Expanded polystyrene (EPS) and extruded polystyrene (XPS) are rigid, closed-cell insulation materials made by Foamex in Australia. EPS is moulded from expanded beads; XPS is extruded as a continuous foam board. Both require a product specification that accounts for thermal resistance, loading and the conditions where the insulation will be installed.
This comparison uses Foamex’s published StyroTherm XPS and conventional Styroboard EPS data. It does not cover graphite-enhanced EPS or establish a universal ranking for every EPS and XPS product.
The short answer
- Consider XPS insulation for moisture-exposed or below-ground positions, and where the required R-value must fit within a limited depth. It is also used in protected-membrane roof systems, with the product and roof build-up specified for that application.
- Consider EPS where a choice of grades, custom-cut dimensions or large blocks suits the project. It is used for insulation, void forming and lightweight fill, with compression performance selected for the intended loading.
- Where either material is suitable, compare the actual grade, thickness, installation requirements and quoted cost. Neither material should be selected on its name alone.
StyroTherm XPS, published performance
| Thickness (mm) | R-value (m²K/W) | K-value λ (W/mK) | Compressive stress at 2% (kPa) | Compressive stress at 10% (kPa) |
|---|---|---|---|---|
| 30 | 0.94 | 0.031 | >90 | >250 |
| 40 | 1.29 | 0.031 | >150 | >350 |
| 50 | 1.55 | 0.033 | >230 | >350 |
| 60 | 1.82 | 0.033 | >240 | >350 |
| 75 | 2.15 | 0.034 | >280 | >350 |
Styroboard EPS, published performance
| Grade | Nominal density (kg/m³) | Compressive stress at 10% deformation, min. (kPa) | Thermal resistance at 50 mm, min. (m²K/W) |
|---|---|---|---|
| L | 11 | 50 | R1.0 |
| SL | 13.5 | 70 | R1.20 |
| S | 16 | 85 | R1.22 |
| M | 19 | 105 | R1.25 |
The EPS figures are from Foamex’s Styroboard EPS technical data sheet. Thermal resistance is reported for a 50 mm sample at a mean temperature of 23°C; compression values are minimum stresses at 10% deformation. The six standard classes run from L to VH. VVH 32, VVH 36 and VVH 38 are additional Foamex grades outside those classes.
Comparing the published figures at 50 mm
A shared thickness makes the thermal comparison easier to read. Compression results must also be compared at the same percentage deformation.
StyroTherm XPS has a published R-value of R1.55 at 50 mm. The EPS grades shown range from R1.0 to R1.50 at 50 mm. XPS therefore has the higher published R-value within this comparison, although the difference is small against the highest EPS grade. Check the exact product where installation depth is limited.
These figures measure the pressure needed to squeeze the insulation by 10% of its original thickness. For 50 mm StyroTherm XPS, the published rating is above 350 kPa. Styroboard EPS has minimum ratings from 50 kPa for L grade to 350 kPa for VVH 38. Because XPS is rated above 350 kPa, the figures do not confirm that the highest EPS grade and XPS are equally strong.
The figures help compare products, but they do not tell you what load the insulation can safely carry in a building. An engineer must check the expected loads and how much the insulation may compress over time before specifying the product.
Choosing by application
| Where it goes | What the position demands | Usual choice |
|---|---|---|
| Under slab | Thermal resistance, design loading and moisture exposure | Eitherwith the product and thickness checked against the slab design |
| Slab edge | Insulation continuity, moisture exposure and protection | EPS or XPSwhere supported by the specified slab-edge detail |
| Cavity and external walls | Thermal performance, fixing, moisture control and fire requirements | Eitherwithin a suitable wall system; do not select on cavity depth alone |
| Roof insulation above the membrane | Exposure to water and the roof’s loading requirements | XPSspecified for the roof system, including drainage and waterproofing details |
| Below ground level | Resistance to moisture uptake and any imposed load | XPSwith appropriate drainage and waterproofing |
| Cold rooms and refrigeration | Thermal continuity, moisture control and any floor loading | Eitherwhere the complete enclosure specification supports it |
| Civil fill and void forming | Low weight, required compression performance and geometry | EPSblocks in an engineered fill or void-forming design |
Moisture
Foamex’s XPS technical data sheet lists maximum water absorption below 0.5% by volume under AS 2498.8. The EPS sheet lists maximum total-immersion absorption from 4% to 1% by volume, depending on grade, under ASTM C272. Because the methods differ, these values should not be presented as a direct numerical comparison.
XPS is used in moisture-exposed positions, but it is not a waterproofing layer. For either material, resolve drainage, waterproofing and vapour control as part of the complete design. An added vapour barrier is not a universal solution for every wall, roof or below-ground installation.
Frequently asked questions
Is XPS insulation better than EPS?
It depends on the application and the products being compared. In the published Foamex range shown here, XPS has a higher R-value at 50 mm and is used in moisture-exposed positions. EPS offers a range of grades and custom-cut formats. Check the thermal, load and moisture requirements together.
What R-value does XPS insulation give?
The StyroTherm product range shown here runs from R0.94 at 30 mm to R2.15 at 75 mm, including R1.55 at 50 mm. These are product R-values, not ratings for a complete wall, roof or floor.
Is XPS stronger than EPS?
Compare the specific grade and test result. At 50 mm, StyroTherm XPS is listed at greater than 350 kPa at 10% deformation. Styroboard EPS grades range from 50 kPa to 350 kPa minimum at the same deformation. These values cannot be treated as identical design capacities.
Which one goes under a concrete slab?
Both EPS and XPS are used for under-slab insulation. The product must meet the slab’s thermal, loading and moisture requirements. Refer to our under slab insulation guide and confirm the specification with the structural engineer.
Can polystyrene insulation be used below ground?
XPS is used in below-ground applications because of its resistance to water absorption. Specify it with the necessary drainage and waterproofing details; it does not replace those layers.
Are EPS and XPS made in Australia?
Yes. Foamex manufactures both EPS and XPS products in Australia. For the ranges compared here, see our XPS insulation sheets and polystyrene sheets. For the EPS production process, read how expanded polystyrene is made.