XPS specification across Australian construction is driven by the position the board will occupy and the conditions it will see in service. Moisture exposure, sustained compressive load, available installation depth and the target R-value each push the selection. The following article sets out the variables that typically resolve an XPS specification in Australia, with reference to the Foamex StyroTherm published performance data and the relevant Australian Standards.
Foamex manufactures StyroTherm extruded polystyrene (XPS) sheets in Australia in five thicknesses, with thermal performance tested in accordance with AS/NZS 4859.1 and ASTM C518 and compressive stress tested in accordance with AS 2498.3 and ASTM D1621. The product is supported by an Australian-Made certification and by published warranty, material safety data and material handling documentation.
What XPS sheets are
Extruded polystyrene (XPS) is a rigid cellular plastic insulation board produced by extruding polystyrene polymer with a blowing agent through a die into a continuous closed-cell foam. The cell structure is dense and uniform, with no bead boundaries, which distinguishes XPS from expanded polystyrene (EPS) in three measurable ways. Water absorption is low under sustained exposure (tested in accordance with AS/NZS 4859.1 Appendix), compressive stress at 10% deformation is high relative to most EPS density grades, and rated R-value is more likely to be retained where moisture is part of the operating environment.
For these reasons, XPS is the typical specification at slab-edge, under-slab, perimeter, basement and selected rooftop positions across Australian construction. Foamex manufactures a product range that maps to position: StyroTherm for foundation and perimeter work, StyroRoof for green roof installations, and StyroLink as a purlin spacer for metal roofing assemblies.
Positions where XPS is typically specified in Australian buildings
XPS is typically specified at positions where moisture exposure or sustained compressive load makes EPS or open-cell insulation unsuitable. Common positions include:
- Under-slab insulation beneath ground slabs
- Slab-edge and perimeter insulation around the building footprint, addressing thermal bridging at the slab edge
- Below-ground insulation against retaining walls and basement walls
- Green roof build-ups using a product engineered for the rooftop conditions
- Cool room, cold store and refrigerated floor systems
- Commercial metal roofing, where an XPS purlin spacer provides a thermal break between the purlin and the roof sheet
Foamex matches product to position. StyroTherm is specified for under-slab, slab-edge, perimeter and wall positions. StyroRoof is specified for green roof installations and uses the same closed-cell XPS chemistry, engineered for the loading, drainage and root-resistance considerations of a rooftop build-up. StyroLink is specified for metal roofing as an XPS purlin spacer.
Specification variables
Four variables typically resolve an XPS specification. Each should be confirmed against the project documentation before order placement.
Target R-value and NCC pathway
R-value targets are typically set by the project designer, accredited NatHERS assessor or building certifier against NCC 2022 Section J for Class 2 to 9 buildings, or against NCC 2022 Volume Two and the NatHERS pathway for Class 1 and 10a buildings. State-based requirements such as BASIX in New South Wales may also apply to residential work. The selected board thickness must be sufficient to deliver the required R-value within the available cavity or build-up depth, and should account for any thermal bridging at slab edges, junctions or purlins.
Compressive stress at 2% and 10% deformation
XPS at under-slab, slab-edge, rooftop or roof-spacer positions sits under sustained load through the design life of the assembly. Compressive stress should be reviewed at both 2% and 10% deformation in accordance with AS 2498.3 and ASTM D1621, and matched against the design load with appropriate allowance for compressive creep. Published nominal compressive stress is not a substitute for a load-specific check at the design stage.
Moisture exposure and water absorption
At positions exposed to ground moisture, hydrostatic pressure or external weather, XPS is preferred over EPS or open-cell insulation because the continuous closed-cell foam restricts water movement through the board. Water absorption performance for the product should be confirmed against the relevant test method in AS/NZS 4859.1 Appendix and assessed against the project’s hygrothermal design.
Available installation depth
The available cavity, slab-edge, perimeter batten, roof-spacer or below-ground depth imposes a practical thickness limit. The combination of thickness selected and the as-installed condition of the board (including gaps, compression and continuity) determines the installed R-value.
Caption: NewCold Storage Project in Truganina, using StyroTherm XPS Sheets.
StyroTherm XPS published technical data
The following thermal and compressive data is published by Foamex for StyroTherm XPS at a sheet size of 2400 mm x 600 mm.
| Thickness | R-value (m2.K/W) | K-value (lambda, W/m.K) | Compressive stress at 2% deformation | Compressive stress at 10% deformation |
| 30 mm | 0.94 | 0.031 | >90 kPa | >250 kPa |
| 40 mm | 1.29 | 0.031 | >150 kPa | >350 kPa |
| 50 mm | 1.55 | 0.033 | >230 kPa | >350 kPa |
| 60 mm | 1.82 | 0.033 | >240 kPa | >350 kPa |
| 75 mm | 2.15 | 0.034 | >280 kPa | >350 kPa |
Test methods: thermal performance is reported in accordance with AS/NZS 4859.1 and ASTM C518; compressive stress is reported in accordance with AS 2498.3 and ASTM D1621. Current values should be confirmed against the latest StyroTherm technical specification sheet before final design.
XPS and EPS at a glance
| Consideration | XPS (StyroTherm) | EPS (StyroSheet) |
| Cell structure | Continuous closed-cell extruded foam | Fused expanded beads |
| Water absorption | Low under sustained exposure | Low to moderate, density-grade dependent |
| Compressive stress | >280 kPa at 2% deformation, >350 kPa at 10% deformation at 75 mm | Up to 350 kPa at 10% deformation in VVH grades per AS/NZS 1366.3 |
| Typical positions | Slab-edge, under-slab, perimeter, green roof, metal roof purlin spacer | Walls, floors, ceilings, garage door panels, packaging, fabrication |
| Custom-cut formats | Available within standard sheet sizes | Wide range of custom-cut and foil-faced options |
Internal, dry and protected positions are typically specified in StyroSheet EPS. Positions with moisture exposure, ground contact or external weather are typically specified in StyroTherm XPS, with StyroRoof or StyroLink selected where the position calls for the green roof or purlin-spacer variant.
Australian manufacturing, recycled content and the StyroCycle recycling pathway
StyroTherm is manufactured in Australia using a process free from chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and other ozone-depleting substances. Under Foamex’s closed-loop manufacturing model, StyroTherm can contain up to 90% recycled polystyrene from EPS packaging recovered through construction offcuts, factory surplus and post-consumer streams collected through the StyroCycle take-back network. The product carries Australian-Made certification.
Where a project is targeting waste-diversion or circular-economy outcomes, the take-back recycling pathway and the recycled content can be referenced in the specification alongside the thermal and compressive data.
Contact the Foamex specification team to confirm StyroTherm XPS thickness, R-value and compressive stress for the project, or to request the current technical specification and material safety data.
Frequently asked questions
What are XPS sheets specified for in Australia?
Common positions include under-slab insulation, slab-edge and perimeter insulation, below-ground installations, green roof build-ups, cool room construction and metal roofing as an XPS purlin spacer. Selection by Foamex product depends on position: StyroTherm for foundation and perimeter, StyroRoof for green roofs, StyroLink for metal roof purlins.
What thickness of XPS should be specified?
Thickness depends on the required R-value, the available installation depth and the sustained compressive load. StyroTherm is manufactured in 30 mm, 40 mm, 50 mm, 60 mm and 75 mm thicknesses, with published R-values from 0.94 to 2.15 (tested to AS/NZS 4859.1 / ASTM C518). Final R-value selection should be confirmed against the relevant NCC pathway with the project designer or accredited NatHERS assessor.
How does XPS compare to EPS for thermal applications?
XPS uses a continuous closed-cell extruded foam structure with low water absorption and high compressive stress relative to most EPS density grades. EPS uses fused beads and is classified by density grade under AS/NZS 1366.3, with grades from SL through to VVH 38. EPS is typically specified for internal, dry, protected positions; XPS is typically specified for positions with moisture exposure or sustained compressive load.
Can XPS be specified below ground or in wet conditions?
Yes. The closed-cell extruded foam structure restricts water movement through the board, which supports use at under-slab, slab-edge, retaining wall and green roof positions. Water absorption performance should be confirmed against AS/NZS 4859.1 Appendix and against the project’s hygrothermal design.
Is Foamex XPS manufactured in Australia?
Yes. StyroTherm XPS is manufactured in Australia from Foamex Styroboard XPS, free from CFCs, HCFCs and other ozone-depleting substances, and carries Australian-Made certification.
Where can XPS sheets be sourced in Australia?
Foamex supplies StyroTherm XPS nationally through the Foamex specification team. Technical data, warranty, material safety data and Australian-Made certification documentation are published and can be requested at order stage.