What is the difference between XPS and EPS?

24 April 2017 Kingspan Insulation Middle East
Kingspan Styrozone XPS insulation board

The different manufacturing processes of the two types of polystyrene insulation can have a huge effect on the long-term performance of the two products.

It is important to note that the differences can result in a building that does not perform as designed.

Extruded polystyrene insulation or XPS Insulation is manufactured through an extrusion process. This manufacturing process involves melting together the plastic resin and other ingredients. The liquid formed is then continuously extruded through a die and expands during the cooling process. This produces a closed cell rigid insulation.

Expanded polystyrene insulation or EPS insulation is manufactured using a mould to contain small foam beads. Heat or steam is then applied to the mould, which causes the small beads to expand and fuse together. This manufacturing process does not form a closed cell insulation as there can often be voids between each of the beads where they are not touching one another.

What do the different manufacturing processes mean for the insulating properties?

Water Absorption

The interconnected voids between the beads in EPS can allow water to penetrate. This will degrade the insulating performance as water is a good conductor of heat. This is especially true for below-grade and inverted roofing applications, where the insulation will be exposed to water. Also, be mindful in climates where temperatures get below freezing. The freeze - thaw cycle in these areas will cause the water inside the voids to expand and break the bonds between the beads – further reducing the insulating performance. This will open up the EPS insulation. Due to the closed cell structure of XPS,

Thermal Conductivity

The thermal conductivity of XPS begins at lower than the thermal conductivity of EPS. This is because air is a good conductor of heat. The interconnected voids in the EPS foam will conduct heat due to the air inside them, therefore, you would need a much higher density (more beads, so smaller voids) to reduce this form of conduction.

Compressive Strength

The compressive strength of EPS is worse than XPS. For EPS to meet the same compressive strengths as XPS, the density of the foam would need to be a lot higher, thus requiring a more onerous manufacturing process.