ASTM C755-10(2015)e1
(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.
FIG. 1 Dew Point (Dp) Relation to Water Vapor Pressure
4.3.2 As an example of th...
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, 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.
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´1
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.
ε NOTE—Editorially corrected Table 2 in January 2018.
1. Scope C647Guide to Properties and Tests of Mastics and Coating
Finishes for Thermal Insulation
1.1 Thispracticeoutlinesfactorstobeconsidered,describes
C921Practice for Determining the Properties of Jacketing
design principles and procedures for water vapor retarder
Materials for Thermal Insulation
selection, and defines water vapor transmission values appro-
C1136Specification for Flexible, Low Permeance Vapor
priate for established criteria. It is intended for the guidance of
Retarders for Thermal Insulation
design engineers in preparing vapor retarder application speci-
E96/E96MTest Methods for Water Vapor Transmission of
fications for control of water vapor flow through thermal
Materials
insulation. It covers commercial and residential building con-
struction and industrial applications in the service temperature
3. Terminology
range from−40 to+150°F (−40 to+66°C). Emphasis is placed
3.1 For definitions of terms used in this practice, refer to
on the control of moisture penetration by choice of the most
Terminology C168.
suitable components of the system.
4. Significance and Use
1.2 The values stated in inch-pound units are to be regarded
as standard. The values given in parentheses are mathematical
4.1 Experiencehasshownthatuncontrolledwaterentryinto
conversions to SI units that are provided for information only
thermal insulation is the most serious factor causing impaired
and are not considered standard.
performance. Water entry into an insulation system may be
through diffusion of water vapor, air leakage carrying water
1.3 This standard does not purport to address all of the
vapor, and leakage of surface water.Application specifications
safety concerns, if any, associated with its use. It is the
for insulation systems that operate below ambient dew-point
responsibility of the user of this standard to establish appro-
temperatures should include an adequate vapor retarder sys-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use. tem. This may be separate and distinct from the insulation
system or may be an integral part of it. For selection of
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard- adequate retarder systems to control vapor diffusion, it is
necessary to establish acceptable practices and standards.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
4.2 Vapor Retarder Function—Water entry into an insula-
mendations issued by the World Trade Organization Technical
tion system may be through diffusion of water vapor, air
Barriers to Trade (TBT) Committee.
leakage carrying water vapor, and leakage of surface water.
The primary function of a vapor retarder is to control move-
2. Referenced Documents
ment of diffusing water vapor into or through a permeable
insulation system. The vapor retarder system alone is seldom
2.1 ASTM Standards:
C168Terminology Relating to Thermal Insulation intendedtopreventeitherentryofsurfacewaterorairleakage,
but it may be considered as a second line of defense.
4.3 Vapor Retarder Performance—Design choice of retard-
This practice is under the jurisdiction of ASTM Committee C16 on Thermal
erswillbeaffectedbythicknessofretardermaterials,substrate
Insulation and is the direct responsibility of Subcommittee C16.33 on Insulation
to which applied, the number of joints, available length and
Finishes and Moisture.
width of sheet materials, useful life of the system, and
Current edition approved Sept. 1, 2015. Published October 2015. Originally
ɛ1
inspectionprocedures.Eachofthesefactorswillhaveaneffect
approved in 1973. Last previous edition approved in 2010 as C755–10 . DOI:
10.1520/C0755-10R15E01.
on the retarder system performance and each must be consid-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
ered and evaluated by the designer.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
4.3.1 Althoughthispracticeproperlyplacesmajoremphasis
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. onselectingthebestvaporretarders,itmustberecognizedthat
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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C755 − 10 (2015)
faulty installation techniques can impair vapor retarder perfor- insulated structure that is due to the temperature and moisture
mance. The effectiveness of installation or application tech- content of the air on each side of the insulated system or
niques in obtaining design water vapor transmission (WVT)
structure.Thispressuredifferencedeterminesthedirectionand
performance must be considered in the selection of retarder magnitude of the driving force for the diffusion of the water
materials.
vapor through the insulated system or structure. In general, for
4.3.2 As an example of the evaluation required, it may be
a given permeable structure, the greater the water vapor
impractical to specify a lower “as installed” value, because
pressure difference, the greater the rate of diffusion. Water
difficultiesoffieldapplicationoftenwillpreclude“asinstalled”
vapor pressure differences for specific conditions can be
attainment of the inherent WVT values of the vapor retarder
calculated by numerical methods or from psychrometric tables
materials used. The designer could approach this requirement
showing thermodynamic properties of water at saturation.
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
theapplicationoftheretardermaterialmustalsobeconsidered upongeographiclocationandseasonandcanhaveasubstantial
impactonsystemdesignrequirements.Itisthereforenecessary
in this case.
to calculate the actual conditions rather than rely on estimates.
5. Factors to Be Considered in Choosing Water Vapor
As an example, consider the cold-storage application shown in
Retarders
Table 1. The water vapor pressure difference for the facility
5.1 Water Vapor Pressure Difference is the difference in the located in Biloxi, MS is 0.96 in. Hg (3.25 kPa) as compared to
pressure exerted on each side of an insulation system or a 0.001 in. Hg (3 Pa) pressure difference if the facility was
FIG. 1 Dew Point (Dp) Relation to Water Vapor Pressure
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C755 − 10 (2015)
FIG. 2 Magnitude of Water Vapor Pressure Difference for Selected Conditions (Derived from Fig. 1)
TABLE 1 Cold Storage Example
5.2.1 Unidirectional flow exists where the water vapor
Location Biloxi, MS International pressure is constantly higher on one side of the system. With
Season Summer Falls, MN
buildings operated for cold storage or frozen food storage, the
Winter
summer outdoor air conditions will usually determine vapor
Outside Design Conditions
Temperature , °F (°C) 93 (34) -35 (-37) retarder requirements, with retarder placement on the outdoor
Relative Humidity, % 63 67
(warmer) side of the insulation. In heating only buildings for
Dew Point Temperature, °F (°C) 78.4 (26) -42 (-41)
human occupancy, the winter outdoor air conditions would
Water Vapor Pressure .9795 (3.32) .003 (0.01)
in. Hg (kPa)
require retarder placement on the indoor (warmer) side of the
insulation. In cooling only buildings for human occupancy
Inside Design Conditions
(that is, tropic and subtropic locations), the summer outside air
Temperature, °F (°C) -10 -10
Relative Humidity, % 90 90
conditions would require retarder placement on the outdoor
Water Vapor Pressure in. .02 .02
(warmer) side.
Hg (kPa)
5.2.2 Reversible flow can occur where the vapor pressure
System Design Conditions
may be higher on either side of the system, changing usually
Water Vapor Pressure 0.9795 0.001 (0.067)
because of seasonal variations. The inside temperature and
Difference in. Hg (kPa)
Direction of Diffusion From outside From inside vapor pressure of a refrigerated structure may be below the
outsidetemperatureandvaporpressureattimes,andabovethe
outside temperature and vapor pressure at other times. Cooler
rooms with operating temperatures in the range from 35 to
located in International Falls, MN. In the United States the
design dew point temperature seldom exceeds 75°F (24°C) 45°F (2 to 7°C) at 90% relative humidity and located in
northern latitudes will experience an outward vapor flow in
(1).
5.1.5 The expected vapor pressure difference is a very winter and an inward flow in summer. This reversing vapor
flow requires special design consideration.
importantfactorthatmustbebasedonrealisticdesigndata(not
estimated) to determine vapor retarder requirements.
5.3 Properties of Insulating Materials with Respect to
5.2 Service Conditions—The direction and magnitude of Moisture—Insulating materials permeable to water vapor will
water vapor flow are established by the range of ambient
allow moisture to diffuse through at a rate defined by its
atmospheric and design service conditions. These conditions permeance and exposure. The rate of movement is inversely
normally will cause vapor flow to be variable in magnitude,
proportional to the vapor flow resistance in the vapor path.
and either unidirectional or reversible. 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
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this practice. its thermal properties can be significantly affected where
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C755 − 10 (2015)
wetted.Liquidwaterresultingfromcondensationhasathermal region of origin. The airflow may be from a warm region on
conductivity some fifteen times greater than that of a typical one side of the system through to a cold region on the other
low-temperature insulation. Ice conductivity is nearly four side, or it may consist of recirculation between interconnected
times that of water. Condensation reduces the thermal effec- air spaces at different temperatures forming only a part of the
tiveness of the insulation in the zone where it occurs, but if the system. Sufficient airflow rate could virtually eliminate the
zone is thin and perpendicular to the heat flow path, the temperature gradient through the insulation.
reduction is not extreme. Water or ice in insulation joints that
5.5.3 When air flows from a cold region of low vapor
are parallel to the heat flow path provide higher conductance
pressure through the system to the warm side there will be a
paths with consequent increased heat flow. Generally, hygro-
drying effect along the flow path; the accompanying lowering
scopic moisture in insulation can be disregarded.
of temperatures along the flow path, if significant, may be
5.3.2 Thermal insulation materials range in permeability
undesirable.
from essentially 0 perm-in. (0 g/Pa-s-m) to greater than 100
5.5.4 Inanyinsulationsystemwherethereisapossibilityof
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perm-in. (1.45 × 10 g/Pa-s-m) Because insulation is supplied
condensationduetoairleakage,thedesignershouldattemptto
inpiecesofvarioussizeandthickness,vapordiffusionthrough
ensure that there is a continuous unbroken air barrier on the
joints must be considered in the permeance of the materials as
warm side of the insulation. Often this can be provided by the
applied. The effect of temperature changes on dimensions and
vapor retarder system, but sometimes it can best be provided
other physical characteristics of all materials of the assembly
by a separate element. Particular attention should be given to
mustbeconsideredasitrelatestovaporflowintothejointsand
providing airtightness at discontinuities in the system, such as
into the insulation.
at intersections of walls, roofs and floors, at the boundaries of
structural elements forming part of an enclosure, and around
5.4 Properties of Boundary or Finish Materials at the Cold
windowandserviceopenings.Theinsulationsystemshouldbe
Side of Insulation—When a vapor pressure gradient exists the
designedsothatitispracticaltoobtainacontinuousairbarrier
lower vapor pressure value usually will be on the lower
undertheconditionsthatwillprevailonthejobsite,keepingin
temperature side of the system, but not always. (There are few
mind the problem of ensuring good workmanship.
exceptions,butthesemustbeconsideredasspecialcases.)The
5.5.5 Recirculationofairbetweenspacesonthecoldsideof
finish on the cold side of the insulation-enclosing refrigerated
the insulation and a region of low vapor pressure (usually on
spacesshouldhavehighpermeancerelativetothatofthewarm
the cold side of the insulation system) can be utilized advan-
side construction, so that water vapor penetrating the system
tageously to maintain continuity of vapor flow, whether due to
can flow through the insulation system without condensing.
diffusion or air leakage, and thus to avoid condensation. This
This moisture should be free to move to the refrigerating
will often be the only practical approach to the control of
surfaceswhereitisremovedascondensate.Whenthecoldside
condensation and maintenance of dry conditions within the
permeance is zero,
...
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 (Reapproved 2015) 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—Editorially corrected Table 2 in January 2018.
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 safety, health, and healthenvironmental 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:
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
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.
ɛ1
Current edition approved Sept. 1, 2015. Published October 2015. Originally approved in 1973. Last previous edition approved in 2010 as C755 – 10 . DOI:
10.1520/C0755-10R15.10.1520/C0755-10R15E01.
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
´1
C755 − 10 (2015)
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 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).
FIG. 1 Dew Point (Dp) Relation to Water Vapor Pressure
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C755 − 10 (2015)
FIG. 2 Magnitude of Water Vapor Pressure Difference for Selected Conditions (Derived from Fig. 1)
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.
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.
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
The boldface numbers in parentheses refer to the list of references at the end of this practice.
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C755 − 10 (2015)
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.
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 d
...
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