Polyiso vs. XPS

With increasing requirements in continuous insulation (c.i.), Architects, Designers and Specifiers are faced with making a choice: Polyisocyanurate (polyiso) or Extruded Polystyrene (XPS). Both of these foam plastic insulations are governed by Chapter 26 of the International Building Code (IBC), yet they are very different in their physical and thermal properties. This is especially true when it comes to performance under elevated temperatures. One key difference between the polyiso and XPS is the type of material or plastic that classifies each one - thermoset or thermoplastic. There are significant differences between these insulations and understanding them will make it clear – polyiso is the only choice.

What is the difference

XPS is classified as a thermoplastic material. A thermosoftening plastic, or thermoplastic, is a polymer that softens or turns to a liquid when heated.

Polyiso is classified as a thermoset material. A thermosetting plastic, also known as thermoset, is a polymer material that irreversibly cures. Once hardened, a thermoset plastic is permanently rigid and cannot be melted back to a liquid form. Thermoset materials are generally stronger than thermoplastics due to the three dimensional network of bonds (cross-linking) and are also better suited for high-temperature applications.

Under normal fire conditions, thermoplastics, such as XPS, melt and drip while thermosets, such as Rmax polyiso, simply char over - see pictures below. 

insulating building envelopes effectively

The service temperature of XPS is typically limited to about 165°F. This falls in line with the instability of a thermoplastic material. In fact, according to an industry Material Safety Data Sheet (MSDS), XPS begins to melt at 194°F. On the other hand, the 250°F service temperature of Rmax polyiso products is more than 50% higher than that of most XPS products.

Many sources agree, the surface temperature of an exterior wall veneer is going to be considerably higher than ambient. This will be even more exaggerated when a highly conductive material, such as aluminum or steel, is being used as the veneer. Depending on building type, construction type, climate zone, orientation, etc., one has to examine the suitability of the limited service temperatures of XPS. The heat island effects of urban vs. rural areas will certainly play a role in the exterior wall temperature as well.

When the service temperature is reached and thermoplastic materials soften or melt like XPS, they lose shape and thickness. You are not getting the full value you paid for, since a loss in shape and thickness results in a loss of R-value.

The fine print

Standardized test methods that determine the spread of flame and the smoke development of insulation materials are performed by national laboratories and used within building codes as a requirement for the use of various materials within a building. Yet, based on the ASTM E84 Standard, thermoplastic materials, such as XPS, cannot be directly compared to thermoset materials, such as polyiso, due to the limits of the test method itself.

ASTM E84 states, “Testing of materials that melt, drip, or delaminate to such a degree that the continuity of the flame front is destroyed, results in low flame spread indices that do not relate directly to indices obtained by testing materials that remain in place.” In fact, when XPS is subjected to the extreme heat of this test, it melts to the bottom of the tunnel. Since there is no material that “remains in place,” the test is essentially stopped, ignoring the materials that continue to burn. An examination of a testing laboratory certification label reveals the real story, even though many XPS insulations claim a flame spread of less than 25. The following example was taken directly from the UL Certifications Directory1:

Flame Spread 15*
Smoke Developed 165*
*Flame Spread and Smoke Development recorded while material remained in original test position. Ignition of molten residue on the furnace floor resulted in flame travel equivalent to calculated Flame spread classification of 125 and smoke developed classification of over 500.

Cost Advantage

In today's market, polyiso is competitive with XPS when comparing thickness. There is also a savings from reduced energy usage throughout the life of the building. With polyiso being a more efficient insulation, you are actually getting more bang for your buck. Closer examination will show that when comparing R-values, polyiso is less expensive than XPS. 

polyiso: the only choice for continuous insulation

The difference between thermoset and thermoplastic materials is real and should not be ignored. For complete reassurance that the insulation value will remain in place year after year, use Rmax polyiso insulation. Polyiso is the clear choice for many reasons: sustainability and the environment, thermal performance, moisture management, thinner wall profile, fire performance and air infiltration reduction.