EPS is a molded, closed-cell plastic made from petrochemicals derived from crude oil and natural gas. EPS starts out as unexpanded polystyrene beads containing the blowing agent pentane and flame-retardant additives. In a vessel, the beads are exposed
Figure 11.2 CMU core inserts. (.Korfil)
to heat (steam) and are expanded from 30 to 50 times their original size. Following a stabilization period, the beads are injected into a mold and, under more heat and pressure, are further expanded and fused into blocks. After curing, the blocks are cut into the required sizes with hot-wire cutting equipment. The wires are electrically heated to over 400°F. The wires are drawn down through the block to achieve the desired length and width. The blocks are then pushed through additional hot wires via a conveyer to achieve the desired thickness.
EPS is a lightweight, strong, resilient, noncorrosive, dimension – ally stable material that does not cause skin irritation. Typically, expanded polystyrene has a lower R-value than extruded polystyrene because of its lower density and because it does not contain refrigerant gas. The main advantage of this board over extruded polystyrene is that it typically costs 40 percent less.3
EPS is manufactured in varying densities, from less than 1.0 lb/ft3 to over 2.0 lb/ft3 depending on the application. The high-den – sity board is more moisture-resistant and can be used on the exterior of a foundation, provided the surrounding soil is dry and sandy. EPS foam used for roofing materials must be of sufficient density to resist damage from foot traffic (Fig. 11.3).
EPS is water-resistant, although the spaces between the foam beads can absorb water. When used as a sheathing, it is not
considered a vapor barrier. Typical water vapor permeance for Iм of material ranges from 5.0 for the low-density foams to 2.0 for the high-density materials. Although EPS provides a high level of moisture resistance and breathability, recommended design practices for walls and foundations should be followed in the selection of vapor and moisture barriers for severe exposures. A vapor diffusion retarder is necessary if water transmission through the insulation might present a problem for the user.
Although EPS is affected by and possibly can deteriorate from contact with organic solvents, their vapors, and solvent-based adhesives, it can be used for specific low-slope roofing systems. Compatibility always should be confirmed. For example, only low temperature bitumens can be applied to the boards. Cover boards are necessary in asphalt or adhered systems to protect the insulation.4