1、Designation: C755 10Standard Practice forSelection of Water Vapor Retarders for Thermal Insulation1This standard is issued under the fixed designation C755; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision.
2、 A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice outlines factors to be considered, describesdesign principles and procedures for water vapor retarderselection, and de
3、fines water vapor transmission values appro-priate for established criteria. It is intended for the guidance ofdesign engineers in preparing vapor retarder application speci-fications for control of water vapor flow through thermalinsulation. It covers commercial and residential building con-structi
4、on and industrial applications in the service temperaturerange from 40 to +150F (40 to +66C). Emphasis is placedon the control of moisture penetration by choice of the mostsuitable components of the system.1.2 The values stated in inch-pound units are to be regardedas standard. The values given in p
5、arentheses are mathematicalconversions to SI units that are provided for information onlyand are not considered standard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-p
6、riate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C168 Terminology Relating to Thermal InsulationC647 Guide to Properties and Tests of Mastics and CoatingFinishes for Thermal InsulationC921 Practice fo
7、r Determining the Properties of JacketingMaterials for Thermal InsulationC1136 Specification for Flexible, Low Permeance VaporRetarders for Thermal InsulationE96/E96M Test Methods for Water Vapor Transmission ofMaterials3. Terminology3.1 For definitions of terms used in this practice, refer toTermin
8、ology C168.4. Significance and Use4.1 Experience has shown that uncontrolled water entry intothermal insulation is the most serious factor causing impairedperformance. Water entry into an insulation system may bethrough diffusion of water vapor, air leakage carrying watervapor, and leakage of surfac
9、e water. Application specificationsfor insulation systems that operate below ambient dew-pointtemperatures should include an adequate vapor retarder sys-tem. This may be separate and distinct from the insulationsystem or may be an integral part of it. For selection ofadequate retarder systems to con
10、trol vapor diffusion, it isnecessary to establish acceptable practices and standards.4.2 Vapor Retarder FunctionWater entry into an insula-tion system may be through diffusion of water vapor, airleakage carrying water vapor, and leakage of surface water.The primary function of a vapor retarder is to
11、 control move-ment of diffusing water vapor into or through a permeableinsulation system. The vapor retarder system alone is seldomintended 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 PerformanceDesign choice of reta
12、rd-ers will be affected by thickness of retarder materials, substrateto which applied, the number of joints, available length andwidth of sheet materials, useful life of the system, andinspection procedures. Each of these factors will have an effecton the retarder system performance and each must be
13、 consid-ered and evaluated by the designer.4.3.1 Although this practice properly places major emphasison selecting the best vapor retarders, it must be recognized thatfaulty installation techniques can impair vapor retarder perfor-mance. The effectiveness of installation or application tech-niques i
14、n obtaining design water vapor transmission (WVT)performance must be considered in the selection of retardermaterials.4.3.2 As an example of the evaluation required, it may beimpractical to specify a lower “as installed” value, becausedifficulties of field application often will preclude “as install
15、ed”attainment of the inherent WVT values of the vapor retardermaterials used. The designer could approach this requirement1This practice is under the jurisdiction of ASTM Committee C16 on ThermalInsulation and is the direct responsibility of Subcommittee C16.33 on InsulationFinishes and Moisture.Cur
16、rent edition approved Oct. 1, 2010. Published November 2010. Originallyapproved in 1973. Last previous edition approved in 2003 as C755 03. DOI:10.1520/C0755-10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book o
17、f ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.by selecting a membrane retarder material that has a lowerpermeance manufactured in 5
18、-ft (1.5-m) width or a sheetmaterial 20 ft (6.1 m) wide having a higher permeance. Thesealternatives may be approximately equivalent on an installedbasis since the wider material has fewer seams and joints.4.3.3 For another example, when selecting mastic or coatingretarder materials, the choice of a
19、 product having a permeancevalue somewhat higher than the lowest obtainable might bejustified on the basis of its easier application techniques, thusensuring “as installed” system attainment of the specifiedpermeance. The permeance of the substrate and its effects onthe application of the retarder m
20、aterial must also be consideredin this case.5. Factors to Be Considered in Choosing Water VaporRetarders5.1 Water Vapor Pressure Difference is the difference in thepressure exerted on each side of an insulation system orinsulated structure that is due to the temperature and moisturecontent of the ai
21、r on each side of the insulated system orstructure. This pressure difference determines the direction andmagnitude of the driving force for the diffusion of the watervapor through the insulated system or structure. In general, fora given permeable structure, the greater the water vaporpressure diffe
22、rence, the greater the rate of diffusion. Watervapor pressure differences for specific conditions can becalculated by numerical methods or from psychrometric tablesshowing thermodynamic properties of water at saturation.5.1.1 Fig. 1 shows the variation of dew-point temperaturewith water vapor pressu
23、re.5.1.2 Fig. 2 illustrates the magnitude of water vapor pres-sure differences for four ambient air conditions and cold-sideoperating temperatures between +40 and 40F (+4.4and 40C).5.1.3 At a stated temperature the water vapor pressure isproportional to relative humidity but at a stated relativehumi
24、dity the vapor pressure is not proportional to temperature.5.1.4 Outdoor design conditions vary greatly dependingupon geographic location and season and can have a substantialimpact on system design requirements. It is therefore necessaryto calculate the actual conditions rather than rely on estimat
25、es.As an example, consider the cold-storage application shown inTable 1. The water vapor pressure difference for the facilitylocated in Biloxi, MS is 0.96 in. Hg (3.25 kPa) as compared toa 0.001 in. Hg (3 Pa) pressure difference if the facility waslocated in International Falls, MN. In the United St
26、ates thedesign dew point temperature seldom exceeds 75F (24C)(16).35.1.5 The expected vapor pressure difference is a veryimportant factor that must be based on realistic design data (notestimated) to determine vapor retarder requirements.5.2 Service ConditionsThe direction and magnitude ofwater vapo
27、r flow are established by the range of ambientatmospheric and design service conditions. These conditionsnormally will cause vapor flow to be variable in magnitude,and either unidirectional or reversible.3The boldface numbers in parentheses refer to the list of references at the end ofthis practice.
28、FIG. 1 Dew Point (Dp) Relation to Water Vapor PressureC755 1025.2.1 Unidirectional flow exists where the water vaporpressure is constantly higher on one side of the system. Withbuildings operated for cold storage or frozen food storage, thesummer outdoor air conditions will usually determine vaporre
29、tarder requirements, with retarder placement on the outdoor(warmer) side of the insulation. In heating only buildings forhuman occupancy, the winter outdoor air conditions wouldrequire retarder placement on the indoor (warmer) side of theinsulation. In cooling only buildings for human occupancy(that
30、 is, tropic and subtropic locations), the summer outside airconditions would require retarder placement on the outdoor(warmer) side.5.2.2 Reversible flow can occur where the vapor pressuremay be higher on either side of the system, changing usuallybecause of seasonal variations. The inside temperatu
31、re andvapor pressure of a refrigerated structure may be below theoutside temperature and vapor pressure at times, and above theoutside temperature and vapor pressure at other times. Coolerrooms with operating temperatures in the range from 35 to45F (2 to 7C) at 90 % relative humidity and located inn
32、orthern latitudes will experience an outward vapor flow inwinter and an inward flow in summer. This reversing vaporflow requires special design consideration.5.3 Properties of Insulating Materials with Respect toMoistureInsulating materials permeable to water vapor willallow moisture to diffuse thro
33、ugh at a rate defined by itspermeance and exposure. The rate of movement is inverselyproportional to the vapor flow resistance in the vapor path.Insulation having low permeance and vapor-tight joints mayact as a vapor retarder.5.3.1 If condensation of water occurs within the insulationits thermal pr
34、operties can be significantly affected wherewetted. Liquid water resulting from condensation has a thermalconductivity some fifteen times greater than that of a typicallow-temperature insulation. Ice conductivity is nearly fourtimes that of water. Condensation reduces the thermal effec-tiveness of t
35、he insulation in the zone where it occurs, but if thezone is thin and perpendicular to the heat flow path, thereduction is not extreme. Water or ice in insulation joints thatare parallel to the heat flow path provide higher conductancepaths with consequent increased heat flow. Generally, hygro-scopi
36、c moisture in insulation can be disregarded.5.3.2 Thermal insulation materials range in permeabilityfrom essentially 0 perm-in. (0 g/Pa-s-m) to greater than 100perm-in. (1.45 3 10-7g/Pa-s-m) Because insulation is suppliedin pieces of various size and thickness, vapor diffusion throughFIG. 2 Magnitud
37、e of Water Vapor Pressure Difference for Selected Conditions (Derived from Fig. 1)TABLE 1 Cold Storage ExampleLocationSeasonBiloxi, MSSummerInternational Falls, MNWinterOutside Design ConditionsTemperature , F (C) 93 (34) -35 (-37)Relative Humidity, % 63 67Dew Point Temperature, F (C) 78.4 (26) -42
38、(-41)Water Vapor Pressurein. Hg (kPa).9795 (3.32) .003 (0.01)Inside Design ConditionsTemperature, F (C) -10 -10Relative Humidity, % 90 90Water Vapor Pressure in.Hg (kPa).02 .02System Design ConditionsWater Vapor PressureDifference in. Hg (kPa)0.9795 0.001 (0.067)Direction of Diffusion From outside F
39、rom insideC755 103joints must be considered in the permeance of the materials asapplied. The effect of temperature changes on dimensions andother physical characteristics of all materials of the assemblymust be considered as it relates to vapor flow into the joints andinto the insulation.5.4 Propert
40、ies of Boundary or Finish Materials at the ColdSide of InsulationWhen a vapor pressure gradient exists thelower vapor pressure value usually will be on the lowertemperature side of the system, but not always. (There are fewexceptions, but these must be considered as special cases.) Thefinish on the
41、cold side of the insulation-enclosing refrigeratedspaces should have high permeance relative to that of the warmside construction, so that water vapor penetrating the systemcan flow through the insulation system without condensing.This moisture should be free to move to the refrigeratingsurfaces whe
42、re it is removed as condensate. When the cold sidepermeance is zero, as with insulated cold piping, water vaporthat enters the insulation system usually will condense withinthe assembly and remain as an accumulation of water, frost, orice.5.5 Effect of Air LeakageWater vapor can be transportedreadil
43、y as a component of air movement into and out of anair-permeable insulation system. This fact must be taken intoaccount in the design and construction of any system in whichmoisture control is a requirement. The quantity of water vaporthat can be transported by air leakage through cracks orair-perme
44、able construction can easily be several times greaterthan that which occurs by vapor diffusion alone.5.5.1 Air movement occurs as a result of air pressuredifferences. In insulated structures these may be due to windaction, buoyancy forces due to temperature difference betweeninterconnected spaces, v
45、olume changes due to fluctuations intemperature and barometric pressure, and the operation ofmechanical air supply or exhaust systems. Air leakage occursthrough openings or through air-permeable construction acrosswhich the air pressure differences occur. Water vapor in airflowing from a warm humidi
46、fied region to a colder zone in aninsulation system will condense in the same way as watervapor moving only by diffusion.5.5.2 If there is no opportunity for dilution with air at lowervapor pressure along the flow path, there will be no vaporpressure gradient. Condensation may occur when the air str
47、eampasses through a region in the insulation system where thetemperature is equal to or lower than the dew point of the warmregion of origin. The airflow may be from a warm region onone side of the system through to a cold region on the otherside, or it may consist of recirculation between interconn
48、ectedair spaces at different temperatures forming only a part of thesystem. Sufficient airflow rate could virtually eliminate thetemperature gradient through the insulation.5.5.3 When air flows from a cold region of low vaporpressure through the system to the warm side there will be adrying effect a
49、long the flow path; the accompanying loweringof temperatures along the flow path, if significant, may beundesirable.5.5.4 In any insulation system where there is a possibility ofcondensation due to air leakage, the designer should attempt toensure that there is a continuous unbroken air barrier on thewarm side of the insulation. Often this can be provided by thevapor retarder system, but sometimes it can best be providedby a separate element. Particular attention should be given toproviding airtightness at discontinuities in the system, such asat intersecti
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