ASTM C168-24

Standard Terminology Relating to Thermal Insulation

SCOPE
1.1 This standard provides definitions, symbols, units, and abbreviations of terms used in ASTM standards pertaining to thermal insulating materials, and to materials associated with them.  
1.2 This terminology is not intended to be used to classify insulation materials as having particular properties. Rather, classification of insulation materials is to be done by the material standards themselves.  
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 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.

General Information

• Status: Published
• Publication Date: 14-Apr-2024
• ICS: 01.040.91 - Construction materials and building (Vocabularies)
• ICS: 91.100.60 - Thermal and sound insulating materials
• Technical Committee: C16 - Thermal Insulation
• Drafting Committee: C16.94 - Terminology

Relations

• Replaces: ASTM C168-22 - Standard Terminology Relating to Thermal Insulation
• Effective Date: 15-Apr-2024
• Referred By: ASTM C552-22 - Standard Specification for Cellular Glass Thermal Insulation
• Effective Date: 15-Apr-2024
• Referred By: ASTM C534/C534M-23 - Standard Specification for Preformed Flexible Elastomeric Cellular Thermal Insulation in Sheet and Tubular Form
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1775-22 - Standard Specification for Laminate Protective Jacket and Tape for Use over Thermal Insulation for Outdoor Applications
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1902-22a - Standard Specification for Cellular Glass Insulation Used in Building and Roof Applications
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1574-04(2018) - Standard Guide for Determining Blown Density of Pneumatically Applied Loose-Fill Mineral Fiber Thermal Insulation
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1511-15(2021) - Standard Test Method for Determining the Water Retention (Repellency) Characteristics of Fibrous Glass Insulation (Aircraft Type)
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1484-10(2018) - Standard Specification for Vacuum Insulation Panels
• Effective Date: 15-Apr-2024
• Referred By: ASTM E3127-24 - Standard Guide for Specifying Water Vapor Transmission Material Properties of Water-Resistive Barriers and Air Barriers
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1410-17(2023) - Standard Specification for Cellular Melamine Thermal and Sound-Absorbing Insulation
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1136-23 - Standard Specification for Flexible, Low Permeance Vapor Retarders for Thermal Insulation
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1071-19 - Standard Specification for Fibrous Glass Duct Lining Insulation (Thermal and Sound Absorbing Material)
• Effective Date: 15-Apr-2024
• Referred By: ASTM C1427-21 - Standard Specification for Extruded Preformed Flexible Cellular Polyolefin Thermal Insulation in Sheet and Tubular Form
• Effective Date: 15-Apr-2024
• Referred By: ASTM C167-22 - Standard Test Methods for Thickness and Density of Blanket or Batt Thermal Insulations
• Effective Date: 15-Apr-2024
• Referred By: ASTM C196-00(2019) - Standard Specification for Expanded or Exfoliated Vermiculite Thermal Insulating Cement
• Effective Date: 15-Apr-2024

Overview

ASTM C168-24: Standard Terminology Relating to Thermal Insulation serves as a comprehensive reference for definitions, symbols, units, and abbreviations commonly used in ASTM standards on thermal insulating materials. Developed and maintained by ASTM Committee C16, this terminology standard ensures consistent language across ASTM publications related to thermal insulation and associated materials. ASTM C168 does not classify insulation materials by properties but instead provides clarity for terms and concepts referenced across international standards, supporting global trade and compliance efforts.

Key Topics

ASTM C168-24 covers a broad range of technical terms and standardized language critical to those specifying, designing, testing, or manufacturing thermal insulation. Major topics include:

• Thermal Conductivity and Resistivity: Definitions and discussions on intrinsic material properties and the concept of “apparent” values when multiple heat transfer modes are involved.
• Types of Insulating Materials: Terms for various materials such as aerogel, fibrous glass, mineral wool, perlite, cellular glass, polyurethane, and others, each with definitions encapsulating their structure and origin.
• Physical Properties: Standardized language for density, area weight, specific heat, diffusivity, and the methodologies for their measurement.
• Insulation Forms and Accessories: Precise terminology for forms like batts, blankets, block, loose fill, pipe insulation, and associated products such as facings, jackets, vapor barriers, and lagging.
• Heat Transfer Concepts: Definitions for concepts including conductance, heat flux, heat flow, emittance, reflectance, and steady-state conditions.
• Water Vapor Control: Comprehensive terms related to vapor permeability, permeance, moisture barriers, vapor stops, and associated concepts.

Applications

ASTM C168-24 is a foundational document for:

• Specification and Procurement: Ensuring clarity in construction and industrial project documentation by using standardized insulation terminology.
• Product Testing and Certification: Maintaining uniform communication regarding measurement methods and property descriptions in laboratory and certification settings.
• Design and Engineering: Allowing architects, engineers, and system designers to interpret and apply ASTM standards without ambiguity.
• Building Codes and Compliance: Facilitating regulatory acceptance and enforcement across jurisdictions by providing a recognized vocabulary.
• Training and Education: Equipping professionals, academics, and trainees with a common language for thermal insulation materials and systems.

By promoting consistent terminology, ASTM C168-24 improves technical communication and minimizes misinterpretation, thereby supporting accuracy and efficiency in the global insulation industry.

Related Standards

ASTM C168-24 references several key standards to align definitions and facilitate multi-disciplinary integration, including:

• ASTM D3574: Test Methods for Flexible Cellular Materials-Slab, Bonded, and Molded Urethane Foams.
• ASTM E456: Terminology Relating to Quality and Statistics.
• ASTM E2282: Guide for Defining the Test Result of a Test Method.
• ISO 7345: Thermal Insulation-Physical Quantities and Definitions.

Other closely related documents include ASTM standards on material specifications and test methods for various insulation materials and their accessories.

For the latest version and official documentation, users are encouraged to access ASTM C168-24 directly through the ASTM International website.

Keywords: ASTM C168-24, thermal insulation terminology, insulation definitions, ASTM standards, heat transfer, material properties, vapor barrier, insulation materials, technical vocabulary, insulation testing, insulation design.

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: C168 − 24
Standard Terminology Relating to
Thermal Insulation
This standard is issued under the fixed designation C168; 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 absorptance, n—the ratio of the radiant flux absorbed by a
body to that incident upon it.
1.1 This standard provides definitions, symbols, units, and
abbreviations of terms used in ASTM standards pertaining to
absorption, n—transformation of radiant energy to a different
thermal insulating materials, and to materials associated with
form of energy by interaction with matter.
them.
apparent thermal conductivity, λ , k , n—a thermal conduc-
a a
1.2 This terminology is not intended to be used to classify
tivity assigned to a material that exhibits thermal transmis-
insulation materials as having particular properties. Rather,
sion by several modes of heat transfer resulting in property
classification of insulation materials is to be done by the
variation with specimen thickness, or surface emittance. See
material standards themselves.
conductivity, thermal.
DISCUSSION—Thermal conductivity and resistivity are normally con-
1.3 This standard does not purport to address all of the
sidered to be intrinsic or specific properties of materials and, as such,
safety concerns, if any, associated with its use. It is the
should be independent of thickness. When nonconductive modes of
responsibility of the user of this standard to establish appro-
heat transfer are present within the specimen (radiation, free convec-
priate safety, health, and environmental practices and deter-
tion) this may not be the case. To indicate the possible presence of this
mine the applicability of regulatory limitations prior to use.
phenomena (for example, thickness effect) the modifier “apparent” is
1.4 This international standard was developed in accor-
used, as in apparent thermal conductivity.
dance with internationally recognized principles on standard-
DISCUSSION—Test data using the “apparent” modifier must be quoted
ization established in the Decision on Principles for the
only for the conditions of the measurement. Values of thermal conduc-
Development of International Standards, Guides and Recom-
tance (material C) and thermal resistance (material R) calculated from
mendations issued by the World Trade Organization Technical apparent thermal conductivity or resistivity, are valid only for the same
conditions.
Barriers to Trade (TBT) Committee.
DISCUSSION—Test data labeled with “apparent” shall not include any
2. Referenced Documents
equipment related measurement errors induced due to measurement
2.1 ASTM Standards:
attempts beyond an apparatus range or calibration.
D3574 Test Methods for Flexible Cellular Materials—Slab,
DISCUSSION—Use of the “apparent” modifier with system C or system
Bonded, and Molded Urethane Foams
R measurements is not permitted.
E456 Terminology Relating to Quality and Statistics
apparent thermal resistivity, r , n—a thermal resistivity
E2282 Guide for Defining the Test Result of a Test Method a
assigned to a material that exhibits thermal transmission by
2.2 ISO Standard:
several modes of heat transfer resulting in property variation
ISO 7345 Thermal Insulation—Physical Quantities and
with specimen thickness, or surface emittance. See
Definitions
resistivity, thermal.
3. Terminology
DISCUSSION—See entire discussion under apparent thermal conduc-
tivity.
3.1 Definitions:
area weight, n—weight per unit area for a specified sample, in
2 2
This terminology is under the jurisdiction of ASTM Committee C16 on units of lb/ft (kg/m ).
Thermal Insulation and is the direct responsibility of Subcommittee C16.94 on
Terminology. aerogel, n—a homogeneous, low-density solid phase material
Current edition approved April 15, 2024. Published April 2024. Originally
derived from a gel, in which the liquid component of the gel
approved in 1941. Last previous edition approved in 2022 as C168 – 22.
has been replaced with a gas.
DOI:101520/C0168-24.
2 DISCUSSION—The resulting material has a porous structure with an
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
average pore size below the mean free path of air molecules at standard
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on atmospheric pressure and temperature.
the ASTM website.
3 batt, n—blanket insulation manufactured to dimensions as
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. required by a specific application.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C168 − 24
blackbody, n—the ideal, perfect emitter and absorber of closed cell foam, n—a material comprised predominantly of
thermal radiation. It emits radiant energy at each wavelength individual non-interconnecting cellular voids.
at the maximum rate possible as a consequence of its
coating, n—a liquid or semiliquid that dries or cures to form a
temperature, and absorbs all incident radiance.
protective finish, suitable for application to thermal insula-
tion or other surfaces in thickness of 30 mils (0.76 mm) or
blanket, n—flexible insulation product, supplied rolled or flat.
less, per coat.
blanket insulation, n—a relatively flat and flexible insulation
conductance, film, n—the time rate of heat flow from a unit
in coherent sheet form furnished in units of substantial area.
area of a surface to its surroundings, induced by a unit
blanket insulation, metal mesh, n—blanket insulation cov-
temperature difference between the surface and the environ-
ered by flexible metal-mesh facings attached on one or both
ment.
sides.
DISCUSSION—The environment is a fluid (liquids or gases). h depends
on the nature of fluid motion past the surface (laminar or turbulent). (h
block insulation, n—rigid insulation preformed into rectangu-
in SI units: W/m ·K).
lar units.
conductance, thermal, C, n—the time rate of steady state heat
board insulation, n—semirigid insulation preformed into rect-
flow through a unit area of a material or construction induced
angular units having a degree of suppleness particularly
by a unit temperature difference between the body surfaces.
related to their geometrical dimensions.
C 5 q/ΔT
calcium silicate, n—insulation composed principally of hy-
A conductance (C) associated with a material shall be
drous calcium silicate, and which usually contains reinforc-
specified as a material C. A conductance (C) associated with
ing fibers.
a system or construction of materials shall be specified as a
system C. (C in SI units: W/m K.) (C in inch-pound units:
cellular elastomeric, n—insulation composed principally of
2 2
(Btu/h)/ft /F = Btu ⁄h ft F.)
natural or synthetic elastomers, or both, processed to form a
DISCUSSION—The average temperature of a surface is the area-
flexible, semirigid, or rigid foam which has a predominantly
weighted temperature of that surface.
closed-cell structure.
DISCUSSION—When the surfaces of a mass type thermal insulation are
cellular glass, n—insulation composed of glass processed to
not of equal areas, as in the case of thermal transmission in the radial
form a rigid foam having a predominantly closed-cell
direction, or are not of uniform separation (thickness), the surface area
structure. and thickness to which the conductance is assigned must be defined.
DISCUSSION—“Total” or “areal” thermal conductance are often used
cellular polyimide, n—insulation composed of the reaction
as synonyms for thermal conductance.
product in which the bonds formed between monomers
during polymerization are essentially imide units forming a
DISCUSSION—Thermal conductance and thermal resistance are recip-
cellular structure. rocals of one another.
DISCUSSION—See Discussion under resistance, thermal.
cellular polystyrene, n—insulation composed principally of
polymerized styrene resin processed to form a rigid foam
conductivity, thermal, λ or k, n—the time rate of steady state
having a predominantly closed-cell structure.
heat flow through a unit area of a homogeneous material
induced by a unit temperature gradient in a direction
cellular polyurethane, n—insulation composed principally of
perpendicular to that unit area. (λ or k in SI units: (W/m )/
the catalyzed reaction product of polyisocyanate and poly-
(K/m) = W ⁄m K.) (λ or k in inch-pound units: (Btu/h)/ft /(F/
hydroxy compounds, processed usually with fluorocarbon
2 2
ft) = Btu ⁄h ft F) or (Btu/h)/ft /(F/in.) = Btu in./h ft F.) (See
gas to form a rigid foam having a predominantly closed-cell
discussion under apparent thermal conductivity.)
structure.
DISCUSSION—Thermal conductivity testing is usually done in one of
two apparatus/specimen geometries: flat-slab specimens with parallel
cellulosic fiber, n—insulation composed principally of cellu-
heat flux lines, or cylindrical specimens with radial heat flux lines. The
lose fibers usually derived from paper, paperboard stock, or
operational definitions of thermal conductivity for these two cases are
wood, with or without binders.
given as follows:
cement, finishing, n—a mixture of dry fibrous or powdery
Q L
Flat 2 slab geometry λ 5 (1)
materials, or both, that when mixed with water develops a
A ΔT
plastic consistency, and when dried in place forms a rela-
where:
tively hard, protective surface.
Q = heat flow rate,
cement, insulating, n—a mixture of dry granular, flaky,
A = area through which Q passes, and
fibrous, or powdery materials that when mixed with water
L = thickness of the flat-slab specimen across which the
develops a plastic consistency, and when dried in place
temperature difference ΔT exists.
forms a coherent covering that affords substantial resistance
The ΔT/L ratio approximates the temperature gradient.
to heat transmission.
Q r
Cylindrical geometry λ 5 log (2)
e
cladding, n—See jacket (as related to insulation jacketing). 2πlΔT r
C168 − 24
where: the temperature corresponding to saturation (100 % relative
humidity) for a given absolute humidity at constant pressure.
1 = length,
r = the outer radius, and
diatomaceous silica, n—insulation composed principally of
r = the inner radius of the cylinder.
diatomaceous earth with or without binders, and which
Eq 1 and Eq 2 are actually special-case simplifications of the more general
usually contains reinforcing fibers.
definition:
thermal conductivity, λ—a tensor property defined by the tensor equation: diffusivity, thermal, n—the ratio of thermal conductivity of a
substance to the product of its density and specific heat. (In
q 5 2λΔT (3)
3 2
SI units: (W/(m•K))/((kg/m )•(J/(kg•K))) = m /s.) (In inch-
where q is the heat flux vector, and ΔT (grad T) is the temperature gra-
3 2
pound units: (Btu/(hr•ft) F)/((lb/ft )(Btu/(lb•F)) = ft /hr.)
dient vector. Except in theoretical discussions, this generalized form of
the definition is seldom used. For experimental situations, the geometry
emittance, ε, n—the ratio of the radiant flux emitted by a
of the testing apparatus and the specimen are chosen such that Eq 3
specimen to that emitted by a blackbody at the same
reduces to the one-dimensional scalar equation:
temperature and under the same conditions.
dT
Q 5 2Aλ (4)
emittance, directional ε(θ; φ), n—the ratio of the radiance
du
from a surface in a particular direction to the radiance from
where:
a blackbody at the same temperature under the same
Q = heat flow rate,
conditions.
A = area through which Q passes,
emittance, hemispherical ε or ε(2π), n—the average direc-
λ = thermal conductivity, and
H
dT/du = the temperature gradient in the direction of heat tional emittance over a hemispherical envelope covering a
flow. surface.
At steady state, Eq 1 and Eq 2 are consistent with Eq 4 if ΔT is sufficiently
emittance, spectral ε or ε(λ; θ;φ ), n—an emittance based on
λ
small. If ΔT is not sufficiently small, then Eq 1 and Eq 2 define a mean
the radiant energy emitted per unit wavelength interval
thermal conductivity over the ΔT range, and this range in addition to the
(monochromatic radiant energy).
mean temperature should be stated.
DISCUSSION—Where necessary to avoid confusion, emittances should
be designated by subscripts, for example: ε , ε , ε , ε , ε . For
DISCUSSION—If the measured thermal property indicates that other HT Hλ Nλ θλ HT
most engineering purposes, the hemispherical total emittance ε
than conductive heat flows are present, as evidenced by dependence on HT
suffices.
specimen thickness, air flow, or emittance of bounding surfaces, then
this definition does not apply. See also, apparent thermal conductiv-
emittance, total ε or ε(t), n—an emittance that is an
T
ity.
integrated average over all wavelengths of radiant energy
DISCUSSION—Thermal conductivity and thermal resistivity are recip-
emitted.
rocals of one another.
facer, n—the outermost adhered top or bottom, or both, sheet
DISCUSSION—As an additional reference and discussion along similar
(or layer) of an insulation product, rigid or flexible, that is
lines, see the International Standard ISO 7345 Annex.
comprised of a different material than the insulation itself.
corrosion retarder (as related to insulation jacketing), n—See
facing, n—a thin covering adhered to the surface of insulation
moisture barrier (as related to insulation jacketing).
prior to field installation.
coverage, n—the area to be covered per unit volume of coating
felt, n—a textile structure characterized by interlocking and
to obtain specified dry thickness and desired performance.
consolidation of its constituent fibers.
DISCUSSION—Felts can be made with or without binders.
covering capacity, dry, n—the area covered to a dry thickness
of 1 in. (25 mm) by 100 lb (45.4 kg) of dry cement when
fiber degradation, n—an irreversible change in the physical
mixed with the recommended amount of water, molded and
structure of fibers that has a negative impact on performance.
dried to constant weight.
DISCUSSION—Changes observed are typically due to elevated tem-
peratures and are observed as melted, fused, fractured, changed to a
covering capacity, wet, n—the area covered to a wet thickness
crystalline structure, or increased brittleness, or a combination thereof.
of 1 in. (25 mm) by 100 lb (45.4 kg) of dry cement when
mixed with the recommended amount of water, and molded. fibrous glass, n—A synthetic vitreous fiber insulation made by
melting predominantly silica sand and other inorganic
density,ρ, n—the mass per unit volume of a material. (ρ in SI
materials, and then physically forming the melt into fibers.
units: kg/m3.) (ρ in inch-pound units: lb/ft3.)
DISCUSSION—Commonly referred to as fiber glass.
DISCUSSION—The term mass is used and not weight, due to the
DISCUSSION—To form an insulation product, there are often other
buoyancy effect of some low density closed cell insulations.
materials applied to the fibrous glass such as binders, oils, etc.
density, apparent (of applied insulation), n—the mass per
flexible cellular material, n—a cellular material that will not
unit volume of in-place mass thermal insulation.
rupture within a specified time when bent around a mandrel
dewpoint temperature, n—the temperature at which conden-
at a specified uniform temperature and rate.
sation of water vapor in a space begins for a given state of
DISCUSSION—Test Methods D3574 “Standard Test Methods for Flex-
humidity and pressure as the vapor temperature is reduced; ible Cellular Materials – Slab, Bonded and Molded Urethane Foams”
C168 − 24
provides a standard procedure for assessing whether an insulation
knit-line, n—a localized region within a foamed plastic
material is a flexible cellular material.
insulation, typically noted as a “line” in a cross-section, that
forms during manufacturing processes when concurrent
graybody, n—a body having the same spectral emittance at all
flows or streams, or consecutive applications of foam plastic,
wavelengths.
meet.
glass fiber, n—fiber manufactured as continuous filament from
lagging-covering, n—See jacket (as related to insulation
molten glass, normally used for reinforcement, tissue or
jacketing).
textiles.
DISCUSSION—lagging-insulation is usually applied in the form of cut,
pieced together or mitered parts.
glass wool, n—See fibrous glass.
lagging-insulation, n—Insulation used on pipe, tanks, ducts,
heat flow; heat flow rate, Q, n—the quantity of heat trans-
vessels, or other mechanical equipment.
ferred to or from a system in unit time. (Q in SI units: W.) (Q
DISCUSSION—Lagging-insulation is usually applied in the form of cut,
in inch-pound units: Btu/h.)
pieced together or mitered parts.
DISCUSSION—See heat flux for the areal dependence.
laminate jacket—a thin, flexible sheet material intended for
DISCUSSION—This definition is different than that given in some
˙
textbooks, which may use Q, or q to represent heat flow rate. The ISO use as a jacket over thermal insulation on pipe, duct, or
definition uses Φ.
equipment, and consisting of multiple layers of polymer film
and aluminum foil bonded together.
heat flux, q, n—the heat flow rate through a surface of unit area
DISCUSSION—A laminate jacket is available with or without a factory
perpendicular to the direction of heat flow.
applied pressure sensitive adhesive.
(q in SI units: W/m )
2 2
DISCUSSION—Laminate jacket is commercially available in different
(q in inch-pound units: Btu/h/ft = Btu ⁄h ft )
widths, it typically is provided in approximate widths of pipe insulation
DISCUSSION—This definition has been used as heat flux density, or
sect
...

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: C168 − 22 C168 − 24
Standard Terminology Relating to
Thermal Insulation
This standard is issued under the fixed designation C168; 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 standard provides definitions, symbols, units, and abbreviations of terms used in ASTM standards pertaining to thermal
insulating materials, and to materials associated with them.
1.2 This terminology is not intended to be used to classify insulation materials as having particular properties. Rather,
classification of insulation materials is to be done by the material standards themselves.
1.3 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 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:
D3574 Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams
E456 Terminology Relating to Quality and Statistics
E2282 Guide for Defining the Test Result of a Test Method
2.2 ISO Standard:
ISO 7345 Thermal Insulation—Physical Quantities and Definitions
3. Terminology
3.1 Definitions:
absorptance, n—the ratio of the radiant flux absorbed by a body to that incident upon it.
absorption, n—transformation of radiant energy to a different form of energy by interaction with matter.
apparent thermal conductivity, λ ,k ,n—a thermal conductivity assigned to a material that exhibits thermal transmission by
a a
several modes of heat transfer resulting in property variation with specimen thickness, or surface emittance. See conductivity,
thermal.
This terminology is under the jurisdiction of ASTM Committee C16 on Thermal Insulation and is the direct responsibility of Subcommittee C16.94 on Terminology.
Current edition approved May 1, 2022April 15, 2024. Published May 2022April 2024. Originally approved in 1941. Last previous edition approved in 20192022 as
C168 – 19.C168 – 22. DOI:101520DOI:101520/C0168-24.⁄C0168-22.
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.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C168 − 24
DISCUSSION—
Thermal conductivity and resistivity are normally considered to be intrinsic or specific properties of materials and, as such, should be independent of
thickness. When nonconductive modes of heat transfer are present within the specimen (radiation, free convection) this may not be the case. To indicate
the possible presence of this phenomena (for example, thickness effect) the modifier “apparent” is used, as in apparent thermal conductivity.
DISCUSSION—
Test data using the “apparent” modifier must be quoted only for the conditions of the measurement. Values of thermal conductance (material C) and
thermal resistance (material R) calculated from apparent thermal conductivity or resistivity, are valid only for the same conditions.
DISCUSSION—
Test data labeled with “apparent” shall not include any equipment related measurement errors induced due to measurement attempts beyond an
apparatus range or calibration.
DISCUSSION—
Use of the “apparent” modifier with system C or system R measurements is not permitted.
apparent thermal resistivity, r ,n—a thermal resistivity assigned to a material that exhibits thermal transmission by several
a
modes of heat transfer resulting in property variation with specimen thickness, or surface emittance. See resistivity, thermal.
DISCUSSION—
See entire discussion under apparent thermal conductivity.
2 2
area weight, n—weight per unit area for a specified sample, in units of lb/ft (kg/m ).
aerogel, n—a homogeneous, low-density solid phase material derived from a gel, in which the liquid component of the gel has
been replaced with a gas.
DISCUSSION—
The resulting material has a porous structure with an average pore size below the mean free path of air molecules at standard atmospheric pressure
and temperature.
batt, n—blanket insulation manufactured to dimensions as required by a specific application.
blackbody, n—the ideal, perfect emitter and absorber of thermal radiation. It emits radiant energy at each wavelength at the
maximum rate possible as a consequence of its temperature, and absorbs all incident radiance.
blanket, n—flexible insulation product, supplied rolled or flat.
blanket insulation, n—a relatively flat and flexible insulation in coherent sheet form furnished in units of substantial area.
blanket insulation, metal mesh, n—blanket insulation covered by flexible metal-mesh facings attached on one or both sides.
block insulation, n—rigid insulation preformed into rectangular units.
board insulation, n—semirigid insulation preformed into rectangular units having a degree of suppleness particularly related
to their geometrical dimensions.
calcium silicate, n—insulation composed principally of hydrous calcium silicate, and which usually contains reinforcing fibers.
cellular elastomeric, n—insulation composed principally of natural or synthetic elastomers, or both, processed to form a
flexible, semirigid, or rigid foam which has a predominantly closed-cell structure.
cellular glass, n—insulation composed of glass processed to form a rigid foam having a predominantly closed-cell structure.
cellular polyimide, n—insulation composed of the reaction product in which the bonds formed between monomers during
polymerization are essentially imide units forming a cellular structure.
C168 − 24
cellular polystyrene, n—insulation composed principally of polymerized styrene resin processed to form a rigid foam having
a predominantly closed-cell structure.
cellular polyurethane, n—insulation composed principally of the catalyzed reaction product of polyisocyanate and polyhydroxy
compounds, processed usually with fluorocarbon gas to form a rigid foam having a predominantly closed-cell structure.
cellulosic fiber, n—insulation composed principally of cellulose fibers usually derived from paper, paperboard stock, or wood,
with or without binders.
cement, finishing, n—a mixture of dry fibrous or powdery materials, or both, that when mixed with water develops a plastic
consistency, and when dried in place forms a relatively hard, protective surface.
cement, insulating, n—a mixture of dry granular, flaky, fibrous, or powdery materials that when mixed with water develops a
plastic consistency, and when dried in place forms a coherent covering that affords substantial resistance to heat transmission.
cladding,n—See jacket(as related to insulation jacketing).
closed cell foam, n—a material comprised predominantly of individual non-interconnecting cellular voids.
coating, n—a liquid or semiliquid that dries or cures to form a protective finish, suitable for application to thermal insulation
or other surfaces in thickness of 30 mils (0.76 mm) or less, per coat.
conductance, film, n—the time rate of heat flow from a unit area of a surface to its surroundings, induced by a unit temperature
difference between the surface and the environment.
DISCUSSION—
The environment is a fluid (liquids or gases). h depends on the nature of fluid motion past the surface (laminar or turbulent). (h in SI units: W/m ·K).
conductance, thermal, C,n—the time rate of steady state heat flow through a unit area of a material or construction induced
by a unit temperature difference between the body surfaces.
C 5 q/ΔT
A conductance (C) associated with a material shall be specified as a material C. A conductance (C) associated with a system
2 2
or construction of materials shall be specified as a system C. (C in SI units: W/m K.) (C in inch-pound units: (Btu/h)/ft /
F = Btu ⁄h ft F.)
DISCUSSION—
The average temperature of a surface is the area-weighted temperature of that surface.
DISCUSSION—
When the surfaces of a mass type thermal insulation are not of equal areas, as in the case of thermal transmission in the radial direction, or are not
of uniform separation (thickness), the surface area and thickness to which the conductance is assigned must be defined.
DISCUSSION—
“Total” or “areal” thermal conductance are often used as synonyms for thermal conductance.
DISCUSSION—
Thermal conductance and thermal resistance are reciprocals of one another.
DISCUSSION—
See Discussion under resistance, thermal.
conductivity, thermal, λ ork,n—the time rate of steady state heat flow through a unit area of a homogeneous material induced
by a unit temperature gradient in a direction perpendicular to that unit area. (λ or k in SI units: (W/m )/(K/m) = W ⁄m K.) (λ or
2 2 2
k in inch-pound units: (Btu/h)/ft /(F/ft) = Btu ⁄h ft F) or (Btu/h)/ft /(F/in.) = Btu in./h ft F.) (See discussion under apparent
thermal conductivity.)
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DISCUSSION—
Thermal conductivity testing is usually done in one of two apparatus/specimen geometries: flat-slab specimens with parallel heat flux lines, or
cylindrical specimens with radial heat flux lines. The operational definitions of thermal conductivity for these two cases are given as follows:
Q L
Flat 2 slab geometry λ5 (1)
A ΔT
where:
Q = heat flow rate,
A = area through which Q passes, and
L = thickness of the flat-slab specimen across which the temperature difference ΔT exists.
The ΔT/L ratio approximates the temperature gradient.
Q r
Cylindrical geometry λ5 log (2)
e
2πlΔT r
where:
1 = length,
r = the outer radius, and
r = the inner radius of the cylinder.
Eq 1 and Eq 2 are actually special-case simplifications of the more general definition:
thermal conductivity, λ—a tensor property defined by the tensor equation:
q 52λΔT (3)
where q is the heat flux vector, and ΔT (grad T) is the temperature gradient vector. Except in theoretical discussions, this generalized form of the
definition is seldom used. For experimental situations, the geometry of the testing apparatus and the specimen are chosen such that Eq 3 reduces to
the one-dimensional scalar equation:
dT
Q 52Aλ (4)
du
where:
Q = heat flow rate,
A = area through which Q passes,
λ = thermal conductivity, and
dT/du = the temperature gradient in the direction of heat flow.
At steady state, Eq 1 and Eq 2 are consistent with Eq 4 if ΔT is sufficiently small. If ΔT is not sufficiently small, then Eq 1 and Eq 2 define a mean thermal
conductivity over the ΔT range, and this range in addition to the mean temperature should be stated.
DISCUSSION—
If the measured thermal property indicates that other than conductive heat flows are present, as evidenced by dependence on specimen thickness, air
flow, or emittance of bounding surfaces, then this definition does not apply. See also, apparent thermal conductivity.
DISCUSSION—
Thermal conductivity and thermal resistivity are reciprocals of one another.
DISCUSSION—
As an additional reference and discussion along similar lines, see the International Standard ISO 7345 Annex.
corrosion retarder (as related to insulation jacketing),n—See moisture barrier (as related to insulation jacketing).
coverage, n—the area to be covered per unit volume of coating to obtain specified dry thickness and desired performance.
covering capacity, dry, n—the area covered to a dry thickness of 1 in. (25 mm) by 100 lb (45.4 kg) of dry cement when mixed
with the recommended amount of water, molded and dried to constant weight.
covering capacity, wet, n—the area covered to a wet thickness of 1 in. (25 mm) by 100 lb (45.4 kg) of dry cement when mixed
with the recommended amount of water, and molded.
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density,ρ, n—the mass per unit volume of a material. (ρ in SI units: kg/m3.) (ρ in inch-pound units: lb/ft3.)
DISCUSSION—
The term mass is used and not weight, due to the buoyancy effect of some low density closed cell insulations.
density, apparent (of applied insulation), n—the mass per unit volume of in-place mass thermal insulation.
dewpoint temperature, n—the temperature at which condensation of water vapor in a space begins for a given state of humidity
and pressure as the vapor temperature is reduced; the temperature corresponding to saturation (100 % relative humidity) for a
given absolute humidity at constant pressure.
diatomaceous silica, n—insulation composed principally of diatomaceous earth with or without binders, and which usually
contains reinforcing fibers.
diffusivity, thermal, n—the ratio of thermal conductivity of a substance to the product of its density and specific heat. (In SI
3 2 3 2
units: (W/(m•K))/((kg/m )•(J/(kg•K))) = m /s.) (In inch-pound units: (Btu/(hr•ft) F)/((lb/ft )(Btu/(lb•F)) = ft /hr.)
emittance, ε, n—the ratio of the radiant flux emitted by a specimen to that emitted by a blackbody at the same temperature and
under the same conditions.
emittance, directional ε(θ; φ), n—the ratio of the radiance from a surface in a particular direction to the radiance from a
blackbody at the same temperature under the same conditions.
emittance, hemispherical ε or ε(2π), n—the average directional emittance over a hemispherical envelope covering a surface.
H
emittance, spectral ε or ε(λ; θ;φ ), n—an emittance based on the radiant energy emitted per unit wavelength interval
λ
(monochromatic radiant energy).
DISCUSSION—
Where necessary to avoid confusion, emittances should be designated by subscripts, for example: ε , ε , ε , ε , ε . For most engineering
HT Hλ Nλ θλ HT
purposes, the hemispherical total emittance ε suffices.
HT
emittance, total ε or ε(t), n—an emittance that is an integrated average over all wavelengths of radiant energy emitted.
T
facer, n—the outermost adhered top or bottom, or both, sheet (or layer) of an insulation product, rigid or flexible, that is
comprised of a different material than the insulation itself.
facing, n—a thin covering adhered to the surface of insulation prior to field installation.
felt, n—a textile structure characterized by interlocking and consolidation of its constituent fibers.
DISCUSSION—
Felts can be made with or without binders.
fiber degradation, n—an irreversible change in the physical structure of fibers that has a negative impact on performance.
DISCUSSION—
Changes observed are typically due to elevated temperatures and are observed as melted, fused, fractured, changed to a crystalline structure, or
increased brittleness, or a combination thereof.
fibrous glass, n—A synthetic vitreous fiber insulation made by melting predominantly silica sand and other inorganic materials,
and then physically forming the melt into fibers.
DISCUSSION—
Commonly referred to as fiber glass.
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DISCUSSION—
To form an insulation product, there are often other materials applied to the fibrous glass such as binders, oils, etc.
flexible cellular material, n—a cellular material that will not rupture within a specified time when bent around a mandrel at a
specified uniform temperature and rate.
DISCUSSION—
Test Methods D3574 “Standard Test Methods for Flexible Cellular Materials – Slab, Bonded and Molded Urethane Foams” provides a standard
procedure for assessing whether an insulation material is a flexible cellular material.
graybody, n—a body having the same spectral emittance at all wavelengths.
glass fiber, n—fiber manufactured as continuous filament from molten glass, normally used for reinforcement, tissue or textiles.
glass wool,n—See fibrous glass.
heat flow; heat flow rate, Q,n—the quantity of heat transferred to or from a system in unit time. (Q in SI units: W.) (Q in
inch-pound units: Btu/h.)
DISCUSSION—
See heat flux for the areal dependence.
DISCUSSION—
This definition is different than that given in some textbooks, which may use Q˙, or q to represent heat flow rate. The ISO definition uses Φ.
heat flux, q,n—the heat flow rate through a surface of unit area perpendicular to the direction of heat flow.
(q in SI units: W/m )
2 2
(q in inch-pound units: Btu/h/ft = Btu ⁄h ft )
DISCUSSION—
This definition has been used as heat flux density, or density of heat flow rate (defined as areal density of heat flow rate by ISO).
heat flux transducer, HFT, n—a device containing a thermopile (or equivalent) that produces an output which is a function of
the heat flux.
DISCUSSION—
In the past this device may also have been known as a heat flow meter, heat flux meter, heat flow sensor, or heat flux sensor.
DISCUSSION—
The HFT output may also be a function of mean temperature, attachment, application, and environmental situation.
homogeneous material, n—a material in which relevant properties are not a function of the position within the material.
DISCUSSION—
Homogeneity depends on the scale of the volume element used to examine the material. The purposes of Committee C16 are best suited if a
macroscopic viewpoint is taken such that the standard insulating materials are considered homogeneous (for example, fibrous and cellular insulations),
at least in the heat flow direction and time frame involved in a thermal test.
DISCUSSION—
Relevant properties may be a function of such variables
...