The creation of XPS foam begins when solid polystyrene crystals, along with special additives and a blowing agent that forms gas bubbles, 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 molten mass is then forced in a continuous process through a shaping die. As it emerges from the die, it expands to a foam and is shaped, cooled, and cut as required.
XPS products can range in thickness from V2 to 3" (or thicker for SIPs applications) and in board sizes of 2′ X 8′, 4′ X 8′, and 4′ X 9′ for a variety of sidewall, below-grade, and roofing applications. Total R-values per board can range from 2.8 to 20 depending on board thickness. The R-value of XPS is R-5, regardless of density. A number of edge treatments such as square edge, ship lap, and tongue and groove are available. Polyethylene facers are also available for additional board durability.
Figure 11.5 Extruded polystyrene. (Tenneco)
Thin V4 to 3/8" XPS insulation products are also produced in a fanfold bundle design. Typically, 4′ X 50′, the fanfolded bundle unfolds to cover large areas quickly, resulting in reduced overall installation time. These also can be sandwiched between two perforated plastic capsheets or with a laminated inner layer for use as exterior sheathing in residing applications (Figs. 11.6 and 11.7).
Not only is XPS naturally hydrophobic (no chemical affinity for water), but its fine, closed-cell structure and smooth, continuous skin also help the foam resist moisture. This property also results in resis-
Figure 11.6 Fanfolded bundle. (Pactive)
tance to cell damage that would be induced by freezing/thawing temperature fluctuations if water were present in the cells.
Although XPS is more expensive than EPS, it has a higher livable, lower water absorption levels, and a higher compressive strength than EPS. The high-density board product can handle relatively high pressures such as applications under concrete slabs.
Figure 11.7 Residing underlayment. (Pactive)
The foamboard does not cause skin irritation, and when board joints are properly sealed, XPS can act as an air barrier.
XPS is also used for low-slope roofing systems. XPS will deteriorate from contact with organic solvents and adhesives, and only low-temperature bitumens can be applied to the boards. Cover boards are necessary in asphalt or adhered systems to protect the insulation.4
Unlike EPS, the R-value of XPS does not depend on the density of the material. The R-value is consistently 5.0 per inch.