What's the difference between Extruded Polystyrene XPS and Expanded Polystyrene EPS​

The difference lies in the cell matrix of the two types of insulation.

  • Extruded Polystyrene (XPS) is to its "closed cell" and has NO voids between its cells

  • Expanded Polystyrene EPS is made of "balls" and therefore is has VOIDS between the cells

This is illustrated under microscope at 25x zoom below;

xps v eps.png

Why is this important?

The lack of voids in the Extruded Polystyrene (XPS) allows the insulation to perform to a much higher level in almost all aspects. Off course the energy efficiency of a building often depends on the materials that create its envelope. Selecting the best insulation type is crucial in delivering the required insulation performance for your project.

How is XPS and EPS manufactured?

Extruded polystyrene insulation (XPS) is manufactured using a process of extrusion. This continuous process results in the closed cell structure demonstrated above with a smooth skin on the top and bottom of the board. R-Walls XPS board is "blown" using industrial waste CO2 (making good use of a waste gas) and the machines are kept running using renewable wind energy. 

Expanded Polystyrene insulation (EPS) is manufactured using beads of foam within a mould, heat or steam is then applied directly to the beads which causes them to expand and fuse together. This process produces voids (imagine lots of balls fused together) and therefore the structure is not a closed cell insulation , due to voids that can occur between the beads.

How does XPS and EPS perform differently? 

  1. Water Resistance

The closed cell structure of Extruded Polystyrene (XPS) prevents water penetration to the structure of the insulation board. So why is this so important? Because allowing water into insulation dramatically affects its R-Value and therefore deliverable U-Value. Small amounts of moisture cause a dramatic drop in R-value – often by as much as 50 percent or more. According to Steven Badger, Ph.D., water’s thermal conductivity is nearly 20 times higher than the conductivity of the average thermal insulation. Professional Service Industries Inc., a division of construction materials testing firm Intertek states:


“Absorption of only 20 percent moisture can cause up to 55 percent loss of insulation value. As a result, to ensure that insulation does its job, it’s crucial to consider moisture performance when selecting insulation – especially in high moisture exposure applications.”   –   Intertek PSI, Building Envelope Documented Causes and Effect of Unintended Moisture Intrusion June 1st, 2015.

This means XPS performs much better in the build environment, not only above ground but also underground. The board is not saturated with water and therefore the U-Value is not compromised. Insulation which can become saturated is then at the perils of freeze thaw and its associated destruction of the material. The idea that it’s ok for any insulation product to take on excessive amounts of moisture; based on the assumption it will dry out later, is absurd. What’s the point of insulation if it loses 50% of its R-value while its damp, and its damp or wet in the UK 1/2 of the time?


The videos below are not made by R-Wall but demonstrate the difference; 










  1. Increased Compressive Strength

The lack of voids gives the XPS a much higher Compressive strength as well as Flexural strength. R-Walls board has a compressive strength of 300Kpa. This is 3 times stronger than most standard EPS systems! This increased compressive strength gives much more strength during pour and allows R-Wall to pour formwork confidently, more rapidly and to increased heights, saving time and costs.  Also this additional strength prevents onsite build damage. Again the video below demonstrates the difference;




  1. Better Thermal Conductivity 

Insulation is one of the most practical and cost effective ways to improve a building’s energy efficiency, by improving the insulation in new and existing buildings, significant cost savings and reductions in energy usage can be achieved.


XPS is a much better insulator than EPS. This improved thermal conductivity performance is due to the lack of voids in its matrix. EPS has air trapped in the voids which conducts heat reducing its ability to insulate. A much higher density EPS board will be required to match the thermal performance of XPS insulation. Hence why R-Wall can provide much better energy efficiency (U-Values) with narrower wall widths. R-Wall is proud to be the leading ICF for Wall widths to U-Values.

  1. High Water Vapour Diffusion

Water vapour diffusion resistance (μ) of EPS ranges from approximately 30 – 70  compared to the water vapour diffusion resistance (μ) of XPS that ranges from approximately 80 – 250. XPS is often selected over EPS for wetter environments that require a higher water vapour diffusion resistance value. 

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