ASTM C1373/C1373M-23

Standard Practice for Determination of Thermal Resistance of Attic Insulation Systems Under Simulated Winter Conditions

SIGNIFICANCE AND USE
4.1 The thermal resistance of a ceiling system is used to characterize its steady-state thermal performance.  
4.2 The thermal resistance of insulation is related to the density and thickness of the insulation. Test data on thermal resistance are obtained at a thickness and density representative of the end use applications. In addition, the thermal resistance of the insulation system will be different from that of the thermal insulation alone because of the system construction and materials.  
4.3 This practice is needed because the in-service thermal resistance of some permeable attic insulations under winter conditions is different, lower or higher R, than that measured at or close to simulated room temperature conditions utilizing small-scale tests in which the insulation is sandwiched between two isothermal impermeable plates that have a temperature difference (ΔT) of 20 to 30°C [36 to 54°F]. When such insulation is installed in an attic, on top of a ceiling composed of normal building materials such as gypsum board or plywood, with an open top surface exposed to the attic air space, the thermal resistance under winter conditions with heat flow up and large temperature differences is significantly less because of additional heat transfer by natural convection. Fig. 1 illustrates the difference between results from small scale tests and tests under the conditions of this practice. See Ref (1-12) for discussions of this phenomenon.3
FIG. 1 Schematic of Thermal Resistance for a Permeable Attic Insulation Under Simulated Winter Conditions (Heat Flow Up)
Note 1: A constant hot-side temperature (T, hot) is used for both tests and the temperature difference increases as the cold side temperature (T, cold) is decreased. See 5.1.6 for requirements on size of air space.  
4.4 In normal use, the thickness of insulation products ranges from 75 mm [3 in.] to 500 mm [20 in.]. Installed densities will depend upon the product type, the installed thickness, the installa...
SCOPE
1.1 This practice presents a laboratory procedure to determine the thermal resistance of attic insulation systems under simulated steady-state winter conditions. The practice applies only to attic insulation systems that face an open attic air space.  
1.2 The thermal resistance of the insulation is inferred from calculations based on measurements on a ceiling system consisting of components consistent with the system being studied. For example, such a system might consist of a gypsum board or plywood ceiling, wood ceiling joists, and attic insulation with its top exposed to an open air space. The temperature applied to the gypsum board or plywood shall be in the range of 18 to 24°C [64 to 75°F]. The air temperature above the insulation shall correspond to winter conditions and ranges from –46°C to 10°C [–51 to 50°F]. The gypsum board or plywood ceiling shall be sealed to prevent direct airflow between the warm and cold sides of the system.  
1.3 This practice applies to a wide variety of loose-fill or blanket thermal insulation products including fibrous glass, rock/slag wool, or cellulosic fiber materials; granular types including vermiculite and perlite; pelletized products; and any other insulation material that is installed pneumatically or poured in place. The practice considers the effects on heat transfer of structures, specifically the ceiling joists, substrate, for example, gypsum board, air films, and possible facings, films, or other materials that are used in conjunction with the insulation.  
1.4 This practice measures the thermal resistance of the attic/ceiling system in which the insulation material has been preconditioned according to the material Specifications C549, C665, C739, and C764.  
1.5 The specimen preparation techniques outlined in this standard do not cover the characterization of loose-fill materials intended for enclosed applications.  
1.6 This practice is be used to characterize mate...

General Information

• Status: Published
• Publication Date: 31-Aug-2023
• ICS: 91.120.10 - Thermal insulation of buildings
• Technical Committee: C16 - Thermal Insulation
• Drafting Committee: C16.30 - Thermal Measurement

Relations

• Replaces: ASTM C1373/C1373M-11(2017) - Standard Practice for Determination of Thermal Resistance of Attic Insulation Systems Under Simulated Winter Conditions
• Effective Date: 01-Sep-2023
• Refers: ASTM C168-24 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Apr-2024
• Refers: ASTM C739-24 - Standard Specification for Cellulosic Fiber Loose-Fill Thermal Insulation
• Effective Date: 01-Mar-2024
• Refers: ASTM C1363-24 - Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus
• Effective Date: 01-Mar-2024
• Refers: ASTM C549-23 - Standard Specification for Perlite Loose Fill Insulation
• Effective Date: 01-Nov-2023
• Refers: ASTM C168-22 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 01-May-2022
• Refers: ASTM C739-21a - Standard Specification for Cellulosic Fiber Loose-Fill Thermal Insulation
• Effective Date: 01-Sep-2021
• Refers: ASTM C1363-19 - Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus
• Effective Date: 01-Sep-2019
• Refers: ASTM C549-18 - Standard Specification for Perlite Loose Fill Insulation
• Effective Date: 01-Nov-2018

Overview

ASTM C1373/C1373M-23 is a standard practice developed by ASTM International that specifies a laboratory procedure for determining the thermal resistance (R-value) of attic insulation systems under simulated winter conditions. This standard is essential for assessing the actual thermal performance of various attic insulation materials installed above ceilings with open exposure to attic air space, as commonly found in residential and commercial buildings.

Unlike traditional small-scale tests, this practice replicates real-world winter conditions where large temperature differences and natural convection can significantly affect the insulation’s effectiveness. The standard covers a range of insulation products-including loose-fill, blanket, fibrous glass, rock or slag wool, cellulosic fiber, granular types like vermiculite and perlite, as well as pelletized and other pneumatically or poured-in-place insulation materials.

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Key Topics

• Thermal Resistance Measurement:
  Focuses on steady-state evaluation of the R-value for attic insulation systems as installed, considering the effects of ceiling construction, air spaces, joists, and other materials.

• Simulated Winter Testing Conditions:
  Specifies a hot-side ceiling temperature of 18 to 24°C (64 to 75°F) and a cold-side air temperature ranging from –46°C to 10°C (–51 to 50°F), simulating harsh winter climates.

• Variety of Insulation Materials:
  Applies to a wide range of products such as fibrous glass, mineral wool, cellulose, perlite, vermiculite, and more, with guidance on system assembly and specimen preparation.

• System-Level Evaluation:
  Goes beyond material-only testing by evaluating the complete ceiling/insulation system, including joists and substrates like gypsum board or plywood, with thermal resistance inferred from the entire assembly.

• Natural Convection Consideration:
  Accounts for the impact of natural convection within insulation at large temperature differentials, providing more accurate, real-world performance data.

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Applications

• Building Code Compliance:
  Provides reliable thermal resistance data for attic insulation materials to demonstrate compliance with energy efficiency requirements in building codes.

• Product Certification and Labeling:
  Offers manufacturers and regulators a standardized way to determine and confirm R-value performance under simulated winter conditions, supporting accurate labeling and marketing.

• Research and Development:
  Facilitates the evaluation of new or modified insulation materials and systems for use in cold climates, allowing for system-level thermal optimization.

• Quality Assurance and Comparative Testing:
  Assists builders, homeowners, and insulation contractors in selecting the optimal attic insulation system by providing performance data reflective of in-service winter conditions.

• Energy Performance Analysis:
  Useful for energy consultants and auditors conducting building envelope assessments, enabling better predictions of heating costs and thermal comfort.

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Related Standards

• ASTM C167 – Test Methods for Thickness and Density of Blanket or Batt Thermal Insulations
• ASTM C168 – Terminology Relating to Thermal Insulation
• ASTM C177 – Test Method for Steady-State Heat Flux by Guarded-Hot-Plate Apparatus
• ASTM C518 – Test Method for Steady-State Thermal Transmission Properties by Heat Flow Meter
• ASTM C549, C665, C739, C764 – Material specifications for various loose-fill and blanket insulations
• ASTM C1114, C1363 – Methods for testing building material thermal performance at larger scale
• ASTM C1045, C1058 – Calculating and selecting temperatures for evaluating thermal insulation properties

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Keywords: ASTM C1373, attic insulation, thermal resistance, R-value, winter conditions, natural convection, insulation testing, building envelope, energy efficiency, loose-fill insulation, fibrous glass, cellulose, perlite, residential insulation standards, building code compliance

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C1373/C1373M − 23
Standard Practice for
Determination of Thermal Resistance of Attic Insulation
Systems Under Simulated Winter Conditions
This standard is issued under the fixed designation C1373/C1373M; the number immediately following the designation indicates the
year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last
reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.6 This practice is be used to characterize material behav-
ior under controlled steady-state laboratory conditions intended
1.1 This practice presents a laboratory procedure to deter-
to simulate actual temperature conditions of use. The practice
mine the thermal resistance of attic insulation systems under
does not simulate forced air flow conditions.
simulated steady-state winter conditions. The practice applies
only to attic insulation systems that face an open attic air space. 1.7 The values stated in either SI units or inch-pound units
are to be regarded separately as standard. The values stated in
1.2 The thermal resistance of the insulation is inferred from
each system may not be exact equivalents; therefore, each
calculations based on measurements on a ceiling system
system shall be used independently of the other. Combining
consisting of components consistent with the system being
values from the two systems may result in non-conformance
studied. For example, such a system might consist of a gypsum
with the standard.
board or plywood ceiling, wood ceiling joists, and attic
1.7.1 All values shall be reported in both SI and inch-pound
insulation with its top exposed to an open air space. The
units unless specified otherwise by the client.
temperature applied to the gypsum board or plywood shall be
1.8 This standard does not purport to address all of the
in the range of 18 to 24°C [64 to 75°F]. The air temperature
safety concerns, if any, associated with its use. It is the
above the insulation shall correspond to winter conditions and
responsibility of the user of this standard to establish appro-
ranges from –46°C to 10°C [–51 to 50°F]. The gypsum board
priate safety, health, and environmental practices and deter-
or plywood ceiling shall be sealed to prevent direct airflow
mine the applicability of regulatory limitations prior to use.
between the warm and cold sides of the system.
1.9 This international standard was developed in accor-
1.3 This practice applies to a wide variety of loose-fill or
dance with internationally recognized principles on standard-
blanket thermal insulation products including fibrous glass,
ization established in the Decision on Principles for the
rock/slag wool, or cellulosic fiber materials; granular types
Development of International Standards, Guides and Recom-
including vermiculite and perlite; pelletized products; and any
mendations issued by the World Trade Organization Technical
other insulation material that is installed pneumatically or
Barriers to Trade (TBT) Committee.
poured in place. The practice considers the effects on heat
transfer of structures, specifically the ceiling joists, substrate,
2. Referenced Documents
for example, gypsum board, air films, and possible facings,
2.1 ASTM Standards:
films, or other materials that are used in conjunction with the
C167 Test Methods for Thickness and Density of Blanket or
insulation.
Batt Thermal Insulations
1.4 This practice measures the thermal resistance of the
C168 Terminology Relating to Thermal Insulation
attic/ceiling system in which the insulation material has been
C177 Test Method for Steady-State Heat Flux Measure-
preconditioned according to the material Specifications C549,
ments and Thermal Transmission Properties by Means of
C665, C739, and C764.
the Guarded-Hot-Plate Apparatus
1.5 The specimen preparation techniques outlined in this C518 Test Method for Steady-State Thermal Transmission
standard do not cover the characterization of loose-fill materi-
Properties by Means of the Heat Flow Meter Apparatus
als intended for enclosed applications. C520 Test Methods for Density of Granular Loose Fill
Insulations
C549 Specification for Perlite Loose Fill Insulation
This practice is under the jurisdiction of ASTM Committee C16 on Thermal
Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal
Measurement. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Sept. 1, 2023. Published October 2023. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1998. Last previous edition approved in 2017 as C1373/C1373M – 11 Standards volume information, refer to the standard’s Document Summary page on
(2017). DOI: 10.1520/C1373_C1373M-23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1373/C1373M − 23
C665 Specification for Mineral-Fiber Blanket Thermal Insu-
lation for Light Frame Construction and Manufactured
Housing
C687 Practice for Determination of Thermal Resistance of
Loose-Fill Building Insulation
C739 Specification for Cellulosic Fiber Loose-Fill Thermal
Insulation
C764 Specification for Mineral Fiber Loose-Fill Thermal
Insulation
C1045 Practice for Calculating Thermal Transmission Prop-
erties Under Steady-State Conditions
C1058 Practice for Selecting Temperatures for Evaluating
and Reporting Thermal Properties of Thermal Insulation
C1114 Test Method for Steady-State Thermal Transmission
Properties by Means of the Thin-Heater Apparatus
C1363 Test Method for Thermal Performance of Building
NOTE 1—A constant hot-side temperature (T, hot) is used for both tests
Materials and Envelope Assemblies by Means of a Hot
and the temperature difference increases as the cold side temperature (T,
Box Apparatus
cold) is decreased. See 5.1.6 for requirements on size of air space.
FIG. 1 Schematic of Thermal Resistance for a Permeable Attic
3. Terminology Insulation Under Simulated Winter Conditions (Heat Flow Up)
3.1 Definitions— Unless otherwise stated, the definitions
listed in Terminology C168 are applicable herein.
4.5 The onset of natural convection under winter conditions
is a function of specimen thickness for some materials. For
4. Significance and Use
purposes of this practice, the tests shall be carried out at
4.1 The thermal resistance of a ceiling system is used to
thicknesses at which the product is used.
characterize its steady-state thermal performance.
4.6 Since this practice simulates winter conditions, the heat
4.2 The thermal resistance of insulation is related to the
flow direction shall be vertically upwards.
density and thickness of the insulation. Test data on thermal
4.7 Specimens shall be prepared in a manner consistent with
resistance are obtained at a thickness and density representative
the intended installation procedure. Products for pneumatic
of the end use applications. In addition, the thermal resistance
installation shall be pneumatically-applied (blown), and prod-
of the insulation system will be different from that of the
ucts for pour-in-place installation shall be poured into place.
thermal insulation alone because of the system construction
See 5.2.
and materials.
5. Equipment
4.3 This practice is needed because the in-service thermal
resistance of some permeable attic insulations under winter
5.1 Thermal test apparatus used for this practice shall meet
conditions is different, lower or higher R, than that measured at
the following requirements:
or close to simulated room temperature conditions utilizing
5.1.1 Conformance to Standards—The apparatus shall con-
small-scale tests in which the insulation is sandwiched between
form to all requirements of the ASTM thermal test method
two isothermal impermeable plates that have a temperature
used, except as required by 5.1.2 – 5.1.6.
difference (ΔT) of 20 to 30°C [36 to 54°F]. When such
5.1.2 Size—The apparatus shall be capable of testing speci-
insulation is installed in an attic, on top of a ceiling composed
mens at the thickness intended for product use. Length and
of normal building materials such as gypsum board or
width of the metering area shall be at least twice the spacing of
plywood, with an open top surface exposed to the attic air
the wood joists or four times the specimen thickness, which-
space, the thermal resistance under winter conditions with heat
ever is greater (see Fig. 2).
flow up and large temperature differences is significantly less
5.1.3 Temperature—The apparatus shall be capable of test-
because of additional heat transfer by natural convection. Fig.
ing with the hot side surface maintained between 18 and 24°C
1 illustrates the difference between results from small scale
[64 and 75°F], and with the cold side air temperature main-
tests and tests under the conditions of this practice. See Ref
tained near the winter condition for the particular climate being
(1-12) for discussions of this phenomenon.
simulated, which ranges from –46 to 10°C [–51 to 50°F]. In the
absence of specified temperatures, the ambient temperatures
4.4 In normal use, the thickness of insulation products
listed in Table 2 of C1058 on Temperatures for Thermal
ranges from 75 mm [3 in.] to 500 mm [20 in.]. Installed
Transmittance Evaluations is one source of test temperatures.
densities will depend upon the product type, the installed
thickness, the installation equipment used, the installation
NOTE 1—Only those with a hot ambient of 24°C [75°F] are applicable.
technique, and the geometry of the insulated space.
5.1.4 Humidity—The absolute humidity on both sides of the
test apparatus shall be maintained low enough to prevent
condensation within the specimen. See 6.9.6 of Test Method
The boldface numbers in parentheses refer to the list of references at the end of
this standard. C1363 for humidity requirements for the hot box methods, 6.6
C1373/C1373M − 23
ing it with the blower operating. This practice dislodges any
pieces of old insulation that might be caught in the hose.
6. Sampling
6.1 A sample of material shall be selected from a lot
according to sampling plans given in the material
specifications, regulations, or other appropriate documents
when applicable. In the absence of such directions, material
from at least two randomly chosen packages shall be combined
in equal portions (mass) so as to combine materials as
uniformly as practicable.
6.2 The insulation material is preconditioned to a moisture
content in equilibrium with the laboratory conditions prior to
FIG. 2 Requirements on Dimensions of Test Specimen Metering
Area the specimen installation. Preconditioning of materials not only
ensures controlled installation conditions but reduces the time
required to condition the prepared specimen prior to thermal
testing. For conditioning requirements, see the applicable
of Test Method C177 for the guarded hot plate method, and
materials Specifications C520, C549, C665, C739, and C764.
7.10 of Test Method C518 for the heat flow meter apparatus.
5.1.5 Orientation and Direction of Heat Flow—The thermal
7. Specimen Preparation
test specimen shall be oriented horizontally with heat flow up.
7.1 General Instructions:
5.1.6 Thermal Test Specimen and Holder—The test assem-
7.1.1 All specimens shall be prepared to a thickness and unit
bly shall be sized to match the test apparatus and shall be made
area mass that are given for the label R-value specification of
of construction materials representative of the intended appli-
interest for the material under test.
cation. The substrate on which the insulation rests shall be
7.1.2 Specimens shall be prepared in a manner consistent
representative of the intended application, typically gypsum
with the intended installation procedure. All materials shall be
board. The substrate shall be sealed to prevent direct airflow
installed carefully using the manufacturer’s recommended
between the warm and cold sides of the system. Wood joists
installation practice. Batts shall be cut, as required, to fit the
also shall be included. The test assembly shall be constructed
available specimen holder. Products for pneumatic installation
such that the top of the insulation is open to an air space having
shall be pneumatically-applied (blown), and products for
a minimum thickness of 150 mm [6 in.]. Test Methods C1363
pour-in-place installation shall be poured into the specimen
is preferred because of its ability to accommodate a large air
holder. See 7.2.2 for the density of pneumatically-installed
space. Other apparatuses that simulate in-service conditions
insulation. Other materials must be installed at the density
must meet the requirements of this practice, (for example,
suggested by the manufacturer.
modifications of Test Methods C177, C518, or C1114 with
7.1.3 The specimen holder shall represent typical attic
Practice C1045). In all cases, the size requirements given in
frame construction, wherever possible. This requires, as a
5.1.2 shall be met. Fig. 3 shows a schematic of an attic test
minimum, horizontal members representing the bottom chord
module that has been used for these types of tests. Other
of a truss system or rafter framing and an air-tight gypsum
configurations without the roof structure are acceptable as long
board or plywood bottom. The specimen holder shall be clean
as the minimum 150 mm [6 in.] air space is maintained.
and free of insulation residue prior to installation of the sample
5.2 Specimen Preparation Equipment:
insulation.
5.2.1 Blowing Apparatus—A blowing apparatus is required
NOTE 2—For commonly available loose-fill insulation, state and federal
when pneumatically-applied specimens are to be tested.
energy codes, ASTM material specifications and the Federal Trade
Choose the combination of hopper, blower, hose size and
Commission have identified those materials that shall apply a correction
length that is representative of common use for the application
for settling when determining thermal performance. It is beyond the scope
of the material to be tested. The following machine specifica-
of this practice to outline the procedures for this determination.
tions have been developed for use with mineral fiber and NOTE 3—Many factors can influence the characteristics of the loose-fill
insulation. These include blowing rate, machine adjustments, the size and
cellulosic materials.
length of the hose, and the angle and dimensions of the hose outlet in
5.2.1.1 A commercial blowing machine with a design ca-
relation to the specimen holder. Trained operators are required to duplicate
pacity for delivering the subject material at a rate recom-
field-installed conditioning.
mended by the insulation manufacturer shall be used. The
NOTE 4—For these tests, the specimen shall be blown close to the
labeled density. Some operators may wish to establish a target mass of
machine must utilize 46 m [150 ft] of flexible, internally
insulation required to fill the test frame to the desired thickness and
corrugated blowing hose with an appropriate sized diameter as
density as a control during the specimen preparation process. By weighing
specified by the machine manufacturer. At least 30 m [100 ft]
the initial material and that remaining after blowing is complete, the
of the hose must be elevated between 3 and 6 m [10 and 20 ft]
operator can estimate the material in the test frame. Other operators may
above the blowing machine to simulate typical installation
wish to eliminate these extra steps. The reported test density, however, is
obtained from the metering area density measurement conducted after the
configuration. The hose must have no more than eight 90°
thermal test.
bends and no bends less than 1.2 m [4 ft] radius. It is good
practice to clean the hose periodically by mechanically agitat- 7.2 Specimen Preparation–Pneumatic-Application:
C1373/C1373M − 23
FIG. 3 Schematic of Attic Test Module and Large Scale Climate Simulator Used for Tests on Attic Insulation Under Simulated Winter
Conditions
7.2.1 The procedure described in this section is intended for from the bag label, a density for test purposes is established by
all products, which normally are installed pneumatically. For the procedures outlined in Test Method C520 or Specification
materials exhibiting post installation settling, a supplemental
C739.
instruction set is provided in 7.3 to correct the test specimen
7.2.3 Calculate the target mass of insulation required to fill
blown density to accommodate for in-situ settling after instal-
the sample frame to the target thickness and density from the
lation.
equation:
7.2.2 Installed Density—The thermal resistances of loose-
m 5 ρ @ ~L × A ! 2 V # (1)
ins joist
fill insulations are specified using densities selected by manu-
facturers to represent the product settled densities. Generally, it
where:
is necessary to know the product thermal resistance at a
m = target mass of insulation, kg [lb],
3 3
representative density. Some bag labels utilize multiple densi-
ρ = target density, kg/m [lb/ft ],
ties to reflect the fact that greater thickness installations usually
L = target insulation thickness, m [ft],
ins
2 2
result in higher installed densities. The use of multiple densi-
A = area within sample frame, m [ft ], and
3 3
ties is detected from the bag label by calculating the label
V = volume of joists within frame area, m [ft ].
joist
density for several different R-value levels. Label densities for
7.2.4 Assemble the blowing machine, hose and hose length
a given R-value are calculated from the bag label by dividing
combination as appropriate for the material being prepared.
the minimum mass/unit area by the minimum thickness. If the
7.2.5 Set the blowing machine adjustments and select the
calculated densities are significantly different, the multiple
feed rates in accordance with the insulation manufacturer’s
density label has been used. When applicable specifications or
codes do not specify the density to be used for comparison recommendations. If the insulation manufacturer does not
provide this information, consult the machine manufacturer for
purposes, the recommended practice is to use the R-30 label
density (R(SI) – 5.3 m · K/W). If the density is not available recommended settings.
C1373/C1373M − 23
7.2.6 Place the required amount of insulation material 7.5 Installation of Other Insulations—Install the insulation
(7.2.3) into the blowing machine hopper. If the hopper is too following the manufacturer’s instructions as required to fill the
small to hold the entire amount required, fill the hopper to specimen frame to the required unit area mass and thickness.
capacity with the premixed insulation (see 6.1). Additional
8. Test Procedure
material is added as required during the blowing process until
the total amount of needed insulation is blown.
8.1 If the specimen holder is not already built into the
7.2.7 Turn on the blowing machine with the hose outlet
apparatus, transfer the specimen and holder in its horizontal
directed away from the cent
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C1373/C1373M − 11 (Reapproved 2017) C1373/C1373M − 23
Standard Practice for
Determination of Thermal Resistance of Attic Insulation
Systems Under Simulated Winter Conditions
This standard is issued under the fixed designation C1373/C1373M; the number immediately following the designation indicates the
year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last
reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice presents a laboratory procedure to determine the thermal resistance of attic insulation systems under simulated
steady-state winter conditions. The practice applies only to attic insulation systems that face an open attic air space.
1.2 The thermal resistance of the insulation is inferred from calculations based on measurements on a ceiling system consisting
of components consistent with the system being studied. For example, such a system might consist of a gypsum board or plywood
ceiling, wood ceiling joists, and attic insulation with its top exposed to an open air space. The temperature applied to the gypsum
board or plywood shall be in the range of 18 to 24°C [64 to 75°F]. The air temperature above the insulation shall correspond to
winter conditions and ranges from –46°C to 10°C [–51 to 50°F]. The gypsum board or plywood ceiling shall be sealed to prevent
direct airflow between the warm and cold sides of the system.
1.3 This practice applies to a wide variety of loose-fill or blanket thermal insulation products including fibrous glass, rock/slag
wool, or cellulosic fiber materials; granular types including vermiculite and perlite; pelletized products; and any other insulation
material that is installed pneumatically or poured in place. The practice considers the effects on heat transfer of structures,
specifically the ceiling joists, substrate, for example, gypsum board, air films, and possible facings, films, or other materials that
are used in conjunction with the insulation.
1.4 This practice measures the thermal resistance of the attic/ceiling system in which the insulation material has been
preconditioned according to the material Specifications C549, C665, C739, and C764.
1.5 The specimen preparation techniques outlined in this standard do not cover the characterization of loose-fill materials intended
for enclosed applications.
1.6 This practice is be used to characterize material behavior under controlled steady-state laboratory conditions intended to
simulate actual temperature conditions of use. The practice does not simulate forced air flow conditions.
1.7 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the
two systems may result in non-conformance with the standard.
1.7.1 All values shall be reported in both SI and inch-pound units unless specified otherwise by the client.
This practice is under the jurisdiction of ASTM Committee C16 on Thermal Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal Measurement.
Current edition approved May 1, 2017Sept. 1, 2023. Published May 2017October 2023. Originally approved in 1998. Last previous edition approved in 20112017 as
C1373/C1373M – 11.C1373/C1373M – 11 (2017). DOI: 10.1520/C1373_C1373M-11R17.10.1520/C1373_C1373M-23.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1373/C1373M − 23
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and healthsafety, health, and environmental practices and determine
the applicability of regulatory limitations prior to use.
1.9 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C167 Test Methods for Thickness and Density of Blanket or Batt Thermal Insulations
C168 Terminology Relating to Thermal Insulation
C177 Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the
Guarded-Hot-Plate Apparatus
C518 Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
C520 Test Methods for Density of Granular Loose Fill Insulations
C549 Specification for Perlite Loose Fill Insulation
C665 Specification for Mineral-Fiber Blanket Thermal Insulation for Light Frame Construction and Manufactured Housing
C687 Practice for Determination of Thermal Resistance of Loose-Fill Building Insulation
C739 Specification for Cellulosic Fiber Loose-Fill Thermal Insulation
C764 Specification for Mineral Fiber Loose-Fill Thermal Insulation
C1045 Practice for Calculating Thermal Transmission Properties Under Steady-State Conditions
C1058 Practice for Selecting Temperatures for Evaluating and Reporting Thermal Properties of Thermal Insulation
C1114 Test Method for Steady-State Thermal Transmission Properties by Means of the Thin-Heater Apparatus
C1363 Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus
3. Terminology
3.1 Definitions— Unless otherwise stated, the definitions listed in Terminology C168 are applicable herein.
4. Significance and Use
4.1 The thermal resistance of a ceiling system is used to characterize its steady-state thermal performance.
4.2 The thermal resistance of insulation is related to the density and thickness of the insulation. Test data on thermal resistance
are obtained at a thickness and density representative of the end use applications. In addition, the thermal resistance of the
insulation system will be different from that of the thermal insulation alone because of the system construction and materials.
4.3 This practice is needed because the in-service thermal resistance of some permeable attic insulations under winter conditions
is different, lower or higher R, than that measured at or close to simulated room temperature conditions utilizing small-scale tests
in which the insulation is sandwiched between two isothermal impermeable plates that have a temperature difference (ΔT) of 20
to 30°C [36 to 54°F]. When such insulation is installed in an attic, on top of a ceiling composed of normal building materials such
as gypsum board or plywood, with an open top surface exposed to the attic air space, the thermal resistance under winter conditions
with heat flow up and large temperature differences is significantly less because of additional heat transfer by natural convection.
Fig. 1 illustrates the difference between results from small scale tests and tests under the conditions of this practice. See Ref (1-12)
for discussions of this phenomenon.
4.4 In normal use, the thickness of insulation products ranges from 75 mm [3 in.] to 500 mm [20 in.]. Installed densities will
depend upon the product type, the installed thickness, the installation equipment used, the installation technique, and the geometry
of the insulated space.
4.5 The onset of natural convection under winter conditions is a function of specimen thickness for some materials. For purposes
of this practice, the tests shall be carried out at thicknesses at which the product is used.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
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NOTE 1—A constant hot-side temperature (T, hot) is used for both tests and the temperature difference increases as the cold side temperature (T, cold)
is decreased. See 5.1.6 for requirements on size of air space.
FIG. 1 Schematic of Thermal Resistance for a Permeable Attic Insulation Under Simulated Winter Conditions (Heat Flow Up)
4.6 Since this practice simulates winter conditions, the heat flow direction shall be vertically upwards.
4.7 Specimens shall be prepared in a manner consistent with the intended installation procedure. Products for pneumatic
installation shall be pneumatically-applied (blown), and products for pour-in-place installation shall be poured into place. See 5.2.
5. Equipment
5.1 Thermal test apparatus used for this practice shall meet the following requirements:
5.1.1 Conformance to Standards—The apparatus shall conform to all requirements of the ASTM thermal test method used, except
as required by 5.1.2 – 5.1.6.
5.1.2 Size—The apparatus shall be capable of testing specimens at the thickness intended for product use. Length and width of
the metering area shall be at least twice the spacing of the wood joists or four times the specimen thickness, whichever is greater
(see Fig. 2).
5.1.3 Temperature—The apparatus shall be capable of testing with the hot side surface maintained between 18 and 24°C [64 and
75°F], and with the cold side air temperature maintained near the winter condition for the particular climate being simulated, which
ranges from –46 to 10°C [–51 to 50°F]. In the absence of specified temperatures, the ambient temperatures listed in Table 2 of
C1058 on Temperatures for Thermal Transmittance Evaluations is one source of test temperatures.
NOTE 1—Only those with a hot ambient of 24°C [75°F] are applicable.
5.1.4 Humidity—The absolute humidity on both sides of the test apparatus shall be maintained low enough to prevent condensation
within the specimen. See 6.9.6 of Test Method C1363 for humidity requirements for the hot box methods, 6.6 of Test Method
C177 for the guarded hot plate method, and 7.10 of Test Method C518 for the heat flow meter apparatus.
5.1.5 Orientation and Direction of Heat Flow—The thermal test specimen shall be oriented horizontally with heat flow up.
5.1.6 Thermal Test Specimen and Holder—The test assembly shall be sized to match the test apparatus and shall be made of
construction materials representative of the intended application. The substrate on which the insulation rests shall be representative
of the intended application, typically gypsum board. The substrate shall be sealed to prevent direct airflow between the warm and
cold sides of the system. Wood joists also shall be included. The test assembly shall be constructed such that the top of the
insulation is open to an air space having a minimum thickness of 150 mm [6 in.]. Test Methods C1363 is preferred because of its
ability to accommodate a large air space. Other apparatuses that simulate in-service conditions must meet the requirements of this
practice, (for example, modifications of Test Methods C177, C518, or C1114 with Practice C1045). In all cases, the size
C1373/C1373M − 23
FIG. 2 Requirements on Dimensions of Test Specimen Metering Area
requirements given in 5.1.2 shall be met. Fig. 3 shows a schematic of an attic test module that has been used for these types of
tests. Other configurations without the roof structure are acceptable as long as the minimum 150 mm [6 in.] air space is maintained.
5.2 Specimen Preparation Equipment:
5.2.1 Blowing Apparatus—A blowing apparatus is required when pneumatically-applied specimens are to be tested. Choose the
combination of hopper, blower, hose size and length that is representative of common use for the application of the material to be
tested. The following machine specifications have been developed for use with mineral fiber and cellulosic materials.
5.2.1.1 A commercial blowing machine with a design capacity for delivering the subject material at a rate recommended by the
insulation manufacturer shall be used. The machine must utilize 46 m [150 ft] of flexible, internally corrugated blowing hose with
an appropriate sized diameter as specified by the machine manufacturer. At least 30 m [100 ft] of the hose must be elevated between
3 and 6 m [10 and 20 ft] above the blowing machine to simulate typical installation configuration. The hose must have no more
than eight 90° bends and no bends less than 1.2 m [4 ft] radius. It is good practice to clean the hose periodically by mechanically
agitating it with the blower operating. This practice dislodges any pieces of old insulation that might be caught in the hose.
6. Sampling
6.1 A sample of material shall be selected from a lot according to sampling plans given in the material specifications, regulations,
or other appropriate documents when applicable. In the absence of such directions, material from at least two randomly chosen
packages shall be combined in equal portions (mass) so as to combine materials as uniformly as practicable.
6.2 The insulation material is preconditioned to a moisture content in equilibrium with the laboratory conditions prior to the
specimen installation. Preconditioning of materials not only ensures controlled installation conditions but reduces the time required
to condition the prepared specimen prior to thermal testing. For conditioning requirements, see the applicable materials
Specifications C520, C549, C665, C739, and C764.
7. Specimen Preparation
7.1 General Instructions:
7.1.1 All specimens shall be prepared to a thickness and unit area mass that are given for the label R-value specification of interest
for the material under test.
7.1.2 Specimens shall be prepared in a manner consistent with the intended installation procedure. All materials shall be installed
carefully using the manufacturer’s recommended installation practice. Batts shall be cut, as required, to fit the available specimen
holder. Products for pneumatic installation shall be pneumatically-applied (blown), and products for pour-in-place installation shall
be poured into the specimen holder. See 7.2.2 for the density of pneumatically-installed insulation. Other materials must be
installed at the density suggested by the manufacturer.
7.1.3 The specimen holder shall represent typical attic frame construction, wherever possible. This requires, as a minimum,
horizontal members representing the bottom chord of a truss system or rafter framing and an air-tight gypsum board or plywood
bottom. The specimen holder shall be clean and free of insulation residue prior to installation of the sample insulation.
C1373/C1373M − 23
FIG. 3 Schematic of Attic Test Module and Large Scale Climate Simulator Used for Tests on Attic Insulation Under Simulated Winter
Conditions
NOTE 2—For commonly available loose-fill insulation, state and federal energy codes, ASTM material specifications and the Federal Trade Commission
have identified those materials that shall apply a correction for settling when determining thermal performance. It is beyond the scope of this practice
to outline the procedures for this determination.
NOTE 3—Many factors can influence the characteristics of the loose-fill insulation. These include blowing rate, machine adjustments, the size and length
of the hose, and the angle and dimensions of the hose outlet in relation to the specimen holder. Trained operators are required to duplicate field-installed
conditioning.
NOTE 4—For these tests, the specimen shall be blown close to the labeled density. Some operators may wish to establish a target mass of insulation
required to fill the test frame to the desired thickness and density as a control during the specimen preparation process. By weighing the initial material
and that remaining after blowing is complete, the operator can estimate the material in the test frame. Other operators may wish to eliminate these extra
steps. The reported test density, however, is obtained from the metering area density measurement conducted after the thermal test.
7.2 Specimen Preparation–Pneumatic-Application:
7.2.1 The procedure described in this section is intended for all products, which normally are installed pneumatically. For
materials exhibiting post installation settling, a supplemental instruction set is provided in 7.3 to correct the test specimen blown
density to accommodate for in-situ settling after installation.
7.2.2 Installed Density—The thermal resistances of loose-fill insulations are specified using densities selected by manufacturers
to represent the product settled densities. Generally, it is necessary to know the product thermal resistance at a representative
density. Some bag labels utilize multiple densities to reflect the fact that greater thickness installations usually result in higher
installed densities. The use of multiple densities is detected from the bag label by calculating the label density for several different
R-value levels. Label densities for a given R-value are calculated from the bag label by dividing the minimum mass/unit area by
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the minimum thickness. If the calculated densities are significantly different, the multiple density label has been used. When
applicable specifications or codes do not specify the density to be used for comparison purposes, the recommended practice is to
use the R-30 label density (R(SI) – 5.3 m · K/W). If the density is not available from the bag label, a density for test purposes is
established by the procedures outlined in Test Method C520 or Specification C739.
7.2.3 Calculate the target mass of insulation required to fill the sample frame to the target thickness and density from the equation:
m 5 ρ L ×A 2 V (1)
@ ~ ! #
ins joist
where:
m = target mass of insulation, kg [lb],
3 3
ρ = target density, kg/m [lb/ft ],
L = target insulation thickness, m [ft],
ins
2 2
A = area within sample frame, m [ft ], and
3 3
V = volume of joists within frame area, m [ft ].
joist
7.2.4 Assemble the blowing machine, hose and hose length combination as appropriate for the material being prepared.
7.2.5 Set the blowing machine adjustments and select the feed rates in accordance with the insulation manufacturer’s
recommendations. If the insulation manufacturer does not provide this information, consult the machine manufacturer for
recommended settings.
7.2.6 Place the required amount of insulation material (7.2.3) into the blowing machine hopper. If the hopper is too small to hold
the entire amount required, fill the hopper to capacity with the premixed insulation (see 6.1). Additional material is added as
required during the blowing process until the total amount of needed insulation is blown.
7.2.7 Turn on the blowing machine with the hose outlet directed away from the center of the specimen metering area and toward
the far end of the specimen holder. The hose outlet orientation is varied, side to side, as needed to cover uniformly the entire area
of the specimen holder, but shall remain approximately horizontal.
NOTE 5—Since the insulation material has been
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