Use And Care Manual

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Q: Why is rigid foam insulation speciﬁed in exterior wall assemblies?

A: When used in continuous insulation (ci) applications, rigid foam insulation increases the thermal resistance of the wall

assembly by minimizing thermal bridging caused by framing members. For example, in an average wood frame construction,

wood framing accounts for approximately 25 percent of the overall wall surface that is left un-insulated when only the stud

cavities are insulated.₁ Similarly, in steel framed construction, the steel not only interrupts the stud cavity insulation, it is also a

very good conductor of heat.

Rigid foam insulation can be installed directly over framing members to provide continuous insulation coverage across the entire

exterior of the wall. This approach enhances the thermal resistance of the wall and also reduces potential for moisture and mold

issues within the wall assembly.

Q: Why do cold months matter most when it comes to selecting a rigid insulation?

A: During cold winter months, buildings experience signiﬁcant heat ﬂow movement from inside to outside. To address this heat

ﬂow transfer, it’s critical to specify a reliable insulation solution to achieve the desired energy efﬁciency, building performance

and occupant comfort. New third-party research illustrates that Extruded Polystyrene (XPS) provides more reliable thermal

performance than Polyisocyanurate (polyiso).

Q: Can the Federal R-value Rule mislead speciﬁers regarding the actual thermal performance of rigid

insulation?

A: In some cases, yes. Across the industry, the U.S. Federal R-value Rule regulates that all R-value claims labeled and marketed

for insulation products must be based on ASTM C518 tests₂. A standard mean temperature of 75°F is used when performing

ASTM test methods for determining a material’s R-value. In the case of rigid foam insulation, this can be problematic.

XPS has a published R-value of R5.0 per inch and polyiso is listed at a range of R5.6 to R6.0 per inch.₃ These values are true

when tested at a mean temperature of 75°F. However, building science research continues to show that the R-value of each

product is substantially different when tested at colder mean temperatures. In other words, insulation R-value is temperature

dependent.

As shown in the table below, the R-value of polyiso can decrease substantially when tested at mean temperatures less than

75°F. In contrast, the R-value of XPS can actually increase in colder temperatures. In fact, XPS outperforms polyiso in cold,

winter months when it matters most.

Mean Temperature XPS Polyiso

75°F

R5.0 R5.6

15°F

R6.0 R2.0

Q: What new research substantiates that XPS exhibits better thermal performance than polyiso as

temperatures get colder?

A: Recently, Building Science Corporation (BSC) published the Thermal Metric Summary Report₄ regarding their Thermal

Metric Project and Reference Wall testing. The long-term goal of this project was to develop a new metric for the thermal

performance of building enclosures that better accounts for known physical heat ﬂow mechanisms (particularly natural and

forced convection) and operating conditions. This project included a focus on better understanding the thermal conductivity of

insulating materials at different mean temperatures. The report reveals that the thermal performance of XPS and many other

insulation types improves as the temperature drops. However, polyiso doesn’t follow the same pattern. In fact, the BSC report

illustrates that the thermal performance of polyiso declines substantially as the temperature drops.

As part of their research, BSC tested the thermal conductivity of polyiso and several other building materials (See Figure 1). At

75°F, the mean temperature at which insulation R-values are reported on consumer packaging, the polyiso sample (shown as

“PIC Cartridge TB” below) exhibits a level of thermal conductivity which correlates to its published R-value (R-value = 1 divided

by its thermal conductivity, BTU-in/hr-ft2-F).

CAPTURING THE THERMAL PERFORMANCE OF

FOAMULAR

EXTRUDED POLYSTYRENE (XPS)

vs. POLYISOCYANU R ATE (POLYISO) FAQ s

Summary of content (5 pages)

PAGE 1
CAPTURING THE THERMAL PERFORMANCE OF FOAMULAR® EXTRUDED POLYSTYRENE (XPS) vs. POLYISOCYANURATE (POLYISO) FAQs Q: Why is rigid foam insulation specified in exterior wall assemblies? A: When used in continuous insulation (ci) applications, rigid foam insulation increases the thermal resistance of the wall assembly by minimizing thermal bridging caused by framing members.
PAGE 2
However, as the temperature drops below 59°F, its thermal conductivity increases substantially which results in a significant decrease in R-value. In contrast, the thermal conductivity of XPS decreases as the temperature drops, resulting in an R-value that is ultimately greater than R5.0 per inch. Figure 1.
PAGE 3
Q: Since the R-value of polyiso is severely compromised in cold temperatures, how does polyiso ci compare to XPS ci in a typical wall assembly? A: Together with Building Science Corporation (BSC), the Owens Corning building science team used WUFI, a well-respected hygrothermal modeling program, to analyze the performance of XPS and polyiso in a variety of U.S. climates using BSC measurements.
PAGE 4
Q: How do XPS and polyiso water absorption properties compare? A: The material standard that defines properties for all XPS and EPS is ASTM C578.₆ It requires that polystyrene insulation be tested for water absorption in accordance with ASTM C272.₇ C272 requires the polystyrene sample to be immersed in water for 24 hours, and weighed immediately upon removal from immersion to determine the amount of absorbed water. The material standard for polyiso is ASTM C1289.
PAGE 5
Q: Why is this scientific data relevant to specifiers and builders? A: Armed with scientific data, builders and specifiers can avoid getting shortchanged on the intent of their high-performance building design by specifying XPS rather than polyiso.