What is XPS (extruded polystyrene)?

XPS stands for extruded polystyrene: rigid foam made by forcing molten polystyrene with a blowing agent through a die. The result is a board with a closed-cell structure and a smooth skin, often coloured depending on the manufacturer.

Properties

• Thermal conductivity: typically around 0.027 to 0.036 W/(m·K), depending on product and thickness. Use the declared value for calculations.
• Compressive strength: depending on grade, roughly 200 to 700 kPa at 10% deformation.
• Low water absorption thanks to the closed cells.
• Vulnerable to UV light, high temperatures and solvents in some adhesives. It is combustible and must be covered.

Uses

1. Inverted (upside-down) roofs: insulation above the waterproofing, ballasted with gravel or pavers.
2. Perimeter insulation on basement walls and foundations in contact with soil.
3. Under floor slabs and screeds.
4. Thermal bridge breaks, for example as a first course under masonry.

XPS or EPS?

EPS is made of fused beads, is cheaper and absorbs more moisture. XPS is stronger and more moisture-resistant, but more expensive. Use XPS where it will be wet or heavily loaded.

Handling tips

• Lay rebated boards staggered and tightly butted.
• Use only adhesives and sealants approved for polystyrene.
• Do not leave boards exposed to sunlight.

Related terms

• Thermal bridge
• Insulation

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