EPS vs. XPS vs. Polyiso: Comparing Foam Board Insulation

Expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso) are the three main types of rigid foam board insulation used in homes. All three can reduce heat transfer, but they differ in R-value, vapor permeability, moisture behavior, strength, cost, and where they tend to work best.

Those differences matter because foam board is often installed in places where the insulation becomes part of a larger wall, foundation, roof, or crawl-space assembly. Choosing a board based only on its R-value can overlook how it needs to handle water, drying, temperature changes, fastening, and interior fire protection.

This guide compares EPS, XPS, and polyiso so you can understand the tradeoffs and choose a material that fits the part of the home you are insulating.

EPS vs. XPS vs. Polyiso at a Glance

EPS, XPS, and polyiso are all rigid foam products, but they are not interchangeable.

EPS and XPS are both polystyrene products manufactured differently. Polyiso belongs to a different foam family and is commonly sold with foil, fiberglass, or other facings.

EPS vs. XPS vs. Polyiso Comparison

Factor	EPS	XPS	Polyiso
Full name	Expanded polystyrene	Extruded polystyrene	Polyisocyanurate
Typical R-value per inch	About R-3.8 to R-4.4	About R-5	About R-6
Relative vapor permeability	Usually higher	Usually lower than EPS	Often very low when foil-faced
Water absorption	Product-dependent; generally higher than XPS	Generally low	Product and facing dependent
Compressive strength	Varies considerably by density	Commonly moderate to high	Varies by product
Common strengths	Cost, stable thermal performance, multiple densities	Moisture resistance, strength, higher R-value than EPS	High R-value per inch
Common considerations	More thickness may be needed	Usually more expensive than EPS	Facing, temperature performance, and application limitations matter
Common residential uses	Exterior walls, foundations, SIPs, ICFs, some below-grade assemblies	Foundations, basements, crawl spaces, exterior walls	Exterior walls, roofs, selected basement and interior details

The practical difference is that EPS often emphasizes affordability and drying potential, XPS combines higher R-value with relatively strong moisture resistance, and polyiso provides more thermal resistance where available thickness is limited.

None of those traits makes one material universally preferable.

What Is EPS Foam Board?

Expanded polystyrene is made by expanding small polystyrene beads and molding them together into rigid panels.

The bead structure is easy to recognize if you look at a cut edge. EPS is available in several densities, which means its thermal resistance, strength, water absorption, and other properties can vary considerably from one product to another.

Typical EPS products provide approximately R-3.8 to R-4.4 per inch. Higher-density EPS generally provides somewhat greater thermal resistance and compressive strength.

Where EPS Can Make Sense

EPS is commonly considered for:

• Continuous exterior wall insulation
• Foundation assemblies when the specific product is approved for the application
• Insulated concrete forms
• Structural insulated panels
• Roof and wall assemblies
• Projects where additional board thickness is available
• Applications where greater vapor permeability is useful

Main Advantages of EPS

EPS tends to offer a useful balance of cost and thermal performance.

It also remains available in several density and strength classes, allowing designers and homeowners to select a product suited to a particular load or application rather than treating all EPS as identical.

Another important characteristic is vapor permeability. Unfaced EPS is generally more vapor-open than XPS or foil-faced polyiso, which may be useful in assemblies that need greater drying potential.

Main Limitations of EPS

Because EPS typically provides less R-value per inch than XPS or polyiso, a thicker layer may be needed to reach the same nominal R-value.

Water absorption and compressive strength also vary substantially by density and product class. That makes the product specification especially important for foundation, slab, or other moisture-exposed applications.

What Is XPS Foam Board?

Extruded polystyrene is manufactured by melting and extruding polystyrene into a closed-cell foam board.

Compared with common EPS products, XPS generally provides a more uniform structure, lower water absorption, and somewhat higher R-value per inch.

XPS is commonly rated around R-5 per inch.

Where XPS Can Make Sense

Common applications include:

• Basement walls
• Foundation walls
• Crawl-space walls
• Exterior continuous insulation
• Under-slab and perimeter applications when approved by the manufacturer
• Areas where compressive strength or moisture resistance is particularly important

Main Advantages of XPS

The combination of approximately R-5 per inch, relatively low water absorption, and useful compressive strength makes XPS a common choice in foundation-related assemblies.

Building America guidance frequently identifies XPS for basement, crawl-space, and foundation applications because of its insulation value and moisture characteristics.

Main Limitations of XPS

XPS generally costs more than EPS, and selecting it only because its R-value is higher may not always produce the most economical assembly.

It is also less vapor permeable than most unfaced EPS. In some wall assemblies that lower permeance is useful; in others, the designer may want greater drying potential.

Blowing-agent technology has also changed over time. Homeowners comparing environmental characteristics should evaluate the current product they are buying rather than assuming every XPS board has the same formulation.

What Is Polyiso Foam Board?

Polyisocyanurate, usually shortened to polyiso, is a rigid closed-cell foam insulation commonly manufactured with a facing bonded to each side of the board.

Many residential polyiso products provide approximately R-6 per inch, giving polyiso the highest typical R-value per inch among EPS, XPS, and polyiso.

Where Polyiso Can Make Sense

Polyiso is commonly used for:

• Exterior continuous wall insulation
• Roof assemblies
• Wall sheathing
• Interior masonry or foundation applications when the product and assembly allow it
• Locations where limited thickness makes R-value per inch especially important

Main Advantages of Polyiso

The major advantage is thermal resistance within a relatively thin layer.

For example, when an exterior wall has limited room behind siding or trim, a board that provides roughly R-6 per inch can sometimes reach the desired insulation level with less thickness than EPS.

Faced polyiso products may also provide very low vapor permeance, which can be helpful when the wall assembly is intentionally designed around that property.

Main Limitations of Polyiso

Polyiso should not automatically be specified wherever the highest labeled R-value is desired.

Its performance depends on product construction, facing, thickness, temperature, and application. Older Building America guidance also notes that polyiso products are rated using long-term thermal resistance because insulating gases within the foam change over time.

Some products are not intended for soil contact or particular below-grade conditions, so manufacturer approval matters.

Building America guidance emphasizes that the properties of rigid foam products vary and that XPS and polyiso generally have higher R-values and lower permeance than EPS. The practical implication is that homeowners should choose the board as part of the entire wall or foundation assembly rather than comparing only one number on the label.

How Do Their R-Values Compare?

For a basic comparison, typical rigid foam R-values are:

• EPS: approximately R-3.8 to R-4.4 per inch
• XPS: approximately R-5 per inch
• Polyiso: approximately R-6 per inch

These are planning numbers, not a substitute for the R-value printed on the board you buy. XPS is usually labeled about R-5 per inch at a mild test temperature, and that number can drop a little over time as the gas in the cells escapes. EPS generally keeps the R-value on its label. Polyiso’s approximate R-6 is a long-term rating in mild conditions and can be lower when the board is cold.

Approximate R-Value by Thickness

Thickness	EPS	XPS	Polyiso
1 inch	About R-3.8 to R-4.4	About R-5	About R-6
1.5 inches	About R-5.7 to R-6.6	About R-7.5	About R-9
2 inches	About R-7.6 to R-8.8	About R-10	About R-12
3 inches	About R-11.4 to R-13.2	About R-15	About R-18

This comparison explains why polyiso is attractive when space is limited. But more R-value per inch does not automatically mean better performance in every location.

The assembly still has to control water, air leakage, vapor movement, and fire exposure.

Moisture and Vapor Differences

Foam board is often chosen for foundations and exterior walls partly because it handles moisture differently than fiber insulation.

However, “moisture resistant” and “waterproof” are not the same thing.

EPS

Unfaced EPS is generally more vapor permeable than XPS and foil-faced polyiso. Building America describes typical EPS as a Class III vapor retarder, although exact permeance depends on density and thickness.

That greater permeability can allow more drying through the insulation.

XPS

Typical XPS has lower vapor permeance than EPS and is commonly around the Class II range at typical thicknesses.

Its relatively low water absorption is one reason it has historically been common in basement and foundation applications.

Polyiso

Polyiso vapor behavior depends heavily on the facing.

Foil-faced products can be highly vapor resistant and are typically treated very differently from an unfaced or more permeable board. Building America notes that polyiso’s vapor-retarder classification depends on the facing and is often Class I.

That can be an advantage or a drawback depending on which direction the assembly needs to dry.

Section Summary

When moisture is part of the decision:

• EPS generally allows more vapor diffusion.
• XPS generally provides lower water absorption and lower vapor permeance.
• Faced polyiso can be very vapor resistant.
• None of these products should be expected to solve bulk-water leakage.
• Drainage, flashing, capillary control, and exterior water management still need to be correct.

Strength and Compressive Resistance

Rigid foam sometimes has to support loads from slabs, foundations, roofing components, siding attachments, or other construction materials.

That means compressive strength may matter as much as R-value.

EPS is available in multiple densities and strength classes. Higher-density products can offer significantly greater compressive strength than lightweight EPS.

XPS is commonly available in several compressive-strength ratings and has traditionally been used where a relatively strong, moisture-resistant board is needed.

Polyiso strength also varies by product and intended application.

The important homeowner takeaway is not that one foam family is always strongest. It is that the specific board must be rated for the loads and installation location.

Do not substitute a wall sheathing product under a slab, foundation, or other loaded assembly unless the manufacturer lists it for that use.

Which Foam Board Works for Exterior Walls?

All three foam families can be used as exterior continuous insulation when the product and wall assembly are designed for it.

Continuous rigid insulation installed over wall framing reduces thermal bridging because the insulation crosses over the studs rather than leaving the framing exposed as a direct heat-flow path.

EPS on Exterior Walls

EPS can be attractive where:

• Cost matters
• Additional thickness is acceptable
• Greater vapor permeability is beneficial
• The selected product is approved for exterior sheathing

XPS on Exterior Walls

XPS may be considered where:

• A higher R-value than typical EPS is useful
• Lower water absorption is valuable
• A firmer board simplifies handling or detailing

Polyiso on Exterior Walls

Polyiso often becomes attractive when:

• Space behind the cladding is limited
• Higher R-value per inch is useful
• The wall is intentionally designed around the board’s facer and vapor characteristics

The right exterior board also depends on how the wall handles drainage, flashing, windows, doors, fasteners, and siding.

Some rigid foam products can become part of the drainage plane or air-control layer when installed exactly as their manufacturer specifies. That should never be assumed for every board.

Learn More: See the main Foam Board Insulation guide for a broader explanation of rigid foam applications, installation, R-value, and homeowner considerations.

Which Foam Board Works for Basements and Foundations?

Foundations are one of the areas where product selection becomes especially dependent on moisture exposure.

Building America guidance commonly uses rigid foam directly against concrete or masonry so the insulation can limit heat transfer without putting moisture-sensitive fiber insulation against the foundation.

EPS Below Grade

Some EPS products are specifically rated for foundations and below-grade use.

Because EPS comes in several densities, homeowners should verify:

• Water absorption
• Compressive strength
• Required thickness
• Soil-contact approval
• Protective covering requirements

Do not assume that every white EPS board sold at a home center is approved for below-grade insulation.

XPS Below Grade

XPS has long been commonly used around foundations because of its low water absorption and useful compressive strength.

For homeowners comparing common rigid boards for damp foundation conditions, those characteristics are often more important than the difference of one R-value point per inch.

Polyiso Below Grade

Polyiso requires more caution.

Some products or systems may be approved for foundation applications, while others are not intended for soil contact. Current DOE material guidance notes that conventional polyiso should not simply be assumed suitable for direct soil contact.

Follow the instructions for the exact board. Not every polyiso product is approved for the same place. In areas with a very heavy termite risk, foam on the outside of foundation walls or under a slab is restricted unless an approved termite-protection method is used.

Crawl Spaces and Rim Joists

Rigid foam can also be useful in sealed crawl spaces and at rim joists.

In a crawl space, foam board is commonly installed on foundation walls rather than between floor joists when the crawl space is being brought inside the home’s conditioned or semi-conditioned enclosure.

Building America guidance identifies rigid foam as a common option for unvented crawl-space walls, with product thickness depending on climate and required performance.

At rim joists, pieces of rigid foam can be cut to fit between the framing. Seal the edges, or air will leak around the board. Sealing the rim joist, or closing in a crawl space, can change how the house pulls air. Have combustion appliances checked for safe venting before and after the work.

See Rim Joist Insulation for a closer look at rigid foam, spray foam, air sealing, and moisture considerations in this area of the home.

For crawl-space assemblies, see Crawl Space Insulation.

How Cost Fits Into the Comparison

Foam board pricing changes by region, board thickness, density, facing, compressive strength, manufacturer, and retailer.

Because of those variations, comparing only the price of one sheet can be misleading.

A better comparison is the cost of achieving the required R-value and assembly performance.

For example:

• EPS may have a lower sheet price but require greater thickness.
• XPS may cost more per inch while providing approximately R-5 per inch.
• Polyiso may provide greater R-value per inch, potentially reducing required thickness.
• Specialty high-density or foundation-rated products can cost more than ordinary wall boards regardless of foam type.

Installation details can also outweigh relatively small differences in board price.

Exterior wall projects may require longer fasteners, furring, window extensions, flashing changes, or siding details as insulation thickness increases.

Foundation projects may require drainage, waterproofing, protective panels, or termite-inspection details.

For that reason, compare the whole assembly, not simply dollars per sheet.

Which Should You Choose: EPS, XPS or Polyiso?

There is no universal winner.

The choice depends on what you need the board to do.

Consider EPS When

EPS may be a good fit when:

• Cost is an important consideration
• You have room for slightly greater thickness
• A more vapor-permeable rigid foam is useful
• The required compressive strength is available in an appropriate EPS density
• The specific board is rated for the intended wall, roof, or foundation application

Consider XPS When

XPS may fit when:

• The insulation will be exposed to damp foundation conditions
• Lower water absorption matters
• You want approximately R-5 per inch
• Moderate or higher compressive strength is required
• The assembly benefits from lower vapor permeance than typical EPS

Consider Polyiso When

Polyiso may fit when:

• Space is limited
• High R-value per inch is important
• You are insulating an exterior wall or roof assembly
• The facer and vapor properties work with the wall design
• The specific product is approved for the intended location

If the location is wet, load-bearing, vapor-sensitive, exposed to temperature extremes, or tightly constrained for thickness, those conditions can narrow the material choices before price or nominal R-value enters the decision.

Installation Details Matter as Much as Foam Type

Rigid foam performs best when it is installed as part of a continuous control layer.

Gaps between sheets, unsealed edges, poorly detailed penetrations, and discontinuities around framing can allow air movement around the insulation even when the foam itself has a high R-value.

Building America guidance for exterior rigid insulation emphasizes sealed seams and compatible tapes where the board is intended to form part of an air- or water-control layer.

Important details include:

1. Fix bulk-water leaks before insulating.
2. Select a board approved for the application.
3. Size thickness for the required R-value and climate.
4. Fit sheets tightly.
5. Seal seams and transitions when the design uses the foam as an air-control layer.
6. Use compatible tapes, sealants, adhesives, and fasteners.
7. Detail windows, doors, penetrations, and flashing correctly.
8. Provide required interior fire protection.

Fire Protection and Interior Foam Board

Foam plastic insulation generally cannot simply remain exposed inside occupied portions of a home.

Foam board inside living space usually has to be covered with a fire barrier, most often 1/2-inch drywall, unless that exact product is approved to be left exposed. In an attic or crawl space that people enter only to service equipment, a lighter ignition barrier may be allowed instead. A foil face or a coat of paint does not count as that cover unless the product is specifically approved for it.

Building America guidance for basement assemblies specifically notes the need for appropriate thermal protection over exposed foam plastics.

Fire-protection requirements depend on the product, location, building code, and assembly.

Follow the board manufacturer’s listing and local code requirements rather than assuming a foil facing or painted surface is sufficient.