Understanding Sandwich Panels

used in Prefabricated & Modular Buildings

Polystyrene Sandwich Panels (EPS) vs Polyurethane Sandwich Panels (PU)

When enquiring about Prefabricated or Modular buildings, many people simply look at the issued architectural drawing and price of the building. The assumption is that all the suppliers offer the same thing.  But they don’t. Not all Prefabricated & Modular Buildings are the same. One of the key components that differ is the panel type used for the walls and in some cases the roof.

Two sandwich panels widely used for Prefabricated & Modular Buildings in South Africa and Sub-Saharan Africa are:

  1. Expanded Polystyrene Sandwich Panels (EPS) and
  2. Polyurethane Sandwich Panels (PU)

Have a look at the key differences below and then decide which sandwich panel you would want in your Prefabricated & Modular Building.

1.Expanded polystyrene Sandwich Panels (EPS)

EPS  sandwich panels consist primarily out of pre-cut polystyrene sheets glued between two galvanised steel sheets and fed through a roller press. This process is fairly cheap and can be done by hand.

EPS Panels come in various thicknesses and densities, primarily a thickness of 40mm and density of 15DV is used in SA. The 15DV density is an important factor, as this provides the building with its thermal properties.  When compared to the density of a PU panel (discussed later) this is very low.

EPS Panel Specimen Thumb Test

A simple thumb test proves that EPS panels are neither strong, nor vey dense.  Have a look at  how easily the polystyrene breaks under the pressure of my thumb.

EPS Panel Specimen Joining

Other issues found is the joining of panels.  Most sandwich panels have a Male and Female joining system that allow multiple panels to slot into each other and form a single wall.

The panels used below are not new, and each EPS panel manufacturer may manufacture better quality panels than shown below. However, the specimen used, is manufactured by one of the largest EPS panel manufacturers in SA.

From the side, the panel wall seems relatively sealed, but looking from the top you can see that there is no real weather and thermal seal between the panels.

  View from inside  

  View from outside

EPS Panel Specimen Adhesive Test

Peeling the steel sheet away from the polystyrene sheet was easy.

You are able to see from the pictures below as to how the adhesive is applied but notice as to how little of the adhesive is stuck to the EPS Core.

2.PU Polyurethane Sandwich Panels

Polyurethane Sandwich Panels are made by injecting PU Foam under high pressure in-between two galvanized steel sheets within a heated press. The foam is left to cool and set which in turn glues the two sheets together.

The PU Panels we use have a Core density of 40Kg per m³ which produces a strong panel and that has a much higher R Value than that of a EPS 16kgs/m3 Panel.

PU Panel Specimen Thumb Test

As you can see from the thumb test done on the PU panel, it is much stronger than the EPS panel test.

PU Panel Specimen Joining

As you can see from the joint between the male & female joint of the PU Panels in the picture above, there is a clear seal which is both weather tight and much cleaner joint on the side.

PU Panel Specimen Adhesive Test

Trying to pull the steel sheet away from the PU core didn’t work, the only way I would be able to separate the steel sheets from the PU core is to cut it open.

Thermal Insulation Properties of EPS & PU Sandwich Panels

Table comparing the R-Values of PU and EPS panels

Data supplied by an EPS Panel Manufacturer and a PU Panel Manufacturer in SA.

The  R-Value of EPS panels is far below that of PU panels, to such an extent that you would have to use a 80mm EPS panel just to compete with a 40mm PU panel if the supplier is using 15DV (Density Value). Therefore EPS panel suppliers, should be offering a much higher polystyrene sheet density or use a much thicker panel than a 40mm in order to compare with the attributes of PU panels.


Although the tests done are rudimentary and not scientific, they certainly do show some of the key differences between using an EPS panels and PU panels for Prefabricated and Modular Buildings. In my opinion the PU panels are far more superior than the EPS panels in all aspects that was tested. This is why we only use PU panels for our Prefabricated & Modular Buildings at Cube Modular.

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