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Long-Term Thermal Resistance (LTTR) – Five Years Later

May 15, 2007

In 2000, a new, prescriptive test method (CAN/ULC-S770, “Standard Test
Method for Determination of Long-term Thermal Resistance of Closed-Cell
Thermal Insulating Foams”) for determining R-values of certain foam plastic
insulations was adopted as a national standard in Canada, providing a longneeded
definition of “aged” R-value. Aging refers to the gradual change in cell
gas composition and the resulting change in R-value of a foam plastic insulation
that relies on a captive blowing agent for thermal performance. This method,
therefore, estimates a value that is both a 5-year, aged value and a 15-year, timeweighted
thermal design value. It applies to polyurethane, polyiso, and extruded polystyrene,
all of which “age.” Since 2001 in Canada, and 2002 in the U.S., polyiso manufacturers
have been testing products to determine their long-term thermal resistance
This standard derives from ASTM C 1303 (“Standard Test Method for Estimating
the Long-Term Change in the Resistance of Unfaced, Rigid, Closed-cell Plastic Foams
by Slicing and Scaling Under Controlled Laboratory Condition,”) the first “thin slice”
method. This method was developed through a government and industry initiative in
1989, and was a major step forward in estimating aged thermal values of cellular
foam plastic insulations that depend on a captive blowing agent for thermal resistance.
Oak Ridge National Laboratory (ORNL) of the Department of Energy (DOE), the
Polyisocyanurate Insulation Manufacturers Association (PIMA), Society of Plastics
Industry (SPI), and National Roofing Contractors Association (NRCA) joined in the sixyear
research project, culminating in a widely read paper delivered at the 11th
Conference on Roofing Technology in September 1995, in Gaithersburg, MD.
The industry, however, hesitated to embrace the standard because it was initially
designed for research. Because it was not prescriptive, many considered it too complex
and inappropriate for product ratings and comparisons. As a result, the industry
continued to rely on the 6-month conditioning practice then in widespread use for
the previous 15 years. The 6-month conditioning practice establishes a time and conditioning
protocol required before measuring R-value. In its time, it too was a major
step toward standardization and away from the market confusion created in the
12 • I N T E R FA C E MA R C H 2007
absence of an industry-recognized method,
which for several years allowed the measurement
of R-values at time of manufacture.
Although the 6-month conditioning
method became standard practice for most
foam plastic insulations and still appears in
the ASTM standard specifications for polyiso
(C 1289) and polystyrene (C 578), it has
long been criticized for not addressing aging
– the change in R-value over time caused by
changes in cell gas composition. An ongoing
study of extruded polystyrene (XPS) products
provides a good example of this shortcoming.
Ten samples of XPS, ranging in thickness
from 1 to 4 inches, were collected from
the field and submitted – first to an R & D
laboratory for testing over time, and then to
a third-party materials testing laboratory
for independent corroboration.
The samples submitted to the industry
laboratory were 2.5 to 12.5 months in age.
By the time they were submitted to the
Table 1
MA R C H 2007 I N T E R FA C E • 1 3
3.94″ 9/29/04 Unknown 4.85
4/20/05 4.66
4/3/06 4.65
5/19/06 4.54 17.89 20.0 -10.6
3.00″ 9/29/04 8.5 4.87
4/20/05 15 4.76
4/3/06 26.5 4.76
5/19/06 4.68 14.04 15.0 -6.4
2.00″ 9/8/04 5 4.95
4/20/05 12.5 4.81
4/3/06 24 4.81
5/19/06 4.78 9.56 10.0 -4.4
1.55″ 9/29/04 12.5 5.21
4/20/05 19 5.13
4/3/06 30.5 5.13
5/23/06 5.06 7.84 7.5 4.5
0.99″ 10/26/04 3 5.35
5/4/05 8.5 5.21
4/3/06 20 5.18
5/19/06 5.12 5.06 5.0 1.2
2.02″ 9/22/04 2.5 4.78
4/20/05 9.5 4.74
4/3/06 20.5 4.69
5/19/06 4.62 9.33 10.0 -6.7
1.49″ 9/22/04 6 4.83
4/20/05 13 4.74
4/3/06 24.5 4.65
5/19/06 4.62 6.88 7.5 -8.3
1.48″ 9/8/04 Unknown 4.98
4/20/05 4.81
4/3/06 4.76
5/24/06 4.80 7.10 7.5 -5.3
1.01″ 9/22/04 3.5 5.10
4/20/05 10.5 4.98
4/3/06 22 4.88
5/19/06 4.76 4.81 5.0 -3.8
1.03″ 9/8/04 Unknown 4.55
4/20/05 4.46
4/3/06 4.42
5/24/06 4.43 4.56 5.0 -8.8
third-party laboratory, they were approximately 20 to 30.5
months in age. This process provided several R-value data
points over time, showing changes (i.e., aging) for products
of various ages. In three cases, the exact age was unknown
but exceeded 19 months, since the samples had been conditioning
in a laboratory for at least that long. Refer to Table 1
for test dates and results.
Based on the data contained in this table, the importance
of an accurate, long-term thermal resistance test method is
clear. In only two cases, the measured R-value met the 6-
14 • I N T E R FA C E MA R C H 2007
Left and Below:
The “thin slice” method performed by PIMA.
month reported value (R-5.0), which is also
the S770 LTTR-value recommended by XPS
manufacturers in their marketing literature.
In at least two cases, the measured R-values
at 2.5 to 5 months in age failed to meet the
6-month minimum value in the ASTM C 578
polystyrene material standard. In the majority
of cases, the measured R-value was
below the published R-5.0, especially as the
samples aged. Third-party test data are
identified in the table in green.
The aging phenomenon shown in this
table has been recognized for years, and the
“slicing and scaling” methods, such as
ASTM C 1303 and CAN/ULC-S770, have
attempted to account for it. For this reason,
foam plastic insulation, such as polyiso and
extruded polystyrene, is required by
Canadian product specifications to report
long-term thermal resistance values (LTTR)
in accordance with CAN/ULC-S770.
After five years of experience with this
test method, some researchers have identified
potential positive bias (over-predicting),
especially for XPS insulation. XPS manufacturers
have reported over-prediction of 10-
25%, presumably based on the reported
value R-5.0. Based on the data shown in
Table 1, the bias could be even greater if
based on the average of R-4.74. Research
on polyiso insulation, however, indicates a
much smaller bias. The wide difference in
reported bias may be related to the difference
in cell gas diffusion rates between
polyiso and XPS. Since diffusion rates for
XPS are at least an order of magnitude
higher than that for polyiso, researchers
have found it difficult to establish a test
method appropriate for both materials.
LTTR test results and measured R-values of
polyiso samples secured from the field have
already been discussed in previous industry
literature (Graham, 2006).
When the industry agreed to include
CAN/ULC S770 in ASTM C 1289 (“Standard
Specification for Faced Rigid Cellular Polyisocyanurate
Thermal Insulation”) as a
mandatory Annex, the polyiso industry,
through PIMA, initiated a bias study involving
products from two manufacturers. This
study is designed to determine a percentage
of positive (over-prediction) or negative
(under-prediction) bias that results from
the CAN/ULC-S770 test method when compared
to actual aged R-values at full thickness
of insulation boards. After three years,
the average bias according to this data is
approximately +6%.
At the same time that CAN/ULC S770
was gaining recognition, the ASTM C 1303
task group in 2000 undertook revisions to
that standard to add a prescriptive method
to complement the existing research method.
The group hoped that the prescriptive
method would remove some of the less precise
elements of the research method and
would provide a method for widespread use
in product rating for LTTR. In other words,
the standard would provide standardized
“cook book” instructions for users. Similar
to some of the work undertaken in the S770
task group, the C 1303 task group identified
slice thickness and other specimen
preparation practices as potential sources
for the apparent bias. Adjustments to these
elements in the test method have been
finalized, and the standard was recently
balloted successfully at ASTM, ensuring a
new version of C 1303 will be issued soon.
The industry hopes that this new portion
of ASTM C 1303 proves with experience to be
appropriate for both XPS and polyiso, and
helps reduce the +10-25% reported bias for
XPS and the +6% bias for polyiso. However,
just as C 1289 initiated a bias study, C 1303
has undertaken a ruggedness test to help
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MA R C H 2007 I N T E R FA C E • 1 5
answer still outstanding questions about
features of that test method. This ruggedness
test will be completed in 2011, at which
time the industry should have a greater
breadth of data that may lead to further
modifications. This data will be reported as
the bias study concludes and the ruggedness
test progresses and should provide the
industry with useful insights into the success
of the attempted improvements.
Graham, M., “Research Reveals the
LTTR Method May be Over-reporting
Results,” Professional Roofing,
January 2006.
16 • I N T E R FA C E MA R C H 2007
Richard Roe, RRC, CCPR, LEED™ AP, is the director of technical
services for Atlas Roofing Corporation in Atlanta, GA.
Mr. Roe has been an RRC since 1994. He is currently chair of
PIMA’s Technical Committee and a member of PIMA’s LTTR
Task Group, which helped prepare this article. Roe is a past
president of the Atlanta Chapter of CSI and past chair of CSI’s
Southeast Region Technical Committee. His articles have
appeared in several industry journals, and he received CSI’s
national Citation Award for technical writing in 2002. Roe is
also a member of SPRI’s Insulation Subcommittee, ASTM C 16, and CAN/ULC-S704
Task Group, and is currently chair of ASTM C 1289 Task Group. He may be reached at
Richard Roe, RRC, CCPR, LEED™ AP
The National Research Council Canada has renovated
one of its experimental facilities to test construction science.
The Ventilation and Wall Research House has the
“potential to investigate and improve the indoor air quality,
comfort, durability, and energy efficiency of housing,”
according to NRC-IRC’s Construction Innovation newsletter.
The council’s Indoor Environment, Building Envelope, and
Structure programs are initiating research projects to
“integrate indoor climate and building envelope performance
to assess heat, air, and moisture transfers between
the outside, the enclosure, the indoor air, and the HVAC
systems.” They are seeking partnerships with public and
private agencies to investigate issues of hybrid heating,
hybrid ventilation, and hygrothermal performance of wall
assemblies. For more information, visit www.irc.nrccnrc.
In RSI’s latest industry survey,
57% of contractors said roof consultants
had helped them install
better roofs. This contrasts
sharply with the responses from
20 years ago, when 70% said “roof
consultants cause contractors
problems on the job.” Survey
compiler Mark Russo gave a nod
to RCI’s efforts to bridge the gap
in his recent report.
Roof Consultants
Becoming More
Popular With
Demand for roofing materials in China is forecast to rise 4.4% per year through 2010, to 2.6 billion
square meters, for an annual value of 47 billion yuan. This is the fastest-growing market in the world,
according to a recent study by The Freedonia Group Inc.
Concrete and clay tiles and bituminous roofing represent the dominant roofing materials in China, with
over 5/6ths of the demand in 2005 in square meters of roofing. However, EPDM, TPO, and PVC membranes
are continuing to make inroads against conventional BUR.
Demand for roofing in the nonresidential market will increase nearly 5 percent per year through 2010
from continued industrialization and sustained strength in foreign direct investment in China. China’s “Flatto-
Slope Conversion Project” will further spur gains for residential roofing. The initiative seeks to replace
existing flat roofs with steep-slope roofs to resolve leakage issues and improve aesthetic appeal.
— Freedonia Group