Rigid Foam Insulation R-Value Chart: Thickness Guide by Material Type

When selecting rigid foam insulation for your building project, understanding R-values across different material types and thicknesses is critical for achieving optimal thermal performance. This comprehensive guide breaks down the R-values for the three primary rigid foam insulation types – expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso) – helping you make informed decisions based on your specific application needs.

Understanding Rigid Foam Insulation R-Values

R-value measures thermal resistance – the higher the R-value, the better the insulation performs. Unlike spray foam insulation, which is applied as a liquid that expands, rigid foam boards come in standardized thicknesses with predictable R-values per inch. This makes them easier to spec for construction projects where precise thermal calculations matter.

Each type of rigid foam insulation offers different R-values per inch, cost points, and performance characteristics. The material you choose depends on your budget, climate zone, installation location, and specific building code requirements.

Rigid Foam Insulation R-Value Chart by Material and Thickness

This comprehensive chart shows the R-values you can expect from each rigid foam type at common thicknesses:

Note: Polyiso R-values are rated at 75°F. Cold weather performance differs significantly – see temperature considerations below.

Types of Rigid Foam Insulation: R-Value Per Inch Breakdown

Expanded Polystyrene (EPS) – R-4.0 per inch

EPS is the white beaded foam board you often see in construction. A 1 inch foam insulation board of EPS delivers approximately R-4.0, making it the lowest R-value per inch among the three types. However, EPS offers excellent long-term thermal stability – its R-value doesn’t degrade over time like some other foam products.

EPS maintains consistent performance across temperature ranges and doesn’t suffer the cold-weather R-value loss that affects polyiso. It’s also the most breathable of the rigid foam options, which can be advantageous in certain wall assemblies where some vapor permeability is desired.

Extruded Polystyrene (XPS) – R-5.0 per inch

XPS is the colored foam board (typically pink, blue, or green depending on manufacturer) with a smooth, closed-cell structure. A 1 inch foam insulation board of XPS provides R-5.0, offering middle-ground performance between EPS and polyiso.

XPS excels in below-grade applications due to its superior moisture resistance and compressive strength. The closed-cell structure prevents water absorption better than EPS, making it ideal for foundation insulation, under-slab applications, and any location where the foam may encounter moisture.

One important consideration: traditional XPS uses blowing agents that can degrade over time, potentially reducing R-value by 10-20% over several decades. Newer formulations using alternative blowing agents show better long-term stability.

Polyisocyanurate (Polyiso) – R-6.0 to R-6.5 per inch

Polyiso delivers the highest R-value per inch at warm temperatures, with a 2 inch foam insulation board of polyiso achieving R-12 to R-13. This makes it the most space-efficient option when wall or roof cavity depth is limited.

Polyiso boards typically come with foil facings on both sides, which contribute to the R-value and provide a vapor barrier. The foil facing also makes polyiso easy to identify on job sites. When choosing rigid panel insulation, polyiso often represents the best value per R-value for above-grade applications in moderate to warm climates.

Temperature Performance Considerations: The Polyiso Cold Weather Factor

Here’s the critical detail many builders miss: polyiso’s R-value decreases significantly in cold temperatures. While rated at R-6.0 to R-6.5 per inch at 75°F, polyiso can drop to R-5.0 or even R-4.5 per inch at temperatures below 40°F.

This temperature dependence matters tremendously in cold climates. A 2 inch foam insulation board of polyiso might deliver R-13 in laboratory conditions but only R-9 to R-10 during winter in northern climate zones. This performance gap can lead to undersized insulation systems if you spec based solely on labeled R-values.

For cold-climate applications, consider these strategies:

• Use EPS or XPS in cold climates – their R-values remain stable regardless of temperature
• Increase polyiso thickness – spec thicker polyiso to compensate for cold-weather R-value loss
• Create hybrid assemblies – combine polyiso with other insulation types for balanced performance
• Calculate using LTTR values – use Long-Term Thermal Resistance ratings that account for aging and temperature

Understanding how spray foam and energy efficiency work together can provide additional perspective on insulation performance across different climate conditions.

Thickness Selection Guide: Matching R-Value to Application

Choosing the right foam board insulation r-value depends on your specific application and climate zone requirements. Here’s a practical guide:

Foundation and Below-Grade Applications

Code typically requires R-10 to R-15 for foundation walls in most climate zones. A 2 inch foam insulation board of XPS (R-10) meets minimum requirements in moderate climates, while colder zones may need 3 inches (R-15) or more.

For below-grade work, XPS is generally preferred over EPS due to superior moisture resistance, and over polyiso because buried insulation experiences cold ground temperatures year-round.

Above-Grade Wall Sheathing

Continuous exterior insulation requires R-5 to R-15 depending on climate zone and wall assembly type. Common solutions include:

• Climate Zones 1-3: 1 inch XPS (R-5) or 3/4 inch polyiso (R-4.5)
• Climate Zones 4-5: 1.5 to 2 inches XPS (R-7.5 to R-10) or 1.5 inches polyiso (R-9)
• Climate Zones 6-8: 2 to 4 inches XPS (R-10 to R-20) or 2 to 3 inches EPS (R-8 to R-12)

Wall assemblies should coordinate with house wrap products for complete weather protection and proper moisture management.

Roof and Ceiling Applications

Roof assemblies typically require higher R-values: R-30 to R-60 depending on climate. While rigid foam can contribute to these assemblies, achieving code-required R-values often means combining rigid foam with cavity insulation or using multiple layers of foam boards.

For flat or low-slope roofs with rigid foam above the deck, polyiso is common due to its high R-value per inch and relatively low cost. In cold climates, account for temperature derating or consider tapered EPS systems that maintain consistent performance.

Cost Comparison: Getting the Best Value

Price per board varies by region and market conditions, but typical relative costs are:

• EPS: Lowest cost per board, but also lowest R-value per inch. Best cost per R-value in many markets.
• XPS: Mid-range pricing, offering premium moisture protection. Higher cost per R-value than EPS.
• Polyiso: Competitive pricing with highest R-value per inch at warm temperatures. Often best cost per R-value for warm-climate applications.

When calculating true cost, factor in labor and framing implications. Higher R-value per inch means less thickness needed, which can reduce wall depth, preserve interior space, and simplify detailing around windows and doors.

The selection should align with other energy-efficient building materials to create an integrated, high-performance building envelope.

Best Applications for Each Rigid Foam Type

Choose EPS when:

• Working in cold climates where stable R-values matter
• Budget constraints require the most affordable option
• You need predictable long-term thermal performance
• The wall assembly benefits from some vapor permeability

Choose XPS when:

• Insulating below-grade or in moisture-prone locations
• You need superior compressive strength (under slabs, plaza decks)
• Moderate R-value with excellent moisture resistance is required
• Working in freeze-thaw environments

Choose Polyiso when:

• Maximizing R-value in limited space is critical
• Working in warm to moderate climates (zones 1-4)
• Installing roof insulation above the deck
• You need an integrated vapor barrier (foil facing)
• Cost per R-value is the primary decision factor

Installation Tips for Maximum Performance

Regardless of which material and thickness you select, proper installation is crucial for achieving the rated rigid insulation board r value:

• Seal all joints and seams – use compatible tape or foam sealant to prevent air leakage
• Stagger seams – when using multiple layers, offset joints to eliminate thermal bridging paths
• Detail penetrations carefully – seal around all pipes, vents, and fasteners
• Protect from UV exposure – most rigid foams degrade under prolonged sunlight; cover promptly
• Follow manufacturer fastening requirements – over-fastening can compress foam and reduce R-value

For complex projects or if you need expert guidance on insulation selection and installation, contact our team for personalized recommendations based on your specific building requirements and climate zone.

Frequently Asked Questions

What is the R-value of 2 inch foam insulation board?

A 2 inch foam insulation board delivers R-8.0 for EPS, R-10.0 for XPS, or R-12.0 to R-13.0 for polyiso at standard test temperatures. Polyiso performance decreases in cold weather, so actual installed R-values may be lower in northern climates during winter months.

Does rigid foam insulation lose R-value over time?

EPS maintains its R-value indefinitely. XPS made with traditional blowing agents can lose 10-20% of R-value over decades as gases diffuse out, though newer formulations perform better. Polyiso shows minimal long-term R-value loss but experiences significant temporary reduction in cold temperatures.

What thickness of rigid foam insulation do I need for exterior walls?

Required thickness depends on your climate zone. Climate zones 1-3 typically need 1 to 1.5 inches (R-5 to R-7.5), zones 4-5 need 1.5 to 2 inches (R-7.5 to R-10), and zones 6-8 may require 2 to 4 inches (R-10 to R-20) to meet energy code requirements for continuous insulation.

Can I use polyiso rigid foam in cold climates?

Yes, but you should account for cold-weather R-value reduction. Polyiso loses 20-30% of its rated R-value at temperatures below 40°F. In cold climates, either increase thickness to compensate, use EPS or XPS which maintain stable R-values, or create hybrid assemblies combining multiple insulation types.

Which is better for basement walls – EPS or XPS rigid foam?

XPS is generally preferred for below-grade basement applications due to its superior moisture resistance and higher compressive strength. While EPS costs less and offers stable R-values, its higher water absorption rate makes it less ideal for direct soil contact. Use 2 to 3 inches of XPS (R-10 to R-15) for most basement wall applications.