 to R-6.5 per inch. But the exterior surface of a wall or roof in climate zones 5–8 routinely sees mean temperatures of 25°F or below during winter — the season when insulation matters most. The DOE's insulation materials guide notes polyiso's high R-value but does not address its cold-weather behavior — which is exactly why independent testing data matters.

Polyiso R-Value by Temperature (BSC Research)

The cause: The pentane blowing agent trapped in polyiso's closed cells undergoes condensation at lower temperatures. When the blowing agent changes from gas to liquid, it no longer insulates the cell — essentially creating tiny cold spots throughout the foam. Post-2000 polyiso (pentane/CO₂ blown) is more affected than older HCFC-141b formulations.

What this means in practice: One inch of polyiso on an exterior wall in zone 6 during January (mean temperature ~20°F) delivers approximately R-3.5–4.0 — not R-6.0. That's 42% less than the label claims. Two inches delivers R-7.0–8.0, not R-12.0. If you designed your wall assembly to R-12 from 2 inches of polyiso, your actual winter performance is R-7 to R-8. That's the difference between a comfortable house and cold rooms on the north wall.

The NRCA (National Roofing Contractors Association) acknowledges this and recommends using R-5.6/inch for warm climates and R-5.0/inch for cold climates in design calculations. Many building scientists argue even R-5.0 is generous for zones 6–8.

The foam industry association (PIMA) maintains that the label R-value is appropriate for design purposes, citing the thermal mass of the building assembly and the contribution of interior-side insulation. The building science community — led by Building Science Corporation and others — disagrees, pointing to field testing data that consistently shows the derating pattern above. We side with the field data.

Pro Tip: The cold-weather derating applies to the OUTER face of the foam. In a thick polyiso board (3–4 inches), the inner portion stays warmer because the outer portion provides some insulation first. This means thicker polyiso boards perform slightly better per inch than thinner ones in cold conditions. If you're using polyiso in zone 5, 3 inches of polyiso performs closer to R-15 than 3× the single-inch derated value would suggest. But for zones 6–8, the exterior surface is simply too cold for polyiso to deliver its labeled performance.

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Cold-Climate Design Strategies

If you're in zones 5–8 and want to use polyiso, these are the approaches that work:

1. Use polyiso only on the interior (warm) side. Polyiso performs at or near its labeled R-value when the foam stays above 40°F — which it does on the interior side of a wall assembly. Install polyiso on the interior of a stud wall as continuous insulation under drywall. It functions as insulation + vapor barrier. The cold side of the wall gets XPS or EPS.

2. Hybrid layering: XPS or EPS exterior + polyiso interior. Place the cold-tolerant foam on the outside (where it faces winter temperatures) and polyiso on the inside (where it stays warm). Example: 1.5 inches of XPS (R-7.5) on the exterior + 1.5 inches of polyiso (R-9.0 at interior temps) on the interior = R-16.5 total at a lower cost than 3 inches of XPS alone.

3. Derate polyiso to R-5.0/inch in zones 5–6. For moderate cold climates, NRCA's recommendation of R-5.0/inch is a reasonable conservative estimate. Use this value in your R-value calculations, not the label value.

4. Avoid polyiso on the cold-side exterior in zones 7–8. At mean temperatures of 0°F to -15°F, polyiso drops to R-3.0–3.5/inch — comparable to EPS at half the cost. Use XPS or EPS for the exterior in these zones.

5. Thicker polyiso performs better per inch than thinner polyiso in cold. If you must use polyiso in a cold climate, use a single thick board (3–4 inches) rather than multiple thin boards. The inner portion stays warmer, mitigating the derating effect.

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Foil Facing & Vapor Properties

Standard polyiso comes with aluminum foil facers on both sides. This makes the board a Class I vapor barrier at <0.05 perms — effectively impermeable to moisture vapor.

Advantages:

• Controls moisture migration through the wall assembly
• Acts as a radiant barrier when facing an air space (foil reflects radiant heat)
• Protects the foam core during installation

Complications:

• In mixed climates (zones 3–5), a vapor-impermeable layer on the exterior can trap moisture in the wall during summer when vapor drive reverses direction (hot/humid outdoor air pushes moisture inward)
• If the exterior foam doesn't provide sufficient R-value to keep the cavity side warm, moisture can condense on the cold face of the polyiso and be trapped by the foil

For wall assemblies with foil-faced polyiso as exterior continuous insulation, the foam must provide enough R-value to keep the condensing surface above the dew point — the same code requirement that applies to all exterior continuous insulation assemblies. The vapor barrier guide and code requirements pages cover this in detail.

Fiberglass-faced polyiso is available as an alternative. It's more vapor-permeable than foil-faced (~1 perm per inch), allowing some drying through the foam. This is preferred in some wall assemblies where bi-directional drying is desired.

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Best Applications

Roofing: Where Polyiso Dominates

Polyiso is the most widely installed roof insulation in North America for commercial buildings, and it's increasingly common in residential above-deck applications. The roofing application is ideal because:

• The foam sits directly below the roofing membrane — the warmest position in the assembly, where cold-weather derating has the least impact
• Maximum continuous operating temperature of ~250°F accommodates the heat from dark roofing membranes and hot-applied adhesives
• High R-per-inch reduces total roof assembly thickness compared to XPS or EPS
• Foil facers provide a radiant barrier benefit, reflecting heat away from the roof deck
• Available in tapered systems for roof drainage (no structural slope needed)

For residential re-roofing projects, 2–4 inches of polyiso above the sheathing (R-11 to R-26) provides continuous insulation that eliminates thermal bridging through rafters. We recommend this approach for homes in zones 3–5 when a roof replacement is already planned — the incremental cost to add rigid foam while the roof is off is far less than a standalone insulation project.

Residential Wall Sheathing

For exterior continuous insulation on wood-framed walls, polyiso delivers the most R-value per inch of any commonly available rigid foam. One inch of polyiso provides R-5.6 in warm conditions — enough to meet the R-5 ci requirement in zones 4–5 with a single layer. Two inches provides R-11.2 — sufficient for the R-10 ci option in zones 5–8.

The caveat remains: in zones 5+ where winter exterior surface temperatures drop well below freezing, the outer portion of the polyiso board loses significant R-value. For these climates, we recommend either derating to R-5.0/inch in calculations, using a hybrid layering approach (polyiso on the interior, XPS or EPS on the exterior), or switching to XPS or EPS entirely.

Installation notes for wall sheathing: Polyiso boards attach over structural sheathing (OSB or plywood) with cap nails, washer-head screws, or adhesive. Tape all seams with manufacturer-recommended foil tape to create a continuous air barrier — this is one of polyiso's underappreciated advantages. The foil-faced surface with taped seams functions as both an air barrier and a vapor retarder, potentially eliminating the need for a separate housewrap (check local code requirements). Furring strips (1×3 or 1×4) over the foam create a ventilated rain screen cavity and provide a solid nailing surface for siding.

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Product Options & Brands

Most polyiso products for wall applications are foil-faced on both sides. For roofing, fiberglass-faced or coated-glass-faced variants are common (better for adhering roofing membranes). Common thicknesses: 1, 1.5, 2, 2.5, 3, and 4 inches. Product data sheets from major manufacturers provide aged LTTR (Long-Term Thermal Resistance) values per ASTM C1303 — use these for design, not the initial R-value.

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Cost

Polyiso costs roughly the same per board foot as XPS but delivers more R-value per inch in warm applications. In cold climates where derating applies, the effective cost per R-value swings in favor of XPS or EPS. Use the insulation cost calculator for project-specific comparisons.

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Polyiso vs XPS vs EPS

The full comparison lives at rigid foam insulation guide and XPS vs EPS.

Pro Tip: When comparing polyiso to XPS or EPS for a specific project, calculate cost per R-value at the temperature the foam will actually see — not at 75°F. In zone 6, polyiso at R-4.0/inch (derated) costs $0.30–$0.55 per board foot. XPS at R-5.0/inch (stable) costs $0.30–$0.50 per board foot. XPS delivers 25% more R-value per dollar in this scenario because polyiso's premium R-value disappears in the cold.

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Common Mistakes

1. Designing with full label R-value in cold climates. Using R-6.0/inch for polyiso on an exterior wall in zone 6 overestimates performance by 35–40%. The wall will underperform in the coldest months when insulation matters most. Derate to R-5.0/inch for zones 5–6 and switch to XPS or EPS for zones 7–8.

2. Using polyiso below grade. Polyiso absorbs moisture through its foil facers' edges when in contact with soil or concrete, and the cold below-grade temperatures compound the R-value loss. XPS (<0.3% absorption) or EPS (with drainage) are the correct choices for basement and crawl space walls.

3. Ignoring the foil facer's vapor barrier effect. Foil-faced polyiso at <0.05 perms is essentially a vapor barrier. On the exterior of a wall assembly, this prevents outward drying — fine in cold climates (where vapor drives outward and you WANT to slow it) but problematic in hot-humid climates where summer vapor drives inward. Understand your assembly's moisture dynamics before specifying foil-faced polyiso.

4. Not taping seams. Untaped joints between polyiso boards create air bypass paths that reduce the effective R-value of the continuous insulation layer and negate the air barrier benefit. Tape every seam with manufacturer-recommended foil tape for a continuous thermal and air barrier.

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Key Takeaways

• Polyiso has the highest labeled R-value of common rigid foams: R-5.6–6.5/inch at 75°F.
• Cold-weather derating is critical: R-value drops to R-3.5–4.5 at 25°F (38% loss) and R-3.0–4.0 at 0°F (46–54% loss).
• Excellent for roofing, warm-climate wall sheathing (zones 1–4), and interior continuous insulation applications.
• Not recommended for below-grade, cold-side exterior in zones 7–8, or any application where the foam consistently operates below 40°F.
• Foil-faced polyiso is a Class I vapor barrier (<0.05 perms) — a benefit in cold climates but a complication in mixed/warm climates.
• NRCA recommends R-5.0/inch for cold-climate design calculations. We agree — and suggest even lower in zones 7–8.
• For cold-climate exterior ci, XPS or EPS deliver more reliable R-value per dollar.
• Cost: $0.30–$0.55/board foot, $0.70–$1.50/sq ft — comparable to XPS per board foot but effectively more expensive per R-value in cold climates.

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FAQ

Does polyiso lose R-value in cold weather?

Yes — significantly. Building Science Corporation testing shows polyiso drops from R-6.0–6.5/inch at 75°F to R-3.5–4.5 at 25°F and R-3.0–4.0 at 0°F. The cause is condensation of the pentane blowing agent within foam cells at lower temperatures. This applies to all post-2000 polyiso products. The derating is most significant on the exterior of buildings in cold climates (zones 5–8).

Can I use polyiso below grade?

No. Below-grade applications combine the two conditions polyiso handles worst: moisture exposure (from soil and concrete) and cold temperatures (soil temperature in northern climates is 45–55°F year-round, with foundation surfaces potentially much colder in winter). Use XPS (best moisture resistance) or EPS (best value, works with drainage) for basement and crawl space walls.

What R-value should I use for polyiso in design?

In warm climates (zones 1–3): R-5.6/inch (NRCA recommendation). In moderate climates (zones 4–5): R-5.0/inch (conservative). In cold climates (zones 6–8): we recommend avoiding polyiso on the cold-side exterior entirely — use XPS or EPS instead. For interior applications where the foam stays above 50°F, the full label value of R-5.6–6.5/inch is reasonable. The R-value calculator helps with assembly-level calculations.

Does polyiso need a thermal barrier?

Yes. Like all rigid foams, polyiso is combustible and requires a 15-minute thermal barrier (typically ½" drywall) when installed in occupied spaces per IBC/IRC. The foil facers provide some fire resistance (FSI ≤25, SDI ≤50 with facers), but they don't eliminate the thermal barrier requirement. In attics and crawl spaces, an ignition barrier (thinner than a thermal barrier) may be acceptable under specific code provisions. Check fire safety requirements and your local code.

Polyiso vs XPS — which should I use?

Polyiso for warm-side applications (roofing, interior ci, wall sheathing in zones 1–4). XPS for cold-side applications (exterior walls in zones 5+, below-grade, under-slab). The key differentiator is temperature performance: polyiso beats XPS at warm temperatures, XPS beats polyiso at cold temperatures. XPS also has better moisture resistance (<0.3% vs polyiso's edge-absorption risk). The rigid foam guide has the full comparison.