1、Designation: C 755 03Standard Practice forSelection of Water Vapor Retarders for Thermal Insulation1This standard is issued under the fixed designation C 755; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revisio
2、n. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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
3、 defines 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-stru
4、ction 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 This standard does not purport to address all of thesafety concerns, if any, associated
5、 with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C 168 Terminology Relating to Thermal InsulationC 647 Guide to
6、Properties and Test Methods of Mastics andCoating Finishes for Thermal InsulationC 921 Specifications for Jackets for Thermal InsulationC 1136 Specification for Flexible, Low Permeance VaporRetarders for Thermal InsulationE 96 Test Methods for Water Vapor Transmission of Mate-rials3. Terminology3.1
7、For definitions of terms used in this practice, refer toTerminology C 168.4. 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 wate
8、r vapor, air leakage carrying watervapor, and leakage of surface 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 integr
9、al part of it. For selection ofadequate retarder systems to control 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
10、of surface water.The primary function of a vapor retarder is to 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
11、 of defense.4.3 Vapor Retarder PerformanceDesign choice of retard-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 ha
12、ve an effecton the retarder system performance and each must be 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.
13、 The effectiveness of installation or application tech-niques in 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, becausedi
14、fficulties of field application often will preclude “as installed”attainment of the inherent WVT values of the vapor retardermaterials used. The designer could approach this requirementby selecting a membrane retarder material that has a lowerpermeance manufactured in 5-ft (1.5-m) width or a sheetma
15、terial 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 product having a permeanceval
16、ue somewhat higher than the lowest obtainable might be1This practice is under the jurisdiction of ASTM Committee C16 on ThermalInsulation and is the direct responsibility of Subcommittee C16.33 on InsulationFinishes and Moisture.Current edition approved Nov. 1, 2003. Published December 2003. Origina
17、llyapproved in 1973. Last previous edition approved in 2002 as C 755 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe AS
18、TM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.justified on the basis of its easier application techniques, thusensuring “as installed” system attainment of the specifiedpermeance. The permeance of the substrate and its e
19、ffects onthe application of the retarder material 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
20、 temperature and moisturecontent of the air 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,
21、 the greater the water vaporpressure difference, 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 de
22、w-point temperaturewith water vapor pressure.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 rela
23、tive humidity but at a stated relativehumidity 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 act
24、ual conditions rather than rely on estimates.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 as compared to a 0.001 in.Hg pressure difference if the facility was located in Interna-tional F
25、alls, MN. In the United States the design dew pointtemperature 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.3The boldface numbers in parenthese
26、s refer to the list of references at the end ofthis practice.FIG. 1 Dew Point (Dp) Relation to Water Vapor PressureC7550325.2 Service ConditionsThe direction and magnitude ofwater vapor flow are established by the range of ambientatmospheric and design service conditions. These conditionsnormally wi
27、ll cause vapor flow to be variable in magnitude,and either unidirectional or reversible.5.2.1 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
28、 usually determine vaporretarder 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 building
29、s for human occupancy(that 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 variat
30、ions. The inside temperature 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 % relativ
31、e humidity and located innorthern 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 willallo
32、w moisture to diffuse through 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 th
33、e insulationits thermal properties 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 fourFIG. 2 Magnitude of Water Vapor Pressure Differ
34、ence 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 (-41)Water Vapor Pressure in. Hg
35、 .9795 .003Inside Design ConditionsTemperature, F (C) -10 -10Relative Humidity, % 90 90Water Vapor Pressure in. Hg .02 .02System Design ConditionsWater Vapor Pressure Difference in. Hg 0.9795 0.001Direction of Diffusion From outside From insideC755033times that of water. Condensation reduces the the
36、rmal effec-tiveness of the 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
37、. Generally, hygro-scopic moisture in insulation can be disregarded.5.3.2 Thermal insulation materials range in permeabilityfrom essentially 0 perm-in. to greater than 100 perm-in.Because insulation is supplied in pieces of various size andthickness, vapor diffusion through joints must be considered
38、 inthe permeance of the materials as applied. The effect oftemperature changes on dimensions and other physical char-acteristics of all materials of the assembly must be consideredas it relates to vapor flow into the joints and into the insulation.5.4 Properties of Boundary or Finish Materials at th
39、e 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 cold side of the insulation-enclosing ref
40、rigeratedspaces 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 where it is removed as condensate. When the
41、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 transportedreadily as a component of air movement into and
42、 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-permeable construction can easily be several t
43、imes 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, volume changes due to fluctuations intempe
44、rature 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 humidified region to a colder zone in aninsulat
45、ion 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 streampasses through a region in the insulat
46、ion 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 interconnectedair spaces at different temperatures
47、 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 along the flow path; the accompanying lowe
48、ringof 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
49、 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 intersections of walls, roofs and floors, at the boundaries ofstructural elements forming part of an enclosure, and aroundwindow and service openings. The insulation system should bedesigned so that it is practical to obtain a continuous air barrierunder the conditions that will prevail on the job site, keeping inmind the problem of ensuring good workm
