Polystyrene Insulation Board

What Is Polystyrene Insulation?

Polystyrene starts life as the same basic plastic resin, but the manufacturing route splits it into two distinct foams.

EPS (expanded polystyrene) blows the beads up with steam and pentane so they fuse into a big, lightweight block that is about 98% trapped air. XPS (extruded polystyrene) melts the resin, injects a blowing agent, then pushes the mix through a die; the result is a denser, closed-cell sheet with a smooth skin that shrugs off knocks and water.

EPS is lightweight and cost-effective, but its insulating capabilities are modest compared to other materials. It's often used in situations where moderate insulation is acceptable, such as in packaging or construction applications where cost is a primary concern.

Extruded polystyrene (XPS) insulation outperforms expanded polystyrene (EPS) as a result of its denser, closed-cell structure. Manufactured through an extrusion process, XPS forms a continuous, uniform foam with a tough, smooth skin on both sides.

This closed-cell makeup improves its thermal performance and makes it highly resistant to water absorption. Because of these properties, XPS is a popular choice for roofing systems and other applications where the insulation may be exposed to moisture.

To greater understand the difference between both of these materials, take a look at our comprehensive blog on the Differences Between XPS and EPS. 

Compressive Strength & Best Uses

• EPS 70 - 70 kPa | domestic floors, external wall insulation.
• EPS 100/150 - 100-150 kPa | warm-roof decks, light commercial floors.
• XPS 300 - 300 kPa+ | raft slabs, inverted/green roofs, car parks.

Fire Safety

Raw polystyrene foam is combustible. Both EPS and XPS, even with flame-retardant additives, land in Euroclass E under EN 13501-1, meaning they must sit behind plasterboard, render, screed or concrete in occupied spaces.

Treat the foam as part of a wider fire-stopping strategy, not as the fire line itself.

Cost

Labour requirements are much the same for both materials, so the primary cost difference lies in the price of the insulation itself. 

XPS is more expensive per m² due to its higher density and performance. However, its added compressive strength and thinner required build-ups often justify the extra cost in demanding applications like inverted roofs or load-bearing floors.

EPS remains the more economical option where conditions allow.

How to Cut Polystyrene Insulation Boards

• Prepare Your Workspace: Ensure you have a flat surface to work on and wear safety glasses.
• Measure and Mark: Use a straight edge and a marker to outline your desired measurements on the insulation board.
• Use a Sharp Knife: A utility knife or a hot wire cutter can be used. If using a knife, ensure it's sharp for cleaner cuts.
• Score and Snap: Gently score the board along the marked lines and then apply gentle pressure to snap it.
• Smooth Edges: Sand any rough edges for a clean finish.

Installation Tips

• Keep foams away from solvents (PU glue, bitumen, gasoline).
• UV degrades XPS in weeks - cover if left exposed.
• Always over-board with plasterboard/render/concrete for fire.
• Stagger & tape joints to kill wind-washing in warm roofs.