Where polystyrene sheets are used in Australian building

Q: Can polystyrene sheets be used under a concrete slab?

EPS is used for under-slab insulation, but the specified product, grade and thickness must suit the slab design. The structural engineer should confirm loading requirements. A general-purpose sheet should not be substituted without approval. For void-fill for concrete slab construction, consider EPS waffle pods.

Expanded polystyrene (EPS) sheets are used in Australian buildings for wall & floor insulation, including under-slab and slab-edge applications. They are also used in rendered wall cladding systems and cold rooms. Foamex manufactures polystyrene sheets from Styroboard EPS in thicknesses from 10 mm to 600 mm, with grades from SL to VVH 38 listed for StyroSheet.

The right sheet depends on where it will be installed, the thermal resistance required and any load it must carry. Moisture exposure and the requirements of the complete building system also need to be checked.

The applications

Where it goes	What the application asks of the sheet
Cavity and external walls	Thermal resistance within the specified wall system. Confirm sheet thickness, fixing, moisture-control details and fire requirements; the sheet alone does not establish wall performance.
Roofs and ceilings	Thermal resistance within the available depth. Check the roof build-up, service conditions and any loading before selecting the sheet.
Under slab	Thermal resistance and compression performance under the slab’s design loads. Both grade and thickness matter.
Slab edge	Continuous insulation at the perimeter. Check thickness, moisture exposure and protection within the slab-edge detail.
Rendered wall cladding	EPS used as part of a specified cladding system, with compatible fixings, reinforcement and render. General-purpose sheet is not a complete cladding system.
Cold rooms and cool stores	Thermal resistance and control of moisture movement through the complete enclosure, including joints and vapour-control details.
Void forming and protective packaging	A shape and grade suited to the task. Void formers require checks for construction loads; packaging requires cushioning and protection during handling.

Working back to a specification

Start with the project’s thermal and structural requirements, then check the sheet against both.

For thermal performance, select a grade and thickness that deliver the required R-value. Foamex’s StyroSheet range lists minimum R-values at 50 mm from R1.20 for SL grade to R1.50 for VVH 38. Increasing thickness can provide more insulation, while the grade also affects the result. Use the published value for the proposed product, not a single R-value for all EPS.

For compression performance, check the specified grade. Published minimum compressive stress at 10% deformation ranges from 70 kPa for StyroSheet SL grade to 350 kPa for VVH 38. These test figures describe material behaviour; they are not allowable working loads for a slab or other structure.

The broader Styroboard EPS grade table, including L grade, is set out in how expanded polystyrene is made. It brings together nominal density, compression-test results and thermal resistance.

AS 1366.3-1992 sets minimum properties for six EPS classes. Foamex describes Styroboard EPS as manufactured to meet this standard; its additional VVH grades sit outside those six classes. For load-bearing use, the structural engineer should confirm the grade. Check the insulation requirement against the project energy assessment.

Moisture and thermal performance

EPS has a closed-cell structure and resists moisture uptake, but it can absorb some water. The conditions around the sheet matter, particularly below ground or where condensation may occur.

Foamex’s Styroboard EPS data sheet gives maximum water absorption under total immersion from 4% by volume for L, SL and S grades to 1% for VVH 36 and VVH 38, tested to ASTM C272. These are laboratory test results, not predictions of moisture content in a particular building.

Assess the sheet as part of the moisture-control design. Specify any required drainage, waterproofing or vapour-control layers for that location; moisture-resistant insulation does not replace them. Do not assume that adding a vapour barrier is appropriate in every wall or roof.

Sizes and grades

Attribute	Value
Widths	up to 1.2 m
Lengths	2.5 m, 3 m, 5 m
Thickness	10 mm to 600 mm
Grades	SL, S, M, H, VH, VVH 32, VVH 36, VVH 38

Foamex publishes these sizes for StyroSheet and offers custom cutting. Contact Foamex with your required dimensions and grade to confirm supply and lead time. Custom sizes may have different availability from standard stock.

Fire performance

StyroSheet is made from fire-retardant Styroboard EPS. EPS remains combustible, so a flame-retardant formulation does not make the sheet non-combustible or establish the fire rating of a complete building system. Foamex publishes flame-propagation test data under AS 2122.1. Use the relevant system documentation when specifying walls, roofs or cladding.

Recycling and site offcuts

StyroSheet contains recycled material, and clean EPS offcuts can be reprocessed into new products. Keep them separate from general building waste and secure them against wind. Confirm acceptance with the recycler, especially for recovered sheets with render, paint or other attached materials. Recyclability does not mean every demolition offcut can be accepted.

For more information about our EPS Sheets, please visit our StyroSheet product page.

Frequently asked questions

What thickness of polystyrene sheet do I need?

Check the R-value required by the project energy assessment, then select the grade and thickness together. StyroSheet’s published minimum R-values at 50 mm range from R1.20 to R1.50. Also check available space, loading and the installation detail.

What is the difference between the EPS grades?

EPS grades have different physical properties. In the StyroSheet range, SL grade has a published minimum compressive stress of 70 kPa at 10% deformation; VVH 38 has 350 kPa. Thermal resistance also varies. Grade selection should use the performance data, not nominal density alone.

Can polystyrene sheets be used under a concrete slab?

EPS is used for under-slab insulation, but the specified product, grade and thickness must suit the slab design. The structural engineer should confirm loading requirements. A general-purpose sheet should not be substituted without approval. For void-fill for concrete slab construction, consider EPS waffle pods.

Do polystyrene sheets absorb water?

They can absorb some water. Foamex publishes maximum total-immersion absorption figures of 1% to 4% by volume across the Styroboard EPS grades. Check the chosen grade and the project’s moisture-control details.

What sizes do polystyrene sheets come in?

Foamex lists widths up to 1.2 m, lengths of 2.5 m, 3 m and 5 m, and thicknesses from 10 mm to 600 mm. Custom cutting is available; confirm dimensions and supply with Foamex.

Are polystyrene sheets recyclable?

Yes. Clean EPS can be recycled through specialist collection and processing services. Keep offcuts free of contamination and confirm whether coated or recovered building material will be accepted before arranging recycling.

Sources

Thermal Properties

Suitable for use across steel studs, timber studs, concrete and masonry buildings, Styroboard Expanded Polystyrene Sheets provide an efficient and economical solution for insulation. Often used on the outside of the wall stud, cavities are maintained closer to room temperature which reduces the risk of surface and cavity condensation. The thermal mass advantages of concrete and masonry walls are also improved.

Styroboard EPS and the Effects of Moisture

It is recommended that an effective vapour barrier be provided in situations where significant moisture content build-up is likely to occur. Styroboard EPS is generally resistant to moisture gain; however, moisture content will affect its thermal performance as with all insulating material.

There is a linear relationship between thermal resistance (R-value) loss and moisture content increase by volume. There is an approximate 2.5% loss of R-value per 1% of moisture content by volume increase (up to 20% m.c. by volume). Styroboard that has a 2% moisture content by volume will have 95% of its dry R-value.

Our Environment

The manufacture of Styroboard EPS produces no ozone depleting gases and uses no chlorofluorocarbons (CFCs).

For each kilogram of oil used in the manufacture of Styroboard insulation, savings of up to 200kg of heating fuel can be made over the average lifespan of a house. In turn this plays a positive role in reducing carbon dioxide emissions and the effects of global warming.

Styroboard EPS from Foamex does not degrade into harmful substances or contaminate ground water. There have also been no reports of any harmful effects on health that could be due to a causal relationship with Styroboard.

On top of all that, polystyrene sheets are 100% recyclable. Therefore, any leftover sheets, or those found during a demolition, can be broken down and repurposed for future use.

The benefits and uses of Foamex’s Styroboard EPS sheets are wide and varied, making them one of the most versatile building materials in the market.

For further information or to request a quote, please feel free to contact one of our state offices, or fill out our Online Enquiry Form, and we will contact you back as soon as possible.