ASTM_C_755_-_10_2015.pdf

Designation:C75510(Reapproved 2015)Standard 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.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 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-struction 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 parentheses 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-priate 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 for 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 toTerminology 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 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 integral 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 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 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 have 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.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,becausedifficulties of field application often will preclude“as installed”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.Current edition approved Sept.1,2015.Published October 2015.Originallyapproved in 1973.Last previous edition approved in 2010 as C755 101.DOI:10.1520/C0755-10R15.2For 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 ASTM website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 by selecting a membrane retarder