In commercial construction, rigid foam insulation meets a trifecta of requirements: performance, durability, and economics. It has done this consistently since the 1950s, when a Dow Chemical inventor combined polystyrene and isobutylene, inadvertently creating a foamed polystyrene used immediately in flotation devices during World War II. Advancements in manufacturing and quality control have diversified polystyrene’s applications, which include the thermal insulation of buildings. Despite the stigma of foam takeout containers, foam as insulation can save up to 200 times the amount of its embodied energy through its ability to increase the building envelope efficiency.
Material Types
Rigid foam insulation comes in three common forms: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (PIR). Sharing the polystyrene surname, EPS and XPS are both manufactured from the petroleum-based resin, a thermoplastic based on non-cross-linked polymers. Polymers of this type will soften and melt at 212 F, and degrade when in contact with certain solvents, underscoring the importance of its use alongside manufacturer-approved compatible materials, according to the Westford, Mass.–based Building Science Corp.
Colloquially referred to as Styrofoam, a trademark that technically only applies to XPS products made by Dow Chemical, EPS is used in foam coolers, packaging peanuts, cups, and takeout containers. Made of 2% plastic and 98% trapped air, EPS comprises tiny polystyrene beads expanded many times their original size, while XPS consists of polystyrene resin crystals combined with additives and a gas-blowing agent, continuously extruded through a machine and then cut to length.
Meanwhile, PIR is a thermoset plastic based on cross-linked polymers that can tolerate higher temperatures—they char instead of melt—and are more resistant to solvents and chemicals than polystyrene. PIR is typically paired with a black facer when used as insulation.
Thermal Resistance Capabilities
Generally speaking, EPS offers an approximate thermal resistance of R-4 per inch of thickness; XPS provides about R-5 per inch; and PIR offers a long-term thermal resistance (LTTR) of approximately R-6.5 per inch. According to the Polyisocyanurate Insulation Manufacturers Association, PIR’s R-value per inch will uniquely increase in a thicker product. However, PIR can off-gas over time, reducing its thermal resistance; for example, it may start out with a 24% to 30% thermal advantage over XPS and EPS, but drop down to a 5% or 10% advantage a decade later, notes Bill Chaleff, AIA, of Water Mill, N.Y.–based firm Chaleff and Rogers Architects.
ASHRAE 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings lists minimum required R-values, broken down by building-envelope component and project location within one of the U.S.’s eight climate zones. Florida, for example, requires the stud cavity of a wood- or steel-framed wall to achieve R-13 because of year-round temperate conditions; Alaska also requires R-13 in the stud cavity, but also a layer of continuous exterior, interior, or integral insulation achieving R-7.5 to combat the subarctic and arctic winter temperatures.
Appropriate Applications
EPS makes up 90% of the structural insulated panel (SIP) cores market, Chaleff says. The material’s pros include “low costs and ease of making panels of any thickness in the range of 4.5 inches up to 15 inches,” says Jack Armstrong, executive director and chief operating officer of the Structural Insulated Panel Association (SIPA) in Fort Lauderdale, Fla.
XPS is used in the balance of SIP products when the cost can be justified and when “the superior density, which translates to resistance to compression, comes in handy,” Chaleff says. Andy Roehl, design professional at Lebanon, Ohio–based Moonlight Design Studio, adds that “XPS is good for applications where you want the most thermal resistance in the least-thick panel along with low water-vapor permeability” because it is denser than EPS.
At equal thickness, PIR’s more efficient insulating ability can become critical when room size is compromised due to the thickness of its surrounding walls.
Cost
With consideration of insulation efficacy, EPS typically will cost less, and then XPS and PIR, Armstrong says. As with any pricing calculations in construction, look beyond the initial material cost to calculate long-term labor and maintenance costs.
Fire Safety
Petroleum-based products are inherently combustible. Chaleff advises architects to only use insulation products manufactured by established companies that have passed independent fire testing. For example, any SIP panel, whether filled with EPS, XPS, or PIR, made by a manufacturer that belongs to SIPA will have met the standard for Type 5 construction, which applies to buildings four stories or less and requires the installation of fire sprinklers. PIR inherently provides a higher level of fire resistance because of its chemical structure. Relevant standards to check for all EPS and XPS products include ASTM C578, Standard Specification for Rigid, Cellular, Polystyrene Thermal Insulation; for PIR products, ASTM E1730, Standard Specification for Rigid Foam for Use in Structural Sandwich Panel Cores is one guide.
