Expanded vs. Extruded Polystyrene
It is commonly understood that XPS offers more initial R-value per inch than EPS but because of its lower cost, EPS offers more R-value per dollar. It must also be noted that stated R-values of insulations products vary under different conditions and truly credible comparisons must take into consideration the specific brands of foam and specific test conditions.
XPS cells contain insulating gases in addition to air that eventually diffuses out of the cells, thus lowering the insulating value. The Long Term Thermal Resistance (LTTR) of XPS 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 XPS 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 XPS and EPS often tout the fact that their products can be recycled, a complete life cycle analysis shows that EPS has a better overall environmental impact when compared to XPS. 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, XPS is often branded by its manufacturer and frequently comes in a number of different colors which have nothing to do with performance. Since XPS is typically more expensive than EPS, buyers need to closely compare the tables of properties for each product as compared to the needs of the application.