ASTM C755 - 25 隔热用水蒸气阻滞剂的选择标准指南

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Designation: C755 −25
Standard Guide for
Selection of Water Vapor Retarders for Thermal Insulation
1
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.
1. Scope
1.1 This guide outlines factors to be considered, describes
design principles and procedures for water vapor retarder
selection, and suggests water vapor transmission values appro-
priate for established criteria. It is intended for the guidance of
design engineers in preparing vapor retarder application speci-
ﬁcations for control of water vapor ﬂow through thermal
insulation. It covers commercial and residential building con-
struction 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 appro-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
2
C168 Terminology Relating to Thermal Insulation
C647 Guide to Properties and Tests of Mastics and Coating
Finishes for Thermal Insulation
C1136 Speciﬁcation for Flexible, Low Permeance Vapor
Retarders for Thermal Insulation
E96/E96M Test Methods for Gravimetric Determination of
Water Vapor Transmission Rate 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. Several ways exist by which water enters into an
insulation system, the primary ones being 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 necessarily
include an adequate vapor retarder system. A vapor retarder
system is separate and distinct from the insulation, or is
provided by the insulation itself when it is has adequate vapor
resistant properties and all joints are sealed against water vapor
intrusion, in which case a separate vapor retarder system is not
necessary.
4.2 Vapor Retarder Function—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 in some cases is designed to prevent entry of
surface water. When properly functioning as a vapor retarder,
it will also serve as a barrier to air leakage.
4.3 Vapor Retarder Performance—Design choice of retard-
ers 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 consid-
ered and evaluated by the designer.
4.3.1 Although this guide properly places major emphasis
on selecting the best vapor retarders, it must be recognized that
faulty installation is likely to impair vapor retarder perfor-
mance. The effectiveness of installation or application tech-
niques in obtaining design water vapor permeance (WVP)
performance must be considered in the selection of retarder
materials.
1
This guide 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 July 1, 2025. Published July 2025. Originally approved
in 1973. Last previous edition approved in 2020 as C755 – 20. DOI: 10.1520/
C0755-25.
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at www.astm.org/contact. 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
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.
1

4.3.2 It is impractical to specify an “as installed” permeance

value because, due to the nature of ﬁeld application, attainment

of system permeance equivalent to the vapor retarder materials

themselves is assumed not possible. The best approach is to

specify an appropriate vapor retarder and insure that proper

installation and sealing procedures are followed.

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. One is able

to calculate water vapor pressure differences for speciﬁc

conditions 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 pres-

sure 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.

5.1.4 Outdoor design conditions vary greatly depending

upon geographic location and season with the potential to 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 0.98 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.

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

The boldface numbers in parentheses refer to the list of references at the end of

this practice.

FIG. 1 Dew Point (Dp) Relation to Water Vapor Pressure

C755 − 25

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 It is possible that reversible ﬂow will occur where the

higher vapor pressure alternates between sides of the system,

changing because of seasonal variations, for example. It is

possible that the inside temperature and vapor pressure of a

refrigerated structure will be below the outside temperature and

vapor pressure at times, and above them 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 will act

as a vapor retarder to varying degrees, depending on type of

insulation and sealing materials.

5.3.1 When condensed water vapor becomes entrained in

the insulation, the insulation’s thermal properties will be

affected to varying degrees where wetted. The overall effect is

less impactful when the condensation does not become trapped

in the insulation itself, or is dissipated from the system in some

manner. 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

when 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 conduc-

tance paths with consequent increased heat ﬂow. Generally,

hygroscopic moisture in insulation is disregarded.

5.3.2 Thermal insulation materials range in permeability

from essentially 0 (zero) perm-in. (0 g/Pa-s-m) to greater than

100 perm-in. (1.45 × 10

–7

g/Pa-s-m). Because insulation is

supplied in pieces of various size and thickness, vapor diffu-

sion 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

FIG. 2 Magnitude of Water Vapor Pressure Difference for Selected Conditions (Derived from Fig. 1)

TABLE 1 Cold Storage Example

Location

Season

Biloxi, MS

Summer

International

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

in. Hg (kPa)

.9795 (3.32) .003 (0.01)

Inside Design Conditions

Temperature, °F (°C) -10 -10

Relative Humidity, % 90 90

Water Vapor Pressure in.

Hg (kPa)

.02 .02

System Design Conditions

Water Vapor Pressure

Difference in. Hg (kPa)

0.9795 0.001 (0.067)

Direction of Diffusion From outside From inside

C755 − 25

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标签： #隔热 #标准指南 #水蒸气

摘要：

本指南详细介绍了ASTM C755 - 25标准，即隔热用水蒸气阻滞剂的选择标准指南，旨在为建筑和工业隔热系统中水蒸气阻滞剂的正确选用提供系统化的技术依据。该标准涵盖了水蒸气传递的基本原理、材料性能评估、环境条件分析以及应用场景匹配等核心内容，帮助工程师与设计师在防止冷凝、延长隔热材料寿命及提高系统能效方面做出科学决策。通过遵循该指南，用户能够根据不同的温度、湿度与结构要求，选择最适宜的水蒸气阻滞剂类型与安装方式，从而有效控制湿气侵入风险，提升整体隔热系统的可靠性与长期稳定性。

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