Considerations for Specifying Rigid, Cellular Polystyrene Insulations in Various Applications

RIGID, CELLULAR POLYSTYRENE (RCPS)
foam boards are used as thermal insulation in
assemblies for exterior walls and roofs, as well
as cold-climate infrastructure.1-3 Considering all
these applications (Fig. 1–3), it is interesting
that the number of generic “types” could be
reduced to just 14 according to ASTM C578,
Standard Specification for Rigid, Cellular
Considerations for Specifying
Rigid, Cellular Polystyrene
Insulations in Various
Applications
Separating Fact from Fiction on Real-World Construction Projects
By Rob Brooks; Tiffany Coppock, AIA,
NCARB, CSI, CDT, LEED AP; Matt Dillon;
Mike Fischer; Meng Guo, PhD; and
Valentina Woodcraft, PhD
Interface articles may cite trade, brand,
or product names to specify or describe
adequately materials, experimental
procedures, and/or equipment. In no
case does such identification imply
recommendation or endorsement by
the International Institute of Building
Enclosure Consultants (IIBEC).
Polystyrene Thermal Insulation.4 This article
addresses considerations surrounding different
applications of various types of polystyrene
insulation, including caveats related to
specifying insulation thickness based solely on
the R-values listed in ASTM C578.
The main purpose of ASTM C578 is to allow
products from different manufacturers to be
classified according to basic physical properties
that can be measured in a laboratory using
standardized test methods. ASTM C578 provides
guidance for testing physical properties such
as compressive resistance, flexural strength,
and thermal resistance to ensure continued
compliance with the product standard. However,
specifying only as-manufactured physical
properties is not adequate to ensure foam
polystyrene performance in practice. Hence,
this article examines how physical properties
of insulation change as they interact with the
environment.
CAVEATS FOR THE SPECIFIER
It is not possible for a designer to choose
an R-value from a table without further
consideration of end-use conditions. Designers
Figure 1. The architects of this building in downtown Chicago developed a protected membrane
roof assembly (PMRA) blue roof, especially to meet the water detention requirements of their
building permit. The 14-story base of the building accommodates storm water management
requirements for the entire building through the design of blue and green (vegetative) PMRAs.
PHOTO COURTESY OF DUPONT AND AMERICAN HYDROTECH INC.
Feature
22 • IIBEC Interface February 2024
also need knowledge of factors that affect
the long-term performance of insulation.
This knowledge can be gained from science,
experience, and the industry’s best practices.
Guidance from individual manufacturers
and industry associations is useful, but such
information can be limited. “Buyer Beware!”
applies in the RCPS marketplace. There is no
substitute for a detailed understanding of the
long- and short-term material properties of
insulations in specific applications.
These caveats are mentioned in
Appendix X1 of the ASTM C578 material
classification standard. While that appendix is
designated as “nonmandatory information,”
it covers several vital topics, including the
following, which will be discussed in this article:
• X1.3 Water Vapor Transmission
• X1.4 Water Absorption
• X1.7 Thermal Resistance Values at Additional
Mean Temperatures
For a building enclosure consultant, engineer,
or architect who is specifying the insulation, the
advice in Appendix X1 may be as relevant as
information from the body of ASTM C578, which
provides mandatory testing information. The
specifier should keep in mind that R-values are
affected by in-use temperature changes, which
occur daily and seasonally. Thermal performance
of RCPS is also affected by moisture absorption
and outgassing of blowing agents (and inward
diffusion of air) over years and decades.
For example, extruded polystyrene (XPS)
types of insulation are subject to long-term aging
due to diffusion of blowing agents and air. The
heavy molecules of the blowing agent slowly
diffuse out of, and lighter air and water vapor
molecules diffuse into, the insulation. As a result,
the R-value typically decreases by a predictable
amount of 1% to 2% over the product’s life, which
often spans several decades. This effect can be
determined and reported in accordance with
ASTM C1303, Standard Test Method for Predicting
Long-Term Thermal Resistance of Closed-Cell
Foam Insulation,5 or CAN/ULC S770, Standard
Test Method for Predicting Long-Term Thermal
Resistance of Closed-Cell Foam Insulation.6
ASTM C578 requires that the long-term thermal
resistance (LTTR) be reported for five types of XPS
(designated in the standard by roman numerals
IV, V, VI, VII, and X). Expanded polystyrene (EPS)
typically is not subject to this gas exchange
mechanism and the associated accelerated
testing to determine a LTTR value.
Compared with the gas exchange mechanism,
moisture absorption more significantly influences
the thermal resistance of polystyrene (EPS and
XPS) insulations. ASTM C578 prescribes only shortterm
moisture absorption tests, using narrowly
defined laboratory conditions. These tests can be
informative for classification purposes, but they
typically do not provide sufficient information to
design a roof, wall, or cold-climate infrastructure
assembly. This article discusses various aspects of
moisture absorption in polystyrene (EPS and XPS)
insulations and the pitfalls of relying exclusively
on ASTM C578 R-values in real-world designs.
RCPS foam boards are used to manage
the hygrothermal performance of roofs, walls,
foundations, and cold-climate infrastructure. The
thickness and physical properties of the foam
boards influence the location of the dew-point
temperature within the building enclosure
elements. Moisture-absorption properties
are especially important with respect to how
moisture is transported through the insulation as
well as the condensation of moisture within the
bulk of the insulation.
Susceptibility to long-term moisture
absorption is not quantified in ASTM C578.
Short-term measurements of moisture
absorption are based on submersion for 24 hours
in a laboratory environment. As described in
detail for cold climates,3 the moisture absorption
measured on samples retrieved from belowgrade
applications after several years are often
much higher than the short-term maximum
values specified in ASTM C578.
To ensure that appropriate environmental
controls are sustained, an informed designer
will strategically use neutral specifications and
apply knowledge of how RCPS foam boards
interact with the environment over long periods
of time. ASTM C578 currently has no required
test method to characterize the long-term effects
of exposure to moisture. It is up to the designer
to gain additional knowledge about these factors
and develop an appropriate strategy.
The values given in ASTM C578 can be likened
to the rules of a game such as basketball or chess.
The rules of manufacturing and classifying the
various types according to ASTM C578 are not in
dispute. Nonetheless, how these specifications
are applied has more to do with the talent of the
designer in developing an optimal design in
accordance with the end use and the environment.
BASICS OF THERMAL
RESISTANCE
Basic thermal insulation knowledge begins
with a fundamental understanding of heat
flow. Arguably, from the viewpoint of a building
enclosure consultant, engineer, or architect, the
Figure 3. Rigid, cellular polystyrene foam boards can be used in
horizontal floor applications as well as vertical wall applications.
Figure 2. Rigid, cellular polystyrene foam boards are often installed
beneath airfields in cold climates.
PHOTOGRAPH COURTESY OF KINGSPAN
February 2024 IIBEC Interface • 23
most important material property of RCPS foam
board is its thermal resistance.
The heat flow Q per unit area A is directly
proportional to the temperature difference ΔT
divided by the thermal resistance Rth.
Heat flow per unit area: q = Q/A = ΔT/Rth In this equation, Rth equals the R-value. Rth
values add in series, similar to electrical resistors.
R-value is sometimes expressed as R-10, R-20,
and so on, indicating the product of the R-value
per inch and the thickness of the board in inches.
The Federal Trade Commission (FTC)
requires home insulation manufacturers,
professional installers, new home sellers,
and retailers to provide R-value information,
based on the results of standard tests, to help
inform consumers. The FTC’s “R-value rule” is
formally known as the “Trade Regulation Rule
Concerning the Labeling and Advertising of
Home Insulation.”7
The reciprocal of thermal resistivity is thermal
conductivity.8 See the sidebar p 31. “Converting
Thermal Resistance Values from Imperial to Metric
Units,” for an explanation of units, including how to
convert between imperial units (inch-pound) and
metric units for thermal resistance.
MAKING SENSE OF ASTM TYPES
For projects involving polystyrene foam, a
building enclosure consultant, engineer, or
architect requires understanding the 14 RCPS
classification types listed in Table 1 of ASTM
C578. They are numbered I, II, and IV to XV. (Type
III has been discontinued, which explains why
the numbering goes to XV even though there are
only 14 types.)
The columns in ASTM C578 Table 1 are not in
numerical order by type but rather are arranged
as follows:
• Types XI, I, VIII, II, IX, XIV, and XV are listed in
the first seven columns. These are typically
EPS insulations.
• Types XII, X, XIII, IV, VI, VII, and V are listed the
next seven columns. These are typically XPS
insulations.
• Within each set of seven columns, the types
are then listed in order according to their
specified minimum value for compressive
resistance. The first row gives the compressive
resistance for each of the 14 types, ranging
from 5.0 to 60.0 psi (35 to 414 kPa) for the
EPS types and from 15.0 to 100.0 psi (104 to
690 kPa) for the XPS types.
Note that ASTM C578 does not specify
that EPS or XPS products must be assigned
to specific types. For example, there is no
requirement that only XPS products can be
classified as Type X or that only EPS products
can be classified as Type XII. As long as the
product meets the standard’s specifications for
a type, it can qualify as that type. Realistically,
however, there is widespread industry
agreement regarding “EPS types” and “XPS
types.” Manufacturers rarely market an XPS
product as one of the EPS types. This article
ignores those rare exceptions and informally
refers to EPS types and XPS types. (Note:
Type XIII, a specialized type of pipe insulation,
is not relevant to this article.)
The first row in Table 1 of ASTM C578
gives the standard for minimal compressive
resistance because compressive resistance is
an important physical property to specify in
design documents. Compressive resistance
determines how much load can be placed on
the RCPS foam board. Although 100 psi is
only a fraction of the compressive resistance
of steel (25,000 psi [172 MPa]) or concrete
(up to 10,000 psi [69 MPa]), the compressive
resistance of the foam boards could be a key
specification for a protected membrane roof
assembly, a basement floor, or an airport
runway, to give a few examples. Obviously,
RCPS foam board is not meant to be a
structural material. However, when the load is
distributed, a compressive resistance of 100
psi could adequately support vehicular traffic
in plaza decks or airplanes taking off from or
landing on an insulated airport runway. ASTM
C578 only gives the tensile and compressive
stress values. It is not a design standard. It is
up to the designer to calculate other structural
effects, such as creep and foundation modulus,
according to appropriate design standards.
TYPICAL R-VALUES
The second row in ASTM C578 Table 1 lists the
minimal thermal resistance as measured at 75°F
(24°C). Thermal resistance varies significantly
with temperature. In general, the R-value of
RCPS decreases as the temperature increases.
In other words, R-values are consistently higher
at 25°F and 40°F (-4°C and 4°C) compared with
75°F and 110°F (24°C and 43°C) for both EPS
and XPS types (Fig. 4). In ASTM C578, the table
that recommends minimum R-values at 25°F,
40°F, and 110°F is Table X1.1 in Appendix X1.
As noted earlier, that appendix is intended to
collect “nonmandatory” information. However,
these higher R-values at lower temperatures are
relevant when specifying insulation for colder
climate zones or environments.
At each of the test temperatures, five of the
XPS types (X, IV, VI, VII, and V) are specified in
ASTM C578 to have the same minimum R-values
per inch, which plateau at 5.60, 5.4, 5.0, and
4.65 for temperatures of 25°F, 40°F, 75°F, and
110°F (-4°C, 4°C, 24°C, and 43°C), respectively.
That is remarkable considering that other
performance characteristics vary substantially
for these five XPS types. This is because R-values
have less correlation with density for XPS types
and a much stronger correlation for EPS types.
Furthermore, Fig. 5 shows that the R-values
for XPS types are consistently higher than the
R-values for EPS types of similar densities.
Density is perhaps the least relevant
material property of RCPS foam boards in terms
of building enclosure design. The additional
load on a building from the weight of RCPS
foam boards is negligible. Density may only be
Figure 4. R-value per inch increases as temperature decreases for both “EPS types” and “XPS
types” of rigid, cellular polystyrene insulation in the ASTM C5784 classification. Note: EPS =
expanded polystyrene; XPS = extruded polystyrene.
24 • IIBEC Interface February 2024
relevant when calculating the buoyancy of the
foam boards used on a protected membrane
roof assembly or a flotation device. In those
cases, there will need to be enough ballast
to keep the foam boards from floating. Also,
the weight of the protected membrane roof
assembly is relevant to the design for wind
uplift resistance.
Figure 5. R-value increases with density for both “EPS types” and “XPS types” of rigid, cellular polystyrene insulation in the ASTM C5784
classification, although R-value peaks at higher values and relatively low density for XPS types. Note: EPS = expanded polystyrene; XPS = extruded
polystyrene. Values are from ASTM C578.
POROSITY AND STRENGTH
Although the density of RCPS foam boards may
not be of much relevance in building enclosure
design, it is noteworthy that the material’s
strength correlates well with density. There are
two observations worth noting:
• Compressive strength and flexural strength
generally correlate with density.
• For a given density, XPS types are stronger
than EPS types.
For example, XPS Type V and EPS Type XV
both must test to a minimal density of
48 kg/m3 (3.0 lb/ft3), but the compressive
strength of the EPS type is 40% less than that
of the XPS type.
February 2024 IIBEC Interface • 25
Figure 6. EPS foam illustration has considerable open porosity compared with XPS foam insulation. Note: EPS = expanded polystyrene;
XPS = extruded polystyrene.
How could two foam boards of the same
material with the same density have such
different strengths? One possible answer may
involve the porosity, which can be subdivided
into closed porosity and open porosity.
Porosity is defined as 1 minus the ratio of the
density of the foam and the density of the solid:
[1 – ( / solid)]. According to the National Institute
of Standards and Technology,9 the density of
solid polystyrene (not insulation) is 1060 kg/m3
(66 lb/ft3), which is slightly denser than water.
(Of course, solid polystyrene has zero porosity.)
In comparison, the density of polystyrene foam
insulation boards ranges between 12 and 48
kg/m3 (0.75 and 3.0 lb/ft3). Hence, the total
porosity of these types of insulation boards
ranges between 0.99 (least dense) and 0.95
(most dense).
Table 1 of this article shows the density and
calculated total porosity of various types of RCPS
along with the compressive strengths as given
in ASTM C578 Table 1. As density increases,
the porosity decreases; however, that does not
explain the strength differences between EPS
types and XPS types of similar porosity.
Total porosity is the sum of closed porosity and
open porosity. Open porosity can be measured
by the gas adsorption method: the more gas
adsorbed, the greater the open porosity. Open
porosity also explains the greater water absorption
Table 1. Calculated total porosity of “XPS types” and “EPS types” of rigid, cellular polystyrene
insulation based on ASTM C578 minimum density values
EPS types
Type XI I VIII II IX XIV XV
Porosity, [1 – ( / solid)] 0.989 0.986 0.983 0.979 0.972 0.964 0.954
Compressive resistance,
minimum, psi 5 10 13 15 25 40 60
Flexural strength,
minimum, psi 10 25 30 35 50 60 75
XPS types
Type XII X IV VI VII V
Porosity, [1 – ( / solid)] 0.982 0.98 0.978 0.972 0.967 0.954
Compressive resistance,
minimum, psi 15 15 25 40 60 100
Flexural strength,
minimum, psi 40 40 50 60 75 100
Note: EPS = expanded polystyrene; XPS = extruded polystyrene. 1 psi = 6.895 kPa. = density of
foam; solid = density of solid.
and permeability of EPS types compared with XPS
types of the same density. Most of the porosity
of XPS types is closed porosity. In contrast, when
the resin beads in EPS are expanded into a closed
mold, the channels between the beads provide
a substantial proportion of open porosity. Thus,
although the cell-wall thicknesses may be similar
in EPS and XPS samples of similar density, the EPS
sample would have a greater proportion of open
porosity as seen in Fig. 6.
26 • IIBEC Interface February 2024
The open porosity of the EPS bulk matrix has
a deleterious effect on strength and explains
why EPS foam board absorbs more water than
XPS foam does. Porosity is not the only factor
underlying strength. Foam structure on the
scale of the cells also is a factor. Mechanical
strength is believed to come from polystyrene
struts, which offer greater strength than the cell
windows. A detailed discussion of how struts
can strengthen RCPS is beyond the scope of
this article. Interested readers are referred to
the technical literature on this topic.10 What is
important to note here is that the strength of
foam board is important for many applications of
RCPS, and manufacturers are continually seeking
to improve this property (Fig. 7).
THE TRUTH ABOUT WATER
ABSORPTION
The most startling numbers in ASTM C578
Table 1 are found in the row on water absorption.
These numbers represent the maximum water
absorption allowed to meet the standard for each
of the RCPS types. There is no question that EPS
absorbs much more water than XPS, specifically
in short-term testing by total immersion.
Maximum water absorption values of 2%, 3%,
or even 4% by volume are seen for EPS types.
In general, for EPS types, as density increases,
the maximum R-value is reduced but it does not
drop below 2% by volume (Fig. 8).
Water weighs 1,000 kg/m3 (60 lb/ft3). When
2% of the volume of a cubic meter of Type XV
foam is occupied by water, 20 kg (44 lb) of
water are added to the 48 kg (106 lb) of foam.
Values for water absorption per ASTM C272,
Standard Test Method for Water Absorption of
Core Materials for Sandwich Constructions,11 for
EPS Types XI and I (maximum 4% by volume),
EPS Types VIII and II (maximum 3% by volume)
and EPS Types IX, IV, and XV (maximum 2% by
volume) are in sharp contrast to the maximum
water absorption for XPS Types XII, X, IV, VI, VII,
and V (maximum 0.3% by volume). In other
words, depending on the specific type, EPS can
absorb 7 to 10 times as much water as XPS and
still meet the ASTM C578 product standard.
See Table 2.
Figure 7. Aerial view of US Coast Guard Headquarters. XPS insulation must resist compressive loads from the weight of vegetative or “green” roofs.
Note: The appearance of US Department of Defense (DoD) visual information does not imply or constitute DoD endorsement. EPS = expanded
polystyrene; XPS = extruded polystyrene.
Table 2. Water absorption for various “EPS types”
and “XPS types” of rigid, cellular polystyrene
insulation
Water absorption
by total
immersion,
24-hour maximum
absorbed
(% by volume)
EPS types
XI, I 4.0
VIII, II 3.0
IX, XIV, and XV 2.0
XPS types
XII, X, IV, VI, VII, V 0.3
XIII 1.0
Note: EPS = expanded polystyrene;
XPS = extruded polystyrene.
February 2024 IIBEC Interface • 27
24-h max. allowable
Water Absorption by Total Immersion,
24-h maximum allowable, vol. %
R-value per inch
(ft2·􀈙F·h/Btu)
R-value
24-h max. allowable
Water Absorption by Total Immersion,
24-h maximum allowable, vol. %
R-value per inch
(ft2·􀈙F·h/Btu)
R-value
Figure 8. R-value per inch increases as water absorption decreases. The high R-values for XPS could be attributed in part to its low values of water
absorption. Water absorption is measured in percentage by volume. Note: EPS = expanded polystyrene; XPS = extruded polystyrene. Values are
from ASTM C578.4
The water absorption rates for short-term
testing represent a major difference between EPS
and XPS types of RCPS foam. ASTM C578 requires
manufacturers to meet the water absorption limits
for relatively short periods of immersion. Additionally,
ASTM C578 mentions in the Appendix X1.4,
“Water Absorption,” that “this characteristic may
have significance when this specification is used to
purchase material for end-uses requiring extended
exposure to water.” The appendix is considered
“nonmandatory” information, and ASTM C578 does
not quantify the effects of this water absorption on
the thermal performance of the materials. Designers
are left to their own resources.
The reason for this disparity in water
absorption rates has to do with the discontinuous
structure of EPS foam boards, which results in
significant open porosity (as described in the
previous section). The capillary pathways allow
water to enter the EPS types throughout the
bulk of the material, depending on the capillary
sizes. Smaller bead sizes—such as those used
in food-grade EPS—result in smaller and less
permeable capillaries but also limit the density
reduction. On the contrary, relatively little water
enters the bulk of the XPS samples because the
high proportion of closed-cell porosity inhibits
the absorption of water. XPS insulation has a
smooth microstructure that is not interrupted by
the millimeter-scale “bead structure” prevalent
in EPS types.
The consequences of water absorption can
be substantial, depending on the application. Its
water absorption rate is one of the main reasons
why EPS is unsuitable for protected membrane
roofing assemblies.1 It also explains why XPS is
preferred in below-grade applications. Also, XPS
is preferred over EPS for habitable basements,
where polystyrene insulation is commonly
applied exterior to the basement walls and
floor slabs (and thus is often in contact with
groundwater or moist soil).2
February 2024 IIBEC Interface • 29
The mechanisms of water absorption are
reviewed in considerable detail in Brooks et al.12
See also Pakkala and Lahdensivu.13
MYTHS ABOUT TESTING FOR
WATER ABSORPTION
Aside from the water absorption mechanisms,
which in general are not disputed, several
myths and misinterpretations have developed
concerning water absorption testing. Most experts
readily acknowledge that R-values of RCPS foam
boards drop as water is absorbed. This is based
on simple physics. The thermal conductivity of
water or ice is much greater than the thermal
conductivity of air or blowing agents. Performance
has been simulated using computer models.14,15
Nonetheless, the prediction of water absorption
depends on the application, the climate zone, and
other factors. It is incorrect to assume that water
absorption does not really matter because the
insulation quickly dries out. This is untrue in many
cases, especially for below-grade applications.3
Also, it is incorrect to assume that “maximum”
water absorption according to ASTM C578
represents a “maximum possible” value of
long-term water absorption, or that it places an
upper limit on the amount of water that can
be absorbed. This is also not true, according
to findings reported by Cai et al.16,17 The crux
of these misunderstandings is the conflation
of the ASTM C578 standard with performance
expectations. ASTM C578 does not dictate the
thickness of insulation required to achieve
long-term design R-values. That is based on
engineering judgment using various thicknesscorrection
guidelines.
Specifiers must keep in mind that the shortterm
testing for moisture absorption used in
ASTM C578 does not predict how moisture
absorption affects performance in different
applications. There is a heavy energy-waste
penalty due to reduced R-value when insulation
is incorrectly used in wet environments such
as building foundations, protected membrane
roofing assemblies, infrastructure in cold
regions, and other below-grade applications. It is
up to the specifier or consultant to account for the
consequences of material choices in any given
application.
ASTM C578 only gives basic properties of
the various types of EPS and XPS at the time of
manufacture. In the final analysis, the building
enclosure consultant, architect, engineer, or
specifier must exercise “engineering judgment”
in the design of insulation systems suitable for a
particular application and environment. Thermal
stability, moisture control, thickness factor, longterm
R-values, and so on are all relevant to the
design of below-grade structures.
CONVERTING THERMAL RESISTANCE VALUES
FROM IMPERIAL TO METRIC UNITS
A full understanding of R-value (in imperial units) or RSI-value (in metric units) is essential to the
application of ASTM C578. The following is a review of the basics of thermal resistance values and
how to convert from imperial units to metric units.
Heat flow per unit area: Q/A = ΔT/R
where
R = Rth = thermal resistance
Q = heat flow
A = unit area
ΔT = temperature difference
Rearranging the heat flow equation gives the following equation:
R = (ΔT/Q) × A
In imperial units:
Q has units of energy per unit time (Btu/h)
ΔT has units of degrees Fahrenheit (°F)
Area has units of square feet (ft2)
Therefore, the units for R-value per unit length are (ft2 × °F)/(BTU/h) = (°F × ft2 × h/BTU)
In SI units, RSI has units of (°C × m2)/W or, equivalently, K × m2/W.
Converting from Imperial Units to SI Units
To convert from imperial units to SI units, apply the following conversions.
1°F = (5/9)K
1 Btu/h = 0.2931 W
1 ft2 = 0.0929 m2
°F × ft2 × h/Btu = (5/9) × 0.0929/0.2931 K.m2/W
(°F × ft2 × h/Btu)/(K × m2/W) = 0.176
(K × m2/W)/(°F × ft2 × h/Btu) = 5.678
R-value (in imperial units) ≈ RSI-value (in SI) × 5.678
RSI-value (in SI) ≈ R-value (in imperial units) × 0.176
To convert to an RSI value in SI units an R-value in imperial units, multiply by 5.678. To convert an
R-value in imperial units to an RSI value in SI units, multiply by 0.176.
About Thermal Conductivity
The U-value for an insulator is a measure of thermal conductivity. The inverse of the R-value is also
known as the overall heat transfer coefficient.
U-value = (1/R-value) = heat flux/(temperature difference)
For a given temperature difference, a high U-value signifies a high heat flux. The heat flux in
imperial units is expressed as Btu/h/ft2. In SI units, the heat flux is expressed as W/m2. The total
heat transferred would be the heat flux multiplied by the area. Heat flow is greatest through areas
with low R-values. The U-value of an assembly such as a wall or entire building enclosure accounts
for individual U-values and interplay of the assembly components.
A Simple Example
The R-value for 1 in. (25 mm) thickness of many common building materials (such wood, brick, or concrete)
is less than 1 °F × ft2 × h/Btu. Consider a 1 ft × 1 ft (0.3 m × 0.3 m), 1 in. (25 mm) thick block of material
with a 10°F (5.5°C) temperature differential on either side of the block. If the R-value per inch of this
hypothetical building material has a value of 1 °F × ft2 × h/Btu, this block would transfer 10 Btu (10.6 kJ)
every hour. If the block area were 10 ft × 10 ft (3 m × 3 m), the heat loss would be 1,000 Btu/h (1,055 kJ/h).
Increasing this example to 1,000 ft2 (equivalent to the wall area of a small double-wide mobile home) results
in 10,000 Btu/h (10,550 kJ/h) or 240,000 BTU (253,000 kJ) per day. Poorly insulated homes are notoriously
expensive to heat and cool for this reason. In our example, energy loss could add up to $3 to $7 per day
($1,000 to $10,000 per year), accounting for seasonal, fuel type, and regional variability.
At the other extreme, an XPS foam board typically has a thermal resistance five times greater
than our hypothetical material. So, in the example, instead of transferring 1,000 Btu/h (1,055 kJ/h),
it would only transfer 200 Btu/h (211 kJ/h). Moreover, 5 in. (125 mm) would only transfer 40 Btu/h
(42 kJ/h). It is easy to see how polystyrene insulation can dramatically inhibit heat transfer through
walls, floor slabs, and roofs, and reduce energy use and waste. Few building materials offer values
of thermal insulation as high as those for polystyrene insulation.
CONCLUDING REMARKS:
FACTS AND CAVEATS
Insulation products are essential for
improving the energy efficiency and
service life of buildings. Polystyrene foam
insulation boards are among the most
versatile insulation materials available.
However, all types of polystyrene insulation
February 2024 IIBEC Interface • 31
Tiffany Coppock,
AIA, NCARB, CSI,
CDT, LEED AP, is a
commercial building
systems specialist
at Owens Corning
where she provides
leadership in
building science,
system development,
testing, and
documentation.
Matt Dillon is a
technical support
manager with Kingspan
Insulation, Atlanta,
Georgia, a position
he has held for two
and a half years,
using his mechanical
engineering
background in support
of Kingspan customers
as well as sales and
marketing within Kingspan Insulation, a division
of the Kingspan Group. Dillon is available
to respond to technical and product-related
inquiries and issues.
Mike Fischer is the
executive director
of the Extruded
Polystyrene Foam
Association (XPSA). He
is a 40-year veteran
of the building
products industry,
and he has held
positions in logistics,
management, sales,
technical marketing,
and regulatory advocacy. Fischer serves as the
XPSA staff lead, as well as the spokesperson and
advocate for XPSA members and the industry.
Meng Guo, PhD,
graduated with a
doctorate in chemistry
and has more than
10 years of experience
in research and
development with
Owens Corning, where
she currently leads the
Foamular science and
technology laboratory.
Guo applies her
knowledge of materials science, engineering,
are not created equal. ASTM C578 has
raised awareness that there are two distinct
types of polystyrene insulation boards, EPS
and XPS, and, in general, these two classes
of insulation have very different properties.
ASTM C578 provides key facts but
leaves the door open for logical fallacies.
Caveat emptor (Buyer Beware!) rules the
marketplace. Some nontechnical product
representatives may incorrectly assume that
the short-term moisture absorption values
in ASTM C578 represent an upper limit
on moisture absorption, or that long-term
moisture absorption doesn’t matter because
the insulation dries out. However, without
standard testing and in-field observations,
marketing campaigns may advance
anecdotal “evidence” of performance in
insulation-friendly environments. Until
reliable long-term testing and modeling
can be developed, engineering judgment
will continue to play a vital role in the
specification of insulation.
REFERENCES
1. Brooks, R., T. Coppock, M. Dillon, V. Woodcraft, and
J. Woestman. 2022. “Extruded Polystyrene in Protected
Membrane Roof Assemblies.” Extruded Polystyrene
Foam Association (XPSA). https://xpsa.com/wp-content/
uploads/2022/09/PMRA-XPSA-FINAL-APPROVED-with-
Photos-Interleaved-2022-bylines.pdf.
2. Brooks, R., T. Coppock, M. Dillon, M. Guo, V.
Woodcraft, and J. Woestman. 2022. “The Role of
Insulation for Habitable Basements,” XPSA. https://
xpsa.com/wp-content/uploads/2022/11/IP-BG-03-
Habitable-Basements.pdf.
3. Brooks, R, B. Fabian, J. Smith, G. Titley, and
J. Woestman. 2019. “Extruded Polystyrene Delivers
Higher R-Values than Expanded Polystyrene in
Below-Grade Applications, According to New
University of Alaska Fairbanks Study.” XPSA. https://
xpsa.com/wp-content/uploads/2020/05/XPSA-IPBG-
01_Nov.8.2019_Preprint.pdf.
4. ASTM International. 2023. Standard Specification
for Rigid, Cellular Polystyrene Thermal Insulation.
ASTM C578-23. West Conshohocken, PA: ASTM
International.
5. ASTM International. 2022. Standard Test Method for
Predicting Long-Term Thermal Resistance of Closed-Cell
Foam Insulation. ASTM C1303/C1303M-22. West
Conshohocken, PA: ASTM International.
6. Standards Council of Canada. 2022. Standard Test
Method for Determination of Long-Term Thermal
Resistance of Closed-Cell Thermal Insulating Foams.
CAN/ULC S770-S770-15 (R2020). Ottawa, ON:
Standards Council of Canada.
7. Federal Trade Commission. n.d. “R-Value Rule.”
Accessed October 24, 2023. https://www.ftc.gov/
legal-library/browse/rules/r-value-rule.
8. Leaman, J., and C. Hendricks. 2022. “Misleading
R-Value and the Need to Reframe Insulation Scales.”
Journal of Light Construction. https://www.jlconline.
com/how-to/insulation/misleading-r-value-and-theneed-
to-reframe-insulation-scales_o.
9. National Institute for Standards and Technology. n.d.
“Composition of Polystyrene.” Accessed October 24,
2023. https://physics.nist.gov/cgi-bin/Star/compos.
pl?matno=226.
10. Gibson, L. J., and M. F. Ashby. 1997. Cellular Solids:
Structure and Properties. 2nd ed. Cambridge, UK:
Cambridge University Press.
11. ASTM International. 2018. Standard Test Method
for Water Absorption of Core Materials for Sandwich
Constructions. ASTM C272/C272M-18. West
Conshohocken, PA: ASTM International.
12. Woestman, J. 2020. “Moisture Absorption in
Polystyrene Insulation: Effects on In-Service Design
R-Values.” IIBEC Interface. Raleigh, NC: IIBEC.
13. Pakkala, T. A., and J. Lahdensivu. 2014. “Long-Term
Water Absorption Tests for Frost Insulation Materials
Taking into Account Frost Attack.” Case Studies
in Construction Materials 1: 40-45. https://doi.
org/10.1016/j.cscm.2014.02.001.
14. Cai, S., H. Guo, B. Zhang, G. Xu, K. Li, and L. Xia.
2020. “Multi-Scale Simulation Study on the
Hygrothermal Behavior of Closed-Cell Thermal
Insulation.” Energy 196: 117142. https://doi.
org/10.1016/j.energy.2020.117142.
15. Woodcraft, V., G. K. LeBlanc, M. Spinu, and
T. Weston. 2021. “Dynamics and Impact of
Vapor-Driven Moisture on Properties of Insulating
Foams.” In Performance, Properties, and Resiliency
of Thermal Insulations, D. Fisler and M. Pazera,
eds. ASTM STP1629-EB. West Conshohocken,
PA: ASTM International. https://doi.org/10.1520/
STP1629-EB.
16. Cai, S., B. Zhang, and L. Cremaschi. 2018. “Moisture
Behavior of Polystyrene: Insulation in Below-Grade
Application.” Energy and Buildings 159: 24-38.
https://doi.org/10.1016/j.enbuild.2017.10.067.
17. Cai, S., B. Zhang, and L. Cremaschi. 2017. “Review
of Moisture Behavior and Thermal Performance of
Polystyrene Insulation in Building Applications.”
Building and Environment 123: 50-65. https://doi.
org/10.1016/j.buildenv.2017.06.034.
ABOUT THE AUTHORS
Rob Brooks is
a building code
consultant with Rob
Brooks and Associates
LLC and specializes
in North American
building codes
and standards for
construction materials
such as foam plastic
ROB BROOKS insulation.
TIFFANY COPPOCK,
AIA, NCARB, CSI, CDT,
LEED AP
MATT DILLON
MIKE FISCHER
MENG GUO, PHD
32 • IIBEC Interface February 2024
and process chemistry to support the creation of
innovative solutions for Owens Corning Foamular
business. She is available to answer technical
questions on Foamular products.
Valentina Woodcraft,
PhD, is a senior lead
research scientist
within DuPont
Performance Building
Solutions, supporting
the rigid foam
insulation product
lines from the research
and development
VALENTINA perspective.
WOODCRAFT, PHD
www.sunoptics.com
The Originator and Inventor of the
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February 2024 IIBEC Interface • 33