ASTM C755-10(2015)
(Practice)Standard Practice for Selection of Water Vapor Retarders for Thermal Insulation
Standard Practice for Selection of Water Vapor Retarders for Thermal Insulation
SIGNIFICANCE AND USE
4.1 Experience has shown that uncontrolled water entry into thermal insulation is the most serious factor causing impaired performance. Water entry into an insulation system may be through diffusion of water vapor, air leakage carrying water vapor, and leakage of surface water. Application specifications for insulation systems that operate below ambient dew-point temperatures should include an adequate vapor retarder system. This may be separate and distinct from the insulation system or may be an integral part of it. For selection of adequate retarder systems to control vapor diffusion, it is necessary to establish acceptable practices and standards.
4.2 Vapor Retarder Function—Water entry into an insulation system may be through diffusion of water vapor, air leakage carrying water vapor, and leakage of surface water. The primary function of a vapor retarder is to control movement of diffusing water vapor into or through a permeable insulation system. The vapor retarder system alone is seldom intended to prevent either entry of surface water or air leakage, but it may be considered as a second line of defense.
4.3 Vapor Retarder Performance—Design choice of retarders will be affected by thickness of retarder materials, substrate to which applied, the number of joints, available length and width of sheet materials, useful life of the system, and inspection procedures. Each of these factors will have an effect on the retarder system performance and each must be considered and evaluated by the designer.
4.3.1 Although this practice properly places major emphasis on selecting the best vapor retarders, it must be recognized that faulty installation techniques can impair vapor retarder performance. The effectiveness of installation or application techniques in obtaining design water vapor transmission (WVT) performance must be considered in the selection of retarder materials.
4.3.2 As an example of the evaluation required, it may be impractical to specify a...
SCOPE
1.1 This practice outlines factors to be considered, describes design principles and procedures for water vapor retarder selection, and defines water vapor transmission values appropriate for established criteria. It is intended for the guidance of design engineers in preparing vapor retarder application specifications for control of water vapor flow through thermal insulation. It covers commercial and residential building construction and industrial applications in the service temperature range from −40 to +150°F (−40 to +66°C). Emphasis is placed on the control of moisture penetration by choice of the most suitable components of the system.
1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: C755 − 10 (Reapproved 2015)
Standard Practice for
Selection of Water Vapor Retarders for Thermal Insulation
This standard is issued under the fixed designation C755; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 Thispracticeoutlinesfactorstobeconsidered,describes 3.1 For definitions of terms used in this practice, refer to
design principles and procedures for water vapor retarder Terminology C168.
selection, and defines water vapor transmission values appro-
4. Significance and Use
priate for established criteria. It is intended for the guidance of
design engineers in preparing vapor retarder application speci-
4.1 Experiencehasshownthatuncontrolledwaterentryinto
fications for control of water vapor flow through thermal
thermal insulation is the most serious factor causing impaired
insulation. It covers commercial and residential building con-
performance. Water entry into an insulation system may be
struction and industrial applications in the service temperature
through diffusion of water vapor, air leakage carrying water
range from−40 to+150°F (−40 to+66°C). Emphasis is placed
vapor, and leakage of surface water.Application specifications
on the control of moisture penetration by choice of the most
for insulation systems that operate below ambient dew-point
suitable components of the system.
temperatures should include an adequate vapor retarder sys-
tem. This may be separate and distinct from the insulation
1.2 The values stated in inch-pound units are to be regarded
system or may be an integral part of it. For selection of
as standard. The values given in parentheses are mathematical
adequate retarder systems to control vapor diffusion, it is
conversions to SI units that are provided for information only
necessary to establish acceptable practices and standards.
and are not considered standard.
1.3 This standard does not purport to address all of the 4.2 Vapor Retarder Function—Water entry into an insula-
safety concerns, if any, associated with its use. It is the tion system may be through diffusion of water vapor, air
responsibility of the user of this standard to establish appro- leakage carrying water vapor, and leakage of surface water.
priate safety and health practices and determine the applica- The primary function of a vapor retarder is to control move-
bility of regulatory limitations prior to use. ment of diffusing water vapor into or through a permeable
insulation system. The vapor retarder system alone is seldom
2. Referenced Documents
intendedtopreventeitherentryofsurfacewaterorairleakage,
but it may be considered as a second line of defense.
2.1 ASTM Standards:
C168Terminology Relating to Thermal Insulation
4.3 Vapor Retarder Performance—Design choice of retard-
C647Guide to Properties and Tests of Mastics and Coating
erswillbeaffectedbythicknessofretardermaterials,substrate
Finishes for Thermal Insulation
to which applied, the number of joints, available length and
C921Practice for Determining the Properties of Jacketing
width of sheet materials, useful life of the system, and
Materials for Thermal Insulation
inspectionprocedures.Eachofthesefactorswillhaveaneffect
C1136Specification for Flexible, Low Permeance Vapor
on the retarder system performance and each must be consid-
Retarders for Thermal Insulation
ered and evaluated by the designer.
E96/E96MTest Methods for Water Vapor Transmission of
4.3.1 Althoughthispracticeproperlyplacesmajoremphasis
Materials
onselectingthebestvaporretarders,itmustberecognizedthat
faulty installation techniques can impair vapor retarder perfor-
mance. The effectiveness of installation or application tech-
This practice is under the jurisdiction of ASTM Committee C16 on Thermal
niques in obtaining design water vapor transmission (WVT)
Insulation and is the direct responsibility of Subcommittee C16.33 on Insulation
performance must be considered in the selection of retarder
Finishes and Moisture.
materials.
Current edition approved Sept. 1, 2015. Published October 2015. Originally
ɛ1
4.3.2 As an example of the evaluation required, it may be
approved in 1973. Last previous edition approved in 2010 as C755–10 . DOI:
10.1520/C0755-10R15.
impractical to specify a lower “as installed” value, because
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
difficultiesoffieldapplicationoftenwillpreclude“asinstalled”
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
attainment of the inherent WVT values of the vapor retarder
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. materials used. The designer could approach this requirement
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C755 − 10 (2015)
FIG. 1 Dew Point (Dp) Relation to Water Vapor Pressure
by selecting a membrane retarder material that has a lower 5.1.1 Fig. 1 shows the variation of dew-point temperature
permeance manufactured in 5-ft (1.5-m) width or a sheet with water vapor pressure.
material 20 ft (6.1 m) wide having a higher permeance. These 5.1.2 Fig. 2 illustrates the magnitude of water vapor pres-
alternatives may be approximately equivalent on an installed sure differences for four ambient air conditions and cold-side
basis since the wider material has fewer seams and joints. operating temperatures between +40 and −40°F (+4.4
4.3.3 Foranotherexample,whenselectingmasticorcoating and−40°C).
retarder materials, the choice of a product having a permeance 5.1.3 At a stated temperature the water vapor pressure is
value somewhat higher than the lowest obtainable might be proportional to relative humidity but at a stated relative
justified on the basis of its easier application techniques, thus humidity the vapor pressure is not proportional to temperature.
ensuring “as installed” system attainment of the specified 5.1.4 Outdoor design conditions vary greatly depending
permeance. The permeance of the substrate and its effects on upongeographiclocationandseasonandcanhaveasubstantial
theapplicationoftheretardermaterialmustalsobeconsidered impactonsystemdesignrequirements.Itisthereforenecessary
in this case. to calculate the actual conditions rather than rely on estimates.
As an example, consider the cold-storage application shown in
5. Factors to Be Considered in Choosing Water Vapor Table 1. The water vapor pressure difference for the facility
Retarders
located in Biloxi, MS is 0.96 in. Hg (3.25 kPa) as compared to
a 0.001 in. Hg (3 Pa) pressure difference if the facility was
5.1 Water Vapor Pressure Difference is the difference in the
located in International Falls, MN. In the United States the
pressure exerted on each side of an insulation system or
design dew point temperature seldom exceeds 75°F (24°C)
insulated structure that is due to the temperature and moisture
(1).
content of the air on each side of the insulated system or
5.1.5 The expected vapor pressure difference is a very
structure.Thispressuredifferencedeterminesthedirectionand
importantfactorthatmustbebasedonrealisticdesigndata(not
magnitude of the driving force for the diffusion of the water
estimated) to determine vapor retarder requirements.
vapor through the insulated system or structure. In general, for
a given permeable structure, the greater the water vapor
5.2 Service Conditions—The direction and magnitude of
pressure difference, the greater the rate of diffusion. Water water vapor flow are established by the range of ambient
vapor pressure differences for specific conditions can be
calculated by numerical methods or from psychrometric tables
The boldface numbers in parentheses refer to the list of references at the end of
showing thermodynamic properties of water at saturation. this practice.
C755 − 10 (2015)
FIG. 2 Magnitude of Water Vapor Pressure Difference for Selected Conditions (Derived from Fig. 1)
TABLE 1 Cold Storage Example
5.2.2 Reversible flow can occur where the vapor pressure
Location Biloxi, MS International may be higher on either side of the system, changing usually
Season Summer Falls, MN
because of seasonal variations. The inside temperature and
Winter
vapor pressure of a refrigerated structure may be below the
Outside Design Conditions
Temperature , °F (°C) 93 (34) -35 (-37) outsidetemperatureandvaporpressureattimes,andabovethe
Relative Humidity, % 63 67
outside temperature and vapor pressure at other times. Cooler
Dew Point Temperature, °F (°C) 78.4 (26) -42 (-41)
rooms with operating temperatures in the range from 35 to
Water Vapor Pressure .9795 (3.32) .003 (0.01)
in. Hg (kPa)
45°F (2 to 7°C) at 90% relative humidity and located in
northern latitudes will experience an outward vapor flow in
Inside Design Conditions
winter and an inward flow in summer. This reversing vapor
Temperature, °F (°C) -10 -10
Relative Humidity, % 90 90
flow requires special design consideration.
Water Vapor Pressure in. .02 .02
Hg (kPa)
5.3 Properties of Insulating Materials with Respect to
Moisture—Insulating materials permeable to water vapor will
System Design Conditions
allow moisture to diffuse through at a rate defined by its
Water Vapor Pressure 0.9795 0.001 (0.067)
Difference in. Hg (kPa) permeance and exposure. The rate of movement is inversely
Direction of Diffusion From outside From inside
proportional to the vapor flow resistance in the vapor path.
Insulation having low permeance and vapor-tight joints may
act as a vapor retarder.
atmospheric and design service conditions. These conditions
5.3.1 If condensation of water occurs within the insulation
normally will cause vapor flow to be variable in magnitude,
its thermal properties can be significantly affected where
and either unidirectional or reversible.
wetted.Liquidwaterresultingfromcondensationhasathermal
5.2.1 Unidirectional flow exists where the water vapor
conductivity some fifteen times greater than that of a typical
pressure is constantly higher on one side of the system. With
low-temperature insulation. Ice conductivity is nearly four
buildings operated for cold storage or frozen food storage, the
times that of water. Condensation reduces the thermal effec-
summer outdoor air conditions will usually determine vapor
tiveness of the insulation in the zone where it occurs, but if the
retarder requirements, with retarder placement on the outdoor
zone is thin and perpendicular to the heat flow path, the
(warmer) side of the insulation. In heating only buildings for
reduction is not extreme. Water or ice in insulation joints that
human occupancy, the winter outdoor air conditions would
are parallel to the heat flow path provide higher conductance
require retarder placement on the indoor (warmer) side of the
paths with consequent increased heat flow. Generally, hygro-
insulation. In cooling only buildings for human occupancy
scopic moisture in insulation can be disregarded.
(that is, tropic and subtropic locations), the summer outside air
conditions would require retarder placement on the outdoor 5.3.2 Thermal insulation materials range in permeability
(warmer) side. from essentially 0 perm-in. (0 g/Pa-s-m) to greater than 100
C755 − 10 (2015)
-7
perm-in. (1.45 × 10 g/Pa-s-m) Because insulation is supplied 5.5.4 Inanyinsulationsystemwherethereisapossibilityof
inpiecesofvarioussizeandthickness,vapordiffusionthrough condensationduetoairleakage,thedesignershouldattemptto
joints must be considered in the permeance of the materials as ensure that there is a continuous unbroken air barrier on the
applied. The effect of temperature changes on dimensions and warm side of the insulation. Often this can be provided by the
vapor retarder system, but sometimes it can best be provided
other physical characteristics of all materials of the assembly
mustbeconsideredasitrelatestovaporflowintothejointsand by a separate element. Particular attention should be given to
providing airtightness at discontinuities in the system, such as
into the insulation.
at intersections of walls, roofs and floors, at the boundaries of
5.4 Properties of Boundary or Finish Materials at the Cold
structural elements forming part of an enclosure, and around
Side of Insulation—When a vapor pressure gradient exists the
windowandserviceopenings.Theinsulationsystemshouldbe
lower vapor pressure value usually will be on the lower
designedsothatitispracticaltoobtainacontinuousairbarrier
temperature side of the system, but not always. (There are few
undertheconditionsthatwillprevailonthejobsite,keepingin
exceptions,butthesemustbeconsideredasspecialcases.)The
mind the problem of ensuring good workmanship.
finish on the cold side of the insulation-enclosing refrigerated
5.5.5 Recirculationofairbetweenspacesonthecoldsideof
spacesshouldhavehighpermeancerelativetothatofthewarm
the insulation and a region of low vapor pressure (usually on
side construction, so that water vapor penetrating the system
the cold side of the insulation system) can be utilized advan-
can flow through the insulation system without condensing.
tageously to maintain continuity of vapor flow, whether due to
This moisture should be free to move to the refrigerating
diffusion or air leakage, and thus to avoid condensation. This
surfaceswhereitisremovedascondensate.Whenthecoldside
will often be the only practical approach to the control of
permeance is zero, as with insulated cold piping, water vapor
condensation and maintenance of dry conditions within the
that enters the insulation system usually will condense within
system. In thus venting the insulation system, whether by
the assembly and remain as an accumulation of water, frost, or
natural or mechanical means, care must be taken to avoid
ice.
adverse thermal effects.
5.5 Effect of Air Leakage—Water vapor can be transported
5.6 Other Factors—Other physical properties of retarder
readily as a component of air movement into and out of an
material, insulations, and structures that are not within the
air-permeable insulation system. This fact must be taken into
scope of this practice may affect choice of barrier. These
account in the design and construction of any system in which
include such properties as combustibility, compatibility of
moisture control is a requirement. The quantity of water vapor
systemcomponents,damageresistance,andsurfaceroughness.
that can be transported by air leakage through cracks or
air-permeable construction can eas
...
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.
´1
Designation: C755 − 10 C755 − 10 (Reapproved 2015)
Standard Practice for
Selection of Water Vapor Retarders for Thermal Insulation
This standard is issued under the fixed designation C755; 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.
ε NOTE—Table 2 and Table X1.1 were editorially corrected in September 2015.
1. Scope
1.1 This practice outlines factors to be considered, describes design principles and procedures for water vapor retarder selection,
and defines water vapor transmission values appropriate for established criteria. It is intended for the guidance of design engineers
in preparing vapor retarder application specifications for control of water vapor flow through thermal insulation. It covers
commercial and residential building construction and industrial applications in the service temperature range from −40 to +150°F
(−40 to +66°C). Emphasis is placed on the control of moisture penetration by choice of the most suitable components of the
system.
1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
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 and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
C168 Terminology Relating to Thermal Insulation
C647 Guide to Properties and Tests of Mastics and Coating Finishes for Thermal Insulation
C921 Practice for Determining the Properties of Jacketing Materials for Thermal Insulation
C1136 Specification for Flexible, Low Permeance Vapor Retarders for Thermal Insulation
E96/E96M Test Methods for Water Vapor Transmission of Materials
3. Terminology
3.1 For definitions of terms used in this practice, refer to Terminology C168.
4. Significance and Use
4.1 Experience has shown that uncontrolled water entry into thermal insulation is the most serious factor causing impaired
performance. Water entry into an insulation system may be through diffusion of water vapor, air leakage carrying water vapor, and
leakage of surface water. Application specifications for insulation systems that operate below ambient dew-point temperatures
should include an adequate vapor retarder system. This may be separate and distinct from the insulation system or may be an
integral part of it. For selection of adequate retarder systems to control vapor diffusion, it is necessary to establish acceptable
practices and standards.
4.2 Vapor Retarder Function—Water entry into an insulation system may be through diffusion of water vapor, air leakage
carrying water vapor, and leakage of surface water. The primary function of a vapor retarder is to control movement of diffusing
water vapor into or through a permeable insulation system. The vapor retarder system alone is seldom intended to prevent either
entry of surface water or air leakage, but it may be considered as a second line of defense.
This practice is under the jurisdiction of ASTM Committee C16 on Thermal Insulation and is the direct responsibility of Subcommittee C16.33 on Insulation Finishes
and Moisture.
Current edition approved Oct. 1, 2010Sept. 1, 2015. Published November 2010October 2015. Originally approved in 1973. Last previous edition approved in 20032010
ɛ1
as C755 – 03.C755 – 10 . DOI: 10.1520/C0755-10E01.10.1520/C0755-10R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C755 − 10 (2015)
4.3 Vapor Retarder Performance—Design choice of retarders will be affected by thickness of retarder materials, substrate to
which applied, the number of joints, available length and width of sheet materials, useful life of the system, and inspection
procedures. Each of these factors will have an effect on the retarder system performance and each must be considered and evaluated
by the designer.
4.3.1 Although this practice properly places major emphasis on selecting the best vapor retarders, it must be recognized that
faulty installation techniques can impair vapor retarder performance. The effectiveness of installation or application techniques in
obtaining design water vapor transmission (WVT) performance must be considered in the selection of retarder materials.
4.3.2 As an example of the evaluation required, it may be impractical to specify a lower “as installed” value, because difficulties
of field application often will preclude “as installed” attainment of the inherent WVT values of the vapor retarder materials used.
The designer could approach this requirement by selecting a membrane retarder material that has a lower permeance manufactured
in 5-ft (1.5-m) width or a sheet material 20 ft (6.1 m) wide having a higher permeance. These alternatives may be approximately
equivalent on an installed basis since the wider material has fewer seams and joints.
4.3.3 For another example, when selecting mastic or coating retarder materials, the choice of a product having a permeance
value somewhat higher than the lowest obtainable might be justified on the basis of its easier application techniques, thus ensuring
“as installed” system attainment of the specified permeance. The permeance of the substrate and its effects on the application of
the retarder material must also be considered in this case.
5. Factors to Be Considered in Choosing Water Vapor Retarders
5.1 Water Vapor Pressure Difference is the difference in the pressure exerted on each side of an insulation system or insulated
structure that is due to the temperature and moisture content of the air on each side of the insulated system or structure. This
pressure difference determines the direction and magnitude of the driving force for the diffusion of the water vapor through the
insulated system or structure. In general, for a given permeable structure, the greater the water vapor pressure difference, the greater
the rate of diffusion. Water vapor pressure differences for specific conditions can be calculated by numerical methods or from
psychrometric tables showing thermodynamic properties of water at saturation.
5.1.1 Fig. 1 shows the variation of dew-point temperature with water vapor pressure.
5.1.2 Fig. 2 illustrates the magnitude of water vapor pressure differences for four ambient air conditions and cold-side operating
temperatures between +40 and −40°F (+4.4 and −40°C).
5.1.3 At a stated temperature the water vapor pressure is proportional to relative humidity but at a stated relative humidity the
vapor pressure is not proportional to temperature.
FIG. 1 Dew Point (Dp) Relation to Water Vapor Pressure
C755 − 10 (2015)
FIG. 2 Magnitude of Water Vapor Pressure Difference for Selected Conditions (Derived from Fig. 1)
5.1.4 Outdoor design conditions vary greatly depending upon geographic location and season and can have a substantial impact
on system design requirements. It is therefore necessary to calculate the actual conditions rather than rely on estimates. As an
example, consider the cold-storage application shown in Table 1. The water vapor pressure difference for the facility located in
Biloxi, MS is 0.96 in. Hg (3.25 kPa) as compared to a 0.001 in. Hg (3 Pa) pressure difference if the facility was located in
International Falls, MN. In the United States the design dew point temperature seldom exceeds 75°F (24°C) (1).
5.1.5 The expected vapor pressure difference is a very important factor that must be based on realistic design data (not
estimated) to determine vapor retarder requirements.
5.2 Service Conditions—The direction and magnitude of water vapor flow are established by the range of ambient atmospheric
and design service conditions. These conditions normally will cause vapor flow to be variable in magnitude, and either
unidirectional or reversible.
The boldface numbers in parentheses refer to the list of references at the end of this practice.
TABLE 1 Cold Storage Example
Location Biloxi, MS International
Season Summer Falls, MN
Winter
Outside Design Conditions
Temperature , °F (°C) 93 (34) -35 (-37)
Relative Humidity, % 63 67
Dew Point Temperature, °F (°C) 78.4 (26) -42 (-41)
Water Vapor Pressure .9795 (3.32) .003 (0.01)
in. Hg (kPa)
Inside Design Conditions
Temperature, °F (°C) -10 -10
Relative Humidity, % 90 90
Water Vapor Pressure in. .02 .02
Hg (kPa)
System Design Conditions
Water Vapor Pressure 0.9795 0.001 (0.067)
Difference in. Hg (kPa)
Direction of Diffusion From outside From inside
C755 − 10 (2015)
5.2.1 Unidirectional flow exists where the water vapor pressure is constantly higher on one side of the system. With buildings
operated for cold storage or frozen food storage, the summer outdoor air conditions will usually determine vapor retarder
requirements, with retarder placement on the outdoor (warmer) side of the insulation. In heating only buildings for human
occupancy, the winter outdoor air conditions would require retarder placement on the indoor (warmer) side of the insulation. In
cooling only buildings for human occupancy (that is, tropic and subtropic locations), the summer outside air conditions would
require retarder placement on the outdoor (warmer) side.
5.2.2 Reversible flow can occur where the vapor pressure may be higher on either side of the system, changing usually because
of seasonal variations. The inside temperature and vapor pressure of a refrigerated structure may be below the outside temperature
and vapor pressure at times, and above the outside temperature and vapor pressure at other times. Cooler rooms with operating
temperatures in the range from 35 to 45°F (2 to 7°C) at 90 % relative humidity and located in northern latitudes will experience
an outward vapor flow in winter and an inward flow in summer. This reversing vapor flow requires special design consideration.
5.3 Properties of Insulating Materials with Respect to Moisture—Insulating materials permeable to water vapor will allow
moisture to diffuse through at a rate defined by its permeance and exposure. The rate of movement is inversely proportional to the
vapor flow resistance in the vapor path. Insulation having low permeance and vapor-tight joints may act as a vapor retarder.
5.3.1 If condensation of water occurs within the insulation its thermal properties can be significantly affected where wetted.
Liquid water resulting from condensation has a thermal conductivity some fifteen times greater than that of a typical
low-temperature insulation. Ice conductivity is nearly four times that of water. Condensation reduces the thermal effectiveness of
the insulation in the zone where it occurs, but if the zone is thin and perpendicular to the heat flow path, the reduction is not
extreme. Water or ice in insulation joints that are parallel to the heat flow path provide higher conductance paths with consequent
increased heat flow. Generally, hygroscopic moisture in insulation can be disregarded.
5.3.2 Thermal insulation materials range in permeability from essentially 0 perm-in. (0 g/Pa-s-m) to greater than 100 perm-in.
-7
(1.45 × 10 g/Pa-s-m) Because insulation is supplied in pieces of various size and thickness, vapor diffusion through joints must
be considered in the permeance of the materials as applied. The effect of temperature changes on dimensions and other physical
characteristics of all materials of the assembly must be considered as it relates to vapor flow into the joints and into the insulation.
5.4 Properties of Boundary or Finish Materials at the Cold Side of Insulation—When a vapor pressure gradient exists the lower
vapor pressure value usually will be on the lower temperature side of the system, but not always. (There are few exceptions, but
these must be considered as special cases.) The finish on the cold side of the insulation-enclosing refrigerated spaces should have
high permeance relative to that of the warm side construction, so that water vapor penetrating the system can flow through the
insulation system without condensing. This moisture should be free to move to the refrigerating surfaces where it is removed as
condensate. When the cold side permeance is zero, as with insulated cold piping, water vapor that enters the insulation system
usually will condense within the assembly and remain as an accumulation of water, frost, or ice.
5.5 Effect of Air Leakage—Water vapor can be transported readily as a component of air movement into and out of an
air-permeable insulation system. This fact must be taken into account in the design and construction of any system in which
moisture control is a requirement. The quantity of water vapor that can be transported by air leakage through cracks or
air-permeable construction can easily be several times greater than that which occurs by vapor diffusion alone.
5.5.1 Air movement occurs as a result of air pressure differences. In insulated structures these may be due to wind action,
buoyancy forces due to temperature difference between interconnected spaces, volume changes due to fluctuations in temperature
and barometric pressure, and the operation of mechanical air supply or exhaust systems. Air leakage occurs through openings or
through air-permeable construction across which the air pressure differences occur. Water vapor in air flowing from a warm
humidified region to a colder zone in an insulation system will condense in the same way as water vapor moving only by diffusion.
5.5.2 If there is no opportunity for dilution with air at lower vapor pressure along the flow path, there will be no vapor pressure
gradient. Condensation may occur when the air stream passes through a region in the insulation system where the temperature is
equal to or lower than the dew point of the warm regi
...
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