ASTM C168-00

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Designation: C 168 – 00
Standard Terminology Relating to
Thermal Insulation
This standard is issued under the fixed designation C 168; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope apparent thermal resistivity, r , n—a thermal resistivity
a
assigned to a material that exhibits thermal transmission by
1.1 This standard provides definitions, symbols, units, and
several modes of heat transfer resulting in property variation
abbreviations of terms used in ASTM standards pertaining to
with specimen thickness, or surface emittance. See resistiv-
thermal insulating materials, and to materials associated with
ity, thermal.
them.
DISCUSSION—See entire discussion under apparent thermal conduc-
2. Referenced Documents
tivity.
2.1 ISO Standard:
blackbody, n—the ideal, perfect emitter and absorber of
ISO 7345 Thermal Insulation—Physical Quantities and
thermal radiation. It emits radiant energy at each wavelength
Definitions
at the maximum rate possible as a consequence of its
temperature, and absorbs all incident radiance.
3. Terminology
blanket insulation, n—a relatively flat and flexible insulation
3.1 Definitions:
in coherent sheet form furnished in units of substantial area.
absorptance, n—the ratio of the radiant flux absorbed by a
blanket insulation, metal mesh, n— blanket insulation cov-
body to that incident upon it.
ered by flexible metal-mesh facings attached on one or both
absorption, n—transformation of radiant energy to a different
sides.
form of energy by interaction with matter.
block insulation, n—rigid insulation preformed into rectangu-
apparent thermal conductivity, l , k , n—a thermal con-
lar units.
a a
ductivity assigned to a material that exhibits thermal trans-
board insulation, n—semirigid insulation preformed into
mission by several modes of heat transfer resulting in
rectangular units having a degree of suppleness particularly
property variation with specimen thickness, or surface emit-
related to their geometrical dimensions.
tance. See conductivity, thermal.
calcium silicate, n—insulation composed principally of hy-
drous calcium silicate, and which usually contains reinforc-
DISCUSSION—Thermal conductivity and resistivity are normally con-
ing fibers.
sidered to be intrinsic or specific properties of materials and, as such,
cellular elastomeric, n—insulation composed principally of
should be independent of thickness. When nonconductive modes of
heat transfer are present within the specimen (radiation, free convec- natural or synthetic elastomers, or both, processed to form a
tion) this may not be the case. To indicate the possible presence of this
flexible, semirigid, or rigid foam which has a predominantly
phenomena (for example, thickness effect) the modifier “apparent” is
closed-cell structure.
used, as in apparent thermal conductivity.
cellular glass, n—insulation composed of glass processed to
DISCUSSION—Test data using the “apparent” modifier must be quoted
form a rigid foam having a predominantly closed-cell
only for the conditions of the measurement. Values of thermal conduc-
structure.
tance (material C) and thermal resistance (material R) calculated from
apparent thermal conductivity or resistivity, are valid only for the same cellular polystyrene, n—insulation composed principally of
conditions.
polymerized styrene resin processed to form a rigid foam
DISCUSSION—Test data labeled with “apparent” shall not include any
having a predominantly closed-cell structure.
equipment related measurement errors induced due to measurement
cellular polyurethane, n—insulation composed principally of
attempts beyond an apparatus range or calibration.
the catalyzed reaction product of polyisocyanate and poly-
DISCUSSION—Use of the “apparent” modifier with system C or system
hydroxy compounds, processed usually with fluorocarbon
R measurements is not permitted.
gas to form a rigid foam having a predominantly closed-cell
structure.
This terminology is under the jurisdiction of ASTM Committee C16 on
cellulosic fiber, n—insulation composed principally of cellu-
Thermal Insulation and is the direct responsibility of Subcommittee C16.94 on
lose fibers usually derived from paper, paperboard stock, or
Terminology.
Current edition approved June 10, 2000. Published September 2000. Originally wood, with or without binders.
published as C 168 – 41 T. Last previous edition C 168 – 97.
cement, finishing, n—a mixture of dry fibrous or powdery
Available from American National Standards Institute, 11 W. 42nd St., 13th
materials, or both, that when mixed with water develops a
Floor, New York, NY 10036.
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C 168
plastic consistency, and when dried in place forms a rela-
A = area through which Q passes, and
tively hard, protective surface.
L = thickness of the flat-slab specimen across which the
cement, insulating, n—a mixture of dry granular, flaky,
temperature difference DT exists.
fibrous, or powdery materials that when mixed with water
develops a plastic consistency, and when dried in place
The DT/L ratio approximates the temperature gradient.
forms a coherent covering that affords substantial resistance
Q r
to heat transmission. Cylindrical geometryl5 log (2)
e
2plDT r
coating, n—a liquid or semiliquid that dries or cures to form a
protective finish, suitable for application to thermal insula-
where:
tion or other surfaces in thickness of 30 mils (0.76 mm) or
1 = length,
less, per coat.
r = the outer radius, and
conductance, film, n—the time rate of heat flow from a unit
r = the inner radius of the cylinder.
area of a surface to its surroundings, induced by a unit
temperature difference between the surface and the environ-
ment.
Eq 1 and Eq 2 are actually special-case simplifications of the
DISCUSSION—The environment is a fluid (liquids or gases). h depends
more general definition:
on the nature of fluid motion past the surface (laminar or turbulent). (h
thermal conductivity, l—a tensor property defined by the
in SI units: W/m ·K).
tensor equation:
conductance, thermal, C, n—the time rate of steady state heat
q 5 2lDT (3)
flow through a unit area of a material or construction induced
where q is the heat flux vector, and D T (grad T)isthe
by a unit temperature difference between the body surfaces.
temperature gradient vector. Except in theoretical discussions,
C 5 q/DT
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
A conductance (C) associated with a material shall be
one-dimensional scalar equation:
specified as a material C. A conductance ( C) associated with a
system or construction of materials shall be specified as a
dT
Q52Al (4)
du
system C.(C in SI units: W/m K.) (C in inch-pound units:
2 2
(Btu/h)/ft /F = Btu/h ft F.)
DISCUSSION—The average temperature of a surface is the area- where:
weighted temperature of that surface.
Q = heat flow rate,
DISCUSSION—When the surfaces of a mass type thermal insulation are
A = area through which Q passes,
not of equal areas, as in the case of thermal transmission in the radial
l = thermal conductivity, and
direction, or are not of uniform separation (thickness), the surface area
dT/du = the temperature gradient in the direction of heat
and thickness to which the conductance is assigned must be defined.
flow.
DISCUSSION—“Total’’ or “areal’’ thermal conductance are often used
as synonyms for thermal conductance.
At steady state, Eq 1 and Eq 2 are consistent with Eq 4 if D T
DISCUSSION—Thermal conductance and thermal resistance are recip-
is sufficiently small. If DT is not sufficiently small, then Eq 1
rocals of one another.
and Eq 2 define a mean thermal conductivity over the DT
DISCUSSION—See Discussion under resistance, thermal.
range, and this range in addition to the mean temperature
conductivity, thermal, l or k, n—the time rate of steady state should be stated.
heat flow through a unit area of a homogeneous material
DISCUSSION—If the measured thermal property indicates that other
induced by a unit temperature gradient in a direction
than conductive heat flows are present, as evidenced by dependence on
perpendicular to that unit area. (l or k in SI units: (W/m )/
specimen thickness, air flow, or emittance of bounding surfaces, then
(K/m) = W/m K.) (l or k in inch-pound units: (Btu/h)/ft this definition does not apply. See also, apparent thermal conductiv-
2 2
ity.
2/(F/ft) = Btu/h ft F) or (Btu/h)/ft /(F/in.) = Btu in./h ft F.)
DISCUSSION—Thermal conductivity and thermal resistivity are recip-
(See discussion under apparent thermal conductivity.)
rocals of one another.
DISCUSSION—Thermal conductivity testing is usually done in one of
DISCUSSION—As an additional reference and discussion along similar
two apparatus/specimen geometries: flat-slab specimens with parallel
lines, see the International Standard ISO 7345 Annex.
heat flux lines, or cylindrical specimens with radial heat flux lines. The
operational definitions of thermal conductivity for these two cases are
coverage, n—the area to be covered per unit volume of coating
given as follows:
to obtain specified dry thickness and desired performance.
covering capacity, dry, n—the area covered to a dry thickness
Q L
Flat2slab geometryl5 (1)
of 1 in. (25 mm) by 100 lb (45.4 kg) of dry cement when
A DT
mixed with the recommended amount of water, molded and
dried to constant weight.
where:
covering capacity, wet, n—the area covered to a wet thickness
Q = heat flow rate,
of 1 in. (25 mm) by 100 lb (45.4 kg) of dry cement when
C 168
heat flow meter, heat flux meter, heat flow sensor, or heat flux sensor.
mixed with the recommended amount of water, and molded.
DISCUSSION—The HFT output may also be a function of mean
density, apparent (of applied insulation), n—the mass per
temperature, attachment, application, and environmental situation.
unit volume of in-place mass thermal insulation.
dewpoint temperature, n—the temperature at which conden-
homogeneous material, n—a material in which relevant
sation of water vapor in a space begins for a given state of
properties are not a function of the position within the
humidity and pressure as the vapor temperature is reduced;
material.
the temperature corresponding to saturation (100 % relative
DISCUSSION—Homogeneity depends on the scale of the volume
humidity) for a given absolute humidity at constant pressure.
element used to examine the material. The purposes of Committee C-16
diatomaceous silica, n—insulation composed principally of
are best suited if a macroscopic viewpoint is taken such that the
diatomaceous earth with or without binders, and which
standard insulating materials are considered homogeneous (for ex-
usually contains reinforcing fibers.
ample, fibrous and cellular insulations), at least in the heat flow
diffusivity, thermal, n—the ratio of thermal conductivity of a
direction and time frame involved in a thermal test.
substance to the product of its density and specific heat. (In
DISCUSSION—Relevant properties may be a function of such variables
3 2
SI units: (W/(m•K))/((kg/m )•(J/(kg•K))) = m /s.) (In inch- as time, direction, or temperature.
3 2
pound units: (Btu/(hr•ft) F)/((lb/ft )(Btu/(lb•F)) = ft /hr))
humidity, absolute, n—the mass of water vapor per unit
emittance, e, n—the ratio of the radiant flux emitted by a
volume.
specimen to that emitted by a blackbody at the same
humidity, relative, n—the ratio of the mol fraction of water
temperature and under the same conditions.
vapor present in the air to the mol fraction of water vapor
emittance, directional e(u; f), n—the ratio of the radiance
present in saturated air at the same temperature and baro-
from a surface in a particular direction to the radiance from
metric pressure. Approximately, it equals the ratio of the
a blackbody at the same temperature under the same
partial pressure or density of the water vapor in the air to the
conditions.
saturation pressure or density, respectively, at the same
emittance, hemispherical e or e(2p), n—the average direc-
H
temperature.
tional emittance over a hemispherical envelope covering a
jacket, n—a form of facing applied over insulation.
surface.
emittance, spectral e or e(l; u;f ), n—an emittance based
DISCUSSION—It may be integral with the insulation, or field-applied
l
on the radiant energy emitted per unit wavelength interval using sheet materials.
(monochromatic radiant energy).
loose fill insulation, n—insulation in granular, nodular, fi-
DISCUSSION—Where necessary to avoid confusion, emittances should
brous, powdery, or similar form designed to be installed by
be designated by subscripts, for example: e ,e , e , e ,e . For
HT Hl Nl ul HT
pouring, blowing, or hand placement.
most engineering purposes, the hemispherical total emittance e
HT
mastic, n—a material of relatively viscous consistency that
suffices.
dries or cures to form a protective finish, suitable for
emittance, total e or e(t), n—an emittance that is an
T
application to thermal insulation in thickness greater than 30
integrated average over all wavelengths of radiant energy
mils (0.76 mm) per coat.
emitted.
mean specific heat, n—the quantity of heat required to change
facing, n—a protective or decorative (or both) surface applied
the temperature of a unit mass of a substance one degree,
as the outermost layers of an insulation system.
measured as the average quantity over the temperature range
graybody, n—a body having the same spectral emittance at all
specified. (It is distinguished from true specific heat by being
wavelengths.
an average rather than a point value.) (In SI units: J/kg•K)
heat flow; heat flow rate, Q, n—the quantity of heat trans-
(In inch-pound units: Btu/lb•F)
ferred to or from a system in unit time. (Q in SI units: W.) (Q
microporous insulation, n—material in the form of com-
in inch-pound units: Btu/h.)
pacted powder or fibers with an average interconnecting pore
size comparable to or below the mean free path of air
DISCUSSION—See heat flux for the areal dependence.
DISCUSSION—This definition is different than that given in some molecules at standard atmospheric pressure. Microporous
˙
textbooks, which may use Q,or q to represent heat flow rate. The ISO
insulation may contain opacifiers to reduce the amount of
definition uses F.
radiant heat transmitted.
mineral fiber, n—insulation composed principally of fibers
heat flux, q, n—the heat flow rate through a surface of unit
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