XPS is the rigid board that goes where the insulation is going to get wet, walked on or buried. It is extruded polystyrene, and it is a different material from the white expanded polystyrene most people picture, made by a different process and specified for different positions in a building. This page sets out what it is, where it belongs, and what it delivers at each thickness.
What is XPS foam?
Extruded polystyrene is made by melting polystyrene with a blowing agent and forcing it through a die as a continuous board. The result is a closed-cell foam with a uniform structure and a smooth skin on both faces.
Expanded polystyrene, EPS, is made a different way. Steam expands loose beads and fuses them together in a mould, which is why you can see the bead boundaries on an EPS board and why it breaks along them.
The practical differences that follow from that: XPS absorbs less water, holds its dimensions better in ground contact, and takes more compression at a given thickness. EPS is lighter, cheaper per square metre and available in far greater thicknesses, which is why it does the bulk work under slabs and inside panels. They are specified for different jobs.
Foamex manufactures XPS in Australia as StyroTherm®, made from Styroboard® XPS, and Styroboard® EPS as StyroSheet, StyroPod® and StyroBloc.
Where XPS is specified
XPS earns its place wherever moisture, load or ground contact would compromise a lighter board.
- Slab edge insulation. A vertical board at the slab perimeter, in contact with soil and backfill. This is the single most common XPS application in Australian housing and it is what NCC 2022 energy provisions have pushed hardest.
- Under slab, where loads are high. Car parks, industrial floors and cold store bases, where the board carries sustained load and sits against damp ground.
- Inverted and protected membrane roofs. The insulation sits above the waterproofing, so it is permanently exposed to water. Closed cell structure is not optional here.
- Cavity walls and internal linings. Where a thin board has to deliver a set R-value in a fixed cavity depth.
- Cool rooms and refrigerated spaces. Continuous insulation with low water absorption, in a position that will see condensation.
- Decks, balconies and planter boxes. Wet, loaded and hard to access once built.
StyroTherm® specification
StyroTherm® is supplied in five thicknesses. Thermal resistance, thermal conductivity and compressive stress by thickness:
|
Thickness |
R-value | K-value (λ) | Compressive stress at 2% |
Compressive stress at 10% |
| 30 mm | 0.94 | $0.03 | >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 |
How to read this table:
Both compressive figures are published for comparison, not as working loads. The 10 per cent figure is the value data sheets conventionally quote. The working load for a given position should be confirmed with the engineer, because sustained load behaves differently from a short term test.
R-value is a property of the board as installed. A board bridged by fixings, compressed at the joints or laid with gaps will not deliver its rated figure. Where a floor or wall has to reach a specific rating, the assembly is what gets assessed, not the product.
Moisture, and why it decides the material
The closed-cell structure is the whole argument for XPS. Water has no continuous path through the board, so it absorbs very little and keeps its thermal performance in positions where an open structure would not.
That matters in three places in particular: below ground, where the board is against soil that holds water; above a waterproof membrane, where it is deliberately outside the dry envelope; and anywhere condensation forms on a cold surface. In all three, insulation that takes up water stops insulating.
It is worth being precise about the claim. XPS resists water absorption. It is not a waterproofing layer and it does not replace a membrane or a damp proof course.
Specifying XPS in Australia
The material standard for extruded polystyrene is AS 1366.4. Thermal resistance is declared under AS/NZS 4859.1, which is the standard an energy assessor and a certifier will look for.
Three things to confirm before an order goes out. The position, because it sets everything else. The thickness, against the R-value the assessment requires rather than the space available. And the load, resolved with the engineer against the figures in the table above.
StyroTherm® is manufactured in Australia using up to 90 per cent recycled polystyrene recovered from EPS packaging, in a process free from ozone-depleting gases.
Talk to Foamex about the position you are insulating. We manufacture XPS and EPS in Australia and can work through the specification with your engineer, assessor or builder.
Frequently asked questions
What is XPS foam?
Extruded polystyrene: a rigid closed cell insulation board made by extruding polystyrene through a die as a continuous sheet, manufactured to AS 1366.4.
What is the difference between XPS and EPS?
XPS is extruded as a continuous board and EPS is moulded from expanded beads. XPS absorbs less water and takes more compression at a given thickness. EPS is lighter, available in far greater thicknesses and does the bulk insulation work.
What R-value does XPS give?
StyroTherm® runs from R0.94 at 30 mm to R2.15 at 75 mm, with R1.55 at 50 mm. The installed assembly is what gets assessed, so fixings, joints and gaps all affect the figure achieved.
Can XPS be used under a concrete slab?
Yes, and it is specified there where loads are high or the ground is wet. Confirm the thickness against the load with the engineer, and resolve the vapour barrier and pour sequence at the same time.
Is XPS waterproof?
XPS is special in that it resists water absorption because of its closed-cell structure, which is why it is used below ground and above membranes. It is not a waterproofing layer and does not replace one.
Sources
- Material standard: AS 1366.4, Rigid cellular plastics sheets for thermal insulation, rigid cellular polystyrene, extruded (RC/PS-E).
- Declared thermal resistance: AS/NZS 4859.1, Thermal insulation materials for buildings.
- Specification figures: Foamex StyroTherm® product data.
- Energy provisions: National Construction Code 2022.