Expanded versus Extruded Polystyrene

A surprisingly large number of otherwise-informed people know very little about the differences between expanded polystyrene (EPS) and extruded polystyrene (XEPS). Some may even think they're the same thing. The fact is that the diifferences are considerable.

EPS and XEPS are both rigid, closed cell, thermoplastic foam materials. EPS (often called beadboard) is produced from solid beads of polystyrene. Expansion is achieved by virtue of small amounts of pentane gas dissolved into the polystyrene base material during production. The gas expands under the action of heat, applied as steam, to form closed cells of EPS. These cells occupy approximately 40 times the volume of the original polystyrene bead. The beads can be molded to specifications to form insulation boards, blocks or customized shapes for the building insulation or packaging industries.

XEPS foam begins with solid polystyrene crystals. The crystals, along with special additives and a blowing agent, are fed into an extruder. Within the extruder the mixture is combined and melted, under controlled conditions of high temperature and pressure, into a viscous plastic fluid. The hot, thick liquid is then forced in a continuous process through a die. As it emerges from the die it expands to a foam, is shaped, cooled, and trimmed to dimension. Dow Styrofoam is the most well-known brand of XEPS.

Read More:

Two primary differentiators where XEPS out-performs EPS are thermal insulating value (rough initial R=5.5 vs. 4.5) and moisture vapor permeance (approximately 1.5 vs. 3 perm-in). It must be noted that these values vary under different conditions, however, and truly credible comparisons must take into consideration the specific brands of foam and specific test conditions. The density of XEPS is also generally higher with a somewhat higher compressive strength.

On the other hand, XEPS cells contain insulating gases in addition to air that eventually diffuse out of the cells, thus lowering the insulating value. The Long Term Thermal Resistance (LTTR) of XEPS is therefore less than its "initial" R-factor. The R-factor of EPS remains constant over the life of the product because the manufacturing process used in EPS results in normal air, rather than gas, in the voids in the product.

While XEPS is "extruded" into a close approximation of its final form, EPS is manufactured in large blocks that are subsequently cut by hot-wire machines into sheets or cut into virtually any special shape or form by computer-driven systems. EPS is also easily worked in the field during installation.

Although manufacturers of XEPS and EPS often tout the fact that their products can be recyled, a complete life cycle analysis shows that EPS has a better overall environmental impact when compared to XEPS (see reference). EPS can be recycled in many ways once it comes to the end of its life. These include recycling directly into new building products and incineration to recover its inherent energy content. The choice of a recycling method is based on technical, environmental and economic considerations.

Finally, XEPS is often branded by its manufacturer and frequently comes in a number of different colors which have nothing to do with performance. Since XEPS is typically more expensive that EPS, buyers need to closely compare the tables of properties for each product as compared to the needs of the application.