XPS: definition and use under a screed

What is XPS?

XPS, or extruded polystyrene, is a rigid insulation material made by pushing molten polystyrene through a die. The result is a homogeneous, closed-cell foam sold in panels that are often blue, pink, or green, depending on the brand.

Manufacturer technical data sheets indicate a lambda value of approximately 0.034 to 0.036 W/m.K depending on thickness, long-term water absorption by immersion of 0.7% at most, and compressive strength of 300 kPa or more for flooring ranges.

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What is the difference between XPS and EPS?

Both are types of polystyrene, but their manufacturing process makes all the difference. EPS is made of expanded beads that are then fused together; XPS is extruded into a continuous, closed-cell mass.

The result: XPS absorbs much less water and supports higher loads, but it is more expensive. With a similar lambda value, the choice is therefore not based on insulation performance, but on moisture and load requirements.

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When should you choose XPS under a screed?

XPS is justified when the floor is subject to heavy loads, such as in a garage or utility room, or when the insulation is at risk of being exposed to moisture, for example on a slab-on-grade without reliable waterproofing. In a standard, dry living area, EPS is usually sufficient.

Like any panel, XPS requires a flat surface and tight joints: on a slab cluttered with conduits, cutting the panels creates gaps. sprayed PUR is then more suitable. The thermal conductivity of each product can be found on its data sheet.

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How Davide Chape chooses insulation under a screed

We start with three questions: what load will the floor bear, what is the risk of moisture from below, and how much height is available. The choice of panels, spray-applied PUR, or insulating mortar follows from these. See our floor insulation solutions.