ASTM E241-20

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.
Designation: E241 − 20
Standard Guide for
Limiting Water-Induced Damage to Buildings
This standard is issued under the fixed designation E241; 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 and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.1 This guide covers building design, construction,
1.7 This international standard was developed in accor-
commissioning, operation, and maintenance.
dance with internationally recognized principles on standard-
1.2 This guide addresses the need for systematic evaluation
ization established in the Decision on Principles for the
of factors that can result in moisture-induced damage to a
Development of International Standards, Guides and Recom-
building or its components. Although of great potential
mendations issued by the World Trade Organization Technical
importance, serviceability issues which are often, but not
Barriers to Trade (TBT) Committee.
necessarily, related to physical damage of the building or its
2. Referenced Documents
components (for example, indoor air quality or electrical
safety) are not directly addressed in this guide.
2.1 ASTM Standards:
C168 Terminology Relating to Thermal Insulation
1.3 The emphasis of this guide is on low-rise buildings.
C717 Terminology of Building Seals and Sealants
Portions of this guide; in particular Sections 5, 6, and 7; may
C755 Practice for Selection of Water Vapor Retarders for
also be applicable to high-rise buildings.
Thermal Insulation
1.4 This guide is not intended for direct use in codes and
C1193 Guide for Use of Joint Sealants
specifications.Itdoesnotattempttoprescribeacceptablelimits
D1079 Terminology Relating to Roofing and Waterproofing
of damage. Buildings intended for different uses may have
E331 Test Method for Water Penetration of Exterior
different service life expectancies, and expected service lives
Windows, Skylights, Doors, and Curtain Walls by Uni-
of different components within a given building often differ.
form Static Air Pressure Difference
Furthermore, some building owners may be satisfied with
E547 Test Method for Water Penetration of Exterior
substantially shorter service life expectancies of building
Windows, Skylights, Doors, and Curtain Walls by Cyclic
components or of the entire building than other building
Static Air Pressure Difference
owners. Lastly, the level of damage that renders a component
E631 Terminology of Building Constructions
unserviceablemayvarywiththetypeofcomponent,thedegree
E632 Practice for Developing Accelerated Tests to Aid
to which failure of the component is critical (for example,
Prediction of the Service Life of Building Components
whether failure constitutes a life-safety hazard), and the judge-
and Materials
ment (that is, tolerance for damage) of the building owner. For
E1105 Test Method for Field Determination of Water Pen-
the reasons stated in this paragraph, prescribing limits of
etration of Installed Exterior Windows, Skylights, Doors,
damage would require listing many pages of exceptions and
and Curtain Walls, by Uniform or Cyclic Static Air
qualifiers and is beyond the scope of this guide.
Pressure Difference
1.5 The values stated in inch-pound units are to be regarded
E1643 Practice for Selection, Design, Installation, and In-
as standard. The values given in parentheses are mathematical
spection of Water Vapor Retarders Used in Contact with
conversions to SI units that are provided for information only
Earth or Granular Fill Under Concrete Slabs
and are not considered standard.
E1677 SpecificationforAirBarrier(AB)MaterialorAssem-
blies for Low-Rise Framed Building Walls
1.6 This standard does not purport to address the safety
E1745 Specification for Plastic Water Vapor Retarders Used
concerns associated with its use. It is the responsibility of the
in Contact with Soil or Granular Fill under Concrete Slabs
user of this standard to establish appropriate safety, health,
E2112 Practice for Installation of Exterior Windows, Doors
and Skylights
This guide is under the jurisdiction of ASTM Committee E06 on Performance
of Buildings and is the direct responsibility of Subcommittee E06.41 on Air
Leakage and Ventilation Performance For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2020. Published August 2020. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ɛ1
approved in 1964. Last previous edition approved in 2014 as E241 – 09 (2014) . Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E0241-20. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E241 − 20
E2136 Guide for Specifying and Evaluating Performance of 3.2 Other Definitions Found in ASTM Standards:
Single Family Attached and Detached Dwellings—
3.2.1 air barrier, n—a material or system in building con-
Durability
struction that is designed and installed to reduce air leakage
2.2 ASCE/SEI Standard:
either into or through an opaque wall or across a ceiling.
ASCE/SEI 24-05 Flood Resistant Design and Construction
3.2.1.1 Discussion—Source of this definition is Specifica-
2.3 ASHRAE Documents:
tion E1677.
ASHRAE Handbook – Fundamentals
3.2.2 opaque wall, n—exposed areas of a wall that enclose
ASHRAE Handbook – HVAC Applications
conditioned space, except openings for windows, doors and
ASHRAE Handbook – HVAC Systems and Equipment
building service systems.
ASHRAE Standard 62.1 Ventilation for Acceptable Indoor
3.2.2.1 Discussion—Source of this definition is Specifica-
Air Quality
tion E1677.
ASHRAE Standard 62.2 Ventilation for Acceptable Indoor
3.3 Definitions from ASHRAE—The following definitions
Air Quality in Low-rise Residential Buildings
are consistent with those in Chapter 27 of theASHRAE Hand-
ASHRAE Technical Data Bulletin, Vol 10, No. 3 Recom-
book of Fundamentals.
mended Practices for Controlling Moisture in Crawl
3.3.1 exfiltration, n—the uncontrolled flow of indoor air out
Spaces, 1994.
of a building through cracks and other unintentional openings
2.4 ISO Standard:
and through the normal use of exterior doors for entrance and
ISO 6707-1 Building and civil engineering—Vocabulary—
egress.
General Terms
3.3.2 infiltration, n—the uncontrolled flow of outdoor air
3. Terminology
intoabuildingthroughcracksandotherunintentionalopenings
3.1 Standard Definitions—Refer to Terminologies C168,
and through the normal use of exterior doors for entrance and
C717, D1079, and E631 for definitions of general terms.
egress.
3.1.1 perm, n—a measurement unit for time rate of water
3.3.3 ventilation, n—theintentionalintroductionofair,from
vapor migration by diffusion through a material or component.
the outside, into a building.
See Terminology C168 for the explicit definition.
3.4 Definitions from the U.S. Department of Energy:
3.1.2 vapor retarder (barrier), n—As defined in Terminol-
3.4.1 cold climate, n—a climate with between 5400 °F and
ogy C168, a material or system that adequately impedes the
9000 °F heating degree days (HDD) (65 °F basis) (or between
transmission of water vapor under specified conditions.
3000 °K and 5000 °K heating degree days (18.3 °C basis)).
3.1.2.1 Discussion—For low-rise residential construction,
3.4.1.1 Discussion—This definition is consistent with the
materials or components with a water vapor permeance not
climate classification system adopted by the U.S. Department
exceeding approximately one perm (60 ng/(s m Pa) are
of Energy’s Building America program. According to this
generallyconsideredvaporretarders(seePracticeC755).What
classification system, a climate with in excess of 9000 °F HDD
constitutes adequate restriction of water vapor transmission
(5000 °K HDD) is considered a very cold climate.
however depends on vapor pressure difference across the
construction (which in turn depends on interior and exterior
3.4.2 hot-humid climate, n—aclimatewhereannualprecipi-
conditions), ability of the construction to dissipate moisture,
tation exceeds 20 in. (500 mm) and one or both of the
and capacity of the construction to seasonally accumulate
following occur: (1) wet-bulb temperature exceeds 67 °F
moisture without damage.Therefore, a material or system with
(19.5 °C) for 3000 or more hours during the warmest six
a water vapor permeance exceeding approximately one perm
consecutive months of the year, or (2) wet-bulb temperature
(60 ng/(s m Pa) may in some circumstances provide adequate
exceeds 73 °F (23 °C) for 1500 or more hours during the
impedance to vapor transmission.
warmest six consecutive months of the year.
3.1.3 water vapor permeance, n—see Terminology C168. 3.4.2.1 Discussion—This definition is consistent with the
3.1.3.1 Discussion—Permeance is a performance evaluation climate classification system adopted by the U.S. Department
of Energy’s Building America program.
and not a property of a material. Permeance is expressed in
perms (IP units) or in ng/(s m Pa) (SI modified units).
3.5 Definitions of Terms Specific to This Standard:
3.1.4 water vapor permeability, n—see Terminology C168.
3.5.1 air leakage, n—infiltration or exfiltration, in other
3.1.4.1 Discussion—Permeability is a property of a mate-
words uncontrolled air flow into or out of a building through
rial. Permeability is the arithmetic product of permeance and
cracks and other unintentional openings and through normal
thickness.
use of exterior doors for entrance and egress.
3.5.1.1 Discussion—This definition is essentially the same
3 as that inTerminology C168, although expressed with different
Available from American Society of Civil Engineers (ASCE), 1801 Alexander
Bell Dr., Reston, VA 20191, http://www.asce.org. verbiage.
Available from American Society of Heating, Refrigerating, and Air-
3.5.2 building component, n—an inclusive term to collec-
Conditioning Engineers, Inc. (ASHRAE), 1791 Tullie Circle, NE, Atlanta, GA
30329, http://www.ashrae.org. tively refer to building materials, products, or assemblies.
Available from International Organization for Standardization (ISO), ISO
3.5.3 capillary break, n—a term applied to a material or
Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
Geneva, Switzerland, http://www.iso.org. system intended to inhibit liquid water transfer by capillary
E241 − 20
suction. The mechanism for inhibiting liquid water transfer is border of what is acceptable, and beyond which an unaccept-
by insertion of, or provision for, a discontinuity of capillary able level of damage to a building component may be
suction force. expected.
3.5.3.1 Discussion—A capillary break may be a membrane
3.5.10 serviceability, n—in a construction, the capacity of a
capable of blocking liquid water movement regardless of
building component or a construction to perform the func-
direction, or may be a coarse granular material capable of
tion(s) for which it was designed and constructed.
preventing capillary rise, while allowing drainage.An airspace
3.5.10.1 Discussion—This definition is similar to that found
may serve as a capillary break, where it is of such dimension
inTerminologyC168 as a subheading under the term “building
and configuration that bridging of water drops across the
performance.”
airspace is prevented. Membrane capillary breaks are com-
3.5.11 water or moisture, n—water as liquid, vapor, or solid
monly composed of synthetic polymers but may also be
(ice, frost, or snow) in any combination or in transition.
composed of corrosion-resistant sheet metal, asphalt impreg-
nated and coated felt, or, where lesser degrees of resistance to 4. Significance and Use
capillary transfer are required, asphalt-impregnated felt.
4.1 Moisture degradation is frequently a significant factor
3.5.4 critical cumulative exposure time, n—a moisture con- that either limits the useful life of a building or necessitates
dition parameter, this parameter is expressed as a time sum costly repairs. Examples of moisture degradation include: (1)
when moisture conditions are above a level that results in decay of wood-based materials, (2) spalling of masonry caused
cumulative damage to a building component, such that the by freeze-thaw cycles, (3) damage to gypsum plasters by
level of cumulative damage is deemed unacceptable. dissolution, (4) corrosion of metals, (5) damage due to expan-
sion of materials or components (by swelling due to moisture
3.5.4.1 Discussion—cumulative damage to a component
may occur over a range of moisture and temperature pickup,orbyexpansionduetocorrosion,hydration,ordelayed
ettringite formation), (6) spalling and degradation caused by
combinations, and damage is frequently more rapid at some
combinations than at others. The differing rate of damage saltmigration,(7)failureoffinishes,and(8)creepdeformation
and reduction in strength or stiffness.
accumulation at different sets of conditions is accounted for
with intensity factors, which are discussed in Chapter 26 of 4.1.1 Moisture accumulation within construction compo-
ASTM MNL 18 (1). nents or constructions may adversely affect serviceability of a
building, without necessarily causing immediate and serious
3.5.5 critical moisture content, n—a moisture condition
degradation of the construction components. Examples of such
parameter. This parameter is expressed as a moisture content
serviceability issues are: (1) indoor air quality, (2) electrical
level above which immediate or virtually immediate damage
safety, (3) degradation of thermal performance of insulations,
will occur to a building component at a given temperature,
and(4)declineinphysicalappearance.Moldormildewgrowth
such that the level of damage is deemed unacceptable.
can influence indoor air quality and physical appearance. With
3.5.6 durability, n—inconstructions,thecapacityofabuild-
some components, in particular interior surface finishes, mold
ing component or a construction to remain serviceable as
or mildew growth may limit service life of the component.
intended with usual and customary operation and maintenance
Moisture conditions that affect serviceability issues can fre-
during the designed service-life under anticipated internal and
quently be expected, unless corrected, to eventually result in
external environments.
degradation of the building or its components. This guide does
3.5.6.1 Discussion—This definition is similar to that found
not attempt however to address serviceability issues that could
inTerminology C168 as a subheading under the term “building
be corrected by cleaning and change in building operation, and
performance.”
that would not require repair or replacement of components to
3.5.7 flashing, n—a term applied to elements, most com-
return the building (or portions or components of the building)
monly fabricated of sheet metal, but which may also be
to serviceability.
fabricated of synthetic materials, used at interruptions and
4.2 Prevention of water-induced damage must be consid-
terminations of water shedding systems of roofs and walls, and
ered throughout the construction process including the various
intended to prevent intrusion of liquid water at these points.
stages of the design process, construction, and building com-
3.5.7.1 Discussion—This definition is consistent with, al-
missioning. It must also be considered in building operation
though not identical to, that found in ISO 6707-1.
and maintenance, and when the building is renovated, rehabili-
3.5.8 limit, v—to keep the value or level of some parameter,
tated or undergoes a change in use.
which is recognized as being problematic or potentially
4.3 This guide is intended to alert designers and builders,
problematic, below a value or level which is deemed to be
and also building owners and managers, to potential damages
objectionable.
that may be induced by water, regardless of its source. This
3.5.9 limit state, n—a value which expresses a moisture
guide discusses moisture sources and moisture migration.
condition parameter, generally a critical moisture content or a
Limit states (or specific moisture conditions that are likely to
critical cumulative exposure time, that is deemed to be at the
impact construction or component durability) and design
methods are also cursorily discussed. Examples of practices
that enhance durability are listed and discussed, as are ex-
amples of constructions or circumstances to avoid. The ex-
The boldface numbers in parentheses refer to a list of references at the end of
this standard. amples listed are not all-inclusive. Lastly, field check lists are
E241 − 20
given. The checklists are not intended for use as is, but as materials must not be overlooked. With proper design, con-
guides for development of checklists which may vary with struction and operation, moisture from wet building materials
specific building designs and climates. can, within limits, be dissipated without causing damage.
5.1.2.1 When wood frame walls are constructed with wet
building materials or under wet conditions, the walls should be
5. Moisture Sources and Migration
allowed to dry by evaporation before they are enclosed. Wall
5.1 Moisture sources for buildings can be broadly classified
designs that permit more rapid dissipation of moisture can
as follows: (1) surface runoff of precipitation from land areas,
accommodate being enclosed at higher moisture conditions
(2) ground water or wet soil, (3) precipitation or irrigation
than can wall designs with lower capacity to dissipate mois-
water that falls on the building, (4) indoor humidity, (5)
ture. Computer models (7.1.2) can be helpful in predicting
outdoor humidity, (6) moisture from use of wet building
drying rate in walls enclosed at higher than ideal moisture
materials or construction under wet conditions, and (7) errors,
contents.
accidents, and maintenance problems associated with indoor
5.2 Strategies to prevent or control moisture accumulation
plumbing.At a given instant of time the categories are distinct
in buildings fall into three broad categories: (1) limit moisture
from each other. Water can change phase and can be trans-
sources, (2) minimize moisture entry into the building or
portedoverspacebyvariousmechanisms.Watermaytherefore
building envelope, and (3) remove moisture from the building
beexpectedtomovebetweencategoriesovertime,blurringthe
or building envelope. Moisture control strategies that combine
distinctions between categories. Chapter 8 of ASTM MNL 18
these approaches are usually most effective.
(1) provides quantitative estimates of potential moisture load
from various sources.
5.3 Moisture can migrate by a variety of moisture transport
5.1.1 High indoor humidity during winter is often a major mechanisms.Acomprehensive treatment of moisture transport
cause of moisture problems in cold or temperate climates. and storage may be found in Chapter 1 ofASTM MNL 18 (1).
Moisture-induced damage may be expected unless the building The following mechanisms are most significant in building
is designed to tolerate the levels of indoor humidity that occur constructionsandarelistedinorderofpotentialmagnitude:(1)
in use. Conversely, moisture induced damage may be expected liquid flow by gravity, air pressure, surface tension,
unless indoor humidity is kept within limits that the building momentum, and capillary suction; (2) movement of water
will tolerate. Buildings should be designed and built so as to vapor by air movement; and (3) water vapor diffusion by vapor
tolerate indoor humidity levels commensurate with their in- pressure differences. These transport mechanisms can deliver
tended use. For some buildings (for example, those intended moisture into the building or the building envelope, in which
for habitation by persons with certain medical conditions or cases it is desirable that they be controlled. These transport
those housing swimming pools or textile production mechanisms can also act to remove moisture from the building
equipment), the levels of indoor humidity which the building or building envelope, in which cases they may be used to
should be expected to tolerate are moderately high, even if the promote drying.
buildingislocatedinacoldclimate.Converselyhowever,most 5.3.1 In control of moisture delivery to the building or
buildings are not designed nor built to tolerate high indoor
building envelope, the transport mechanisms that have the
humidities during winter. It is therefore unreasonable to expect potential for moving the greatest amounts of moisture should
suchbuildingstoperformadequatelyifoperatedathighindoor
(where practical) be controlled first. In promotion of drying of
humidities during winter. the building or building envelope, the transport mechanisms
that have the potential for moving the greatest amounts of
5.1.1.1 The potential for indoor humidity to cause damage
moisture should (where practical) be utilized first.
depends on the local climate. Occupant density, that is number
of occupants per given unit of space, and occupant activities
5.4 Building assemblies can become wet in three ways: (1)
frequently have a large influence on indoor humidity levels.
moisture can enter from the exterior, (2) moisture can enter
Among occupant activities that influence indoor humidity,
from the interior, or (3) the assembly can start out wet as a
cooking, bathing and laundry activities, and use of unvented
result of using wet building materials or building under wet
combustion appliances are those most likely to be significant.
conditions.
Air exchange between the living space and the exterior can
5.4.1 Moisturetypicallyentersbuildingassembliesfromthe
significantly lower indoor humidity levels during winter in
exterior through three mechanisms: (1) liquid flow by gravity,
temperate climates. Control of indoor humidity is discussed in
air pressure, surface tension, momentum, or capillary suction;
greater detail in 8.3 and its subsections.
(2) movement of water vapor by air movement; or (3) water
5.1.1.2 Mathematical evaluation tools (see 7.1.2 and 7.1.3)
vapor diffusion by vapor pressure differences.
can be used to identify if a given building design in a given
5.4.2 Moisturetypicallyentersbuildingassembliesfromthe
climate will tolerate a given level of indoor humidity or,
interior through two mechanisms: (1) movement of water
alternatively, to estimate tolerable indoor relative humidities
vapor by air movement, or (2) water vapor diffusion by vapor
for a given building design and climate.
pressure differences.
5.1.2 Although use of dry building materials is preferable, 5.4.3 Operation of mechanical equipment has not always
wetbuildingmaterialsarecommonlyused.Withsomebuilding beenrecognizedforitspotentialinfluenceonmoisturetransfer.
materials (for example cast-in-place concrete) a wet initial This potential influence should not be overlooked. Most
condition is an inherent characteristic of the material, and thus notably, air handling equipment can induce a moisture trans-
unavoidable. The influence of moisture from wet building port mechanism that is capable of moving large amounts of
E241 − 20
moisture, namely movement of water vapor by air movement. 7. Design Evaluation Tools
Unplanned pressurization or depressurization of buildings or
7.1 Means for evaluating the design of building envelopes
portions of buildings by air handlers can result in substantial
from the perspective of moisture management can be classified
moisture accumulations in the building envelope.
as follows: (1) conceptual, (2) mathematical using computer
simulation models, and (3) mathematical using calculations
5.5 Moisture can typically be removed (dried) to the exte-
that can be performed without computer software (sometimes
rior or the interior by three mechanisms: (1) liquid flow by
referred to as manual design tools).
gravity (drainage) or capillary suction, (2) movement of water
7.1.1 Conceptual Design Evaluation—This approach in-
vapor by air movement (ventilation), or (3) water vapor
volves the following three-step procedure: (1) determine prob-
diffusion by vapor pressure differences.
able external and internal environmental loads (determine
5.5.1 Where condensation of water vapor or water leaks can
climate and interior design conditions), (2) determine the
occur, weep paths to drain liquid water to a place where it can
potential moisture transport mechanisms in each assembly, and
be dissipated are often effective. Converting liquid water to
(3) select moisture control strategies. This approach provides a
vapor, and dissipating the vapor by air movement may also be
qualitativeperceptionofhowabuildingwillperformunderthe
practical.
influence of all the moisture loads the building is likely to be
subjected to. The Moisture Control Handbook (Lstiburek and
6. Limit States
Carmody, 1991) (3) provides a more comprehensive treatment
of this approach. Conceptual design evaluation can be used to
6.1 Identification of conditions that must be avoided in
select a construction for a given climate, as well as to evaluate
order to prevent degradation of building components is an
how a proposed construction may perform in a given climate.
important step in making design or operating decisions.
However, precise guidelines for identification of such condi- 7.1.2 Computer Hygrothermal Analysis Simulation
Models—These models have been developed to quantitatively
tions are generally lacking. Rather rough estimates based on
predict moisture and temperature conditions within proposed
empirical experience are often used.
assemblies using boundary conditions representative for the
6.2 Time and temperature are factors that are interrelated
climate and interior design conditions. As stated in Chapter 6
with moisture level in the degradation of building components.
ofASTM MNL 40 (4), the more detailed computer simulation
The moisture/temperature/time combinations that result in
models employ finite-element or finite-difference schemes.
material degradation furthermore vary with the type of mate-
Thesemodelsmathematicallymodelmoistureandheattransfer
rial. For example, wood will not decay, even at elevated
mechanisms at the inner and outer surfaces of the assemblies
moisture content when its temperature is near or below
and within the assemblies. Some of the models predict mois-
freezing, and even at temperature conditions conducive to
ture transfer by air movement and liquid water flow as well as
decay, wood can withstand intermittent wettings of short
by vapor diffusion. Use of such models requires knowledge of
durationtoelevatedmoisturecontentswithoutdecaybecoming
buildingphysicsandofthelimitationsofthemodelused.Most
established. Conversely, masonry units can generally be ex-
models allow estimates of the duration of a set of temperature
pected to withstand elevated moisture conditions at tempera-
and moisture conditions within assemblies. A discussion of
tures above freezing for extended time periods (conditions
available models is found in Chapter 2 ofASTM MNL 18 (1),
under which wood decay might be expected), but suffer
in Chapter 6 of ASTM MNL 40 (4), and in Chapter 23 of the
damage if frozen in a saturated condition.
ASHRAE Handbook of Fundamentals.
6.2.1 Many materials or constructions have threshold water 7.1.3 Manual Design Tools—These are termed “simplified
hygrothermal analysis method models” in Chapter 6 of
contents below which deterioration may be slow enough to be
negligible for designed life expectancy. As indicated in 6.1 ASTM MNL 40 (4) and “simplified hygrothermal design cal-
culations and analyses” in Chapter 23 of the ASHRAE Hand-
these threshold values are often rather rough estimates. See
book of Fundamentalss. Manual design tools, like computer
“Humidity and Building Materials” (Connolly, 1993) (2) for
simulation models, provide quantitative estimates of moisture
estimates.
conditions within building envelopes. They only account
6.2.2 The concepts of critical moisture content and critical
however for moisture transfer by vapor diffusion. Their focus
cumulative exposure time (see 3.5.4) are discussed in Chapter
is on predicting the occurrence of sustained condensation
26 of ASTM MNL 18 (1). Although these concepts are gener-
withinbuildingassemblies.Thecalculationsformanualdesign
allyrecognizedbybuildingscientists,organizeduseoftheseas
tools can be easily performed with a handheld calculator or in
limit states by designers has not yet become a well-recognized
a computer spreadsheet. The traditional design tool used in
practice.
NorthAmerica is a manual design tool and is referred to as the
6.3 Alimitstateisfrequentlybasedonavoidanceofdamage
dewpoint method. An example of the dewpoint method is
toacomponentastheresultofitsgettingwet.Alimitstatemay
outlined in Appendix X1.1 of Practice C755. The validity and
also be based on avoidance of damage to a component as a
usefulness of predictions made with manual design tools have
result of moisture conditions in an adjacent component. For
limitations. Most notably, manual design tools do not provide
example, limiting moisture-induced dimensional change of estimatesofthetimeperiodduringwhichpotentiallydamaging
plywood sheathing may be critical to prevent cracking of
conditions may occur. Despite the limitations of manual design
stucco cladding. tools, some relatively unsophisticated analysis procedures, like
E241 − 20
dewpoint analysis, can be useful for rapidly comparing relative 8.2.3.2 As suggested in 8.2.1, roof runoff is usually an
performances of many different proposed constructions. A exceptionally large potential water source. In temperate and
discussion of manual design tools is found in Chapter 11 of cold climates, exposure to roof runoff is one of the most
ASTM MNL 18 (1) and in Chapter 23 of the ASHRAE Hand- common causes of freeze-thaw spalling of masonry cladding
book of Fundamentals.
systems. Wood and wood-based cladding systems are widely
recognized as being incapable of performing adequately if
8. Examples of Practices that Enhance Durability exposed to roof runoff. Among the more common water
intrusion points in walls are the interfaces of walls with roofs,
8.1 Drainage of Precipitation and Surface Runoff:
especially with level or nearly-level roofs. Thresholds of doors
8.1.1 Surface Grading—Ground should slope away from
that open to balconies represent one of the most common sites
wallssothatprecipitationrunofffromlandareasdoesnotpond
of serious water intrusion into walls. Serious water intrusion at
near the foundation.
thesesitescangenerallybeexpectedunlessthebalconysurface
8.1.2 Building External Drains—Discharge from drains at
is pitched to drain water away from the wall. For the reasons
ground level should be carried away from the foundation, and
stated in this paragraph, it is generally accepted that walls of
should flow away from it.
buildings must not be exposed to roof runoff.
8.1.3 Below-Grade Drainage Systems—In some cases
8.2.4 Walls are most susceptible to water intrusion at joints
below-grade drainage systems may be required. In some cases,
in and penetrations of the exterior cladding system. Joints
dissipation of collected water by pumping will be required.
between the cladding system and windows and doors are
Below grade drainage systems are discussed in Chapter 2 of
locations susceptible to water leakage. Junctures of walls with
The Moisture Control Handbook (Lstiburek and Carmody,
large horizontal or sloped surfaces (for example roofs, decks,
1991) (3).
or balconies) are susceptible to leakage. Therefore, particular
8.2 Limiting Intrusion of Precipitation:
care is required at these locations.
8.2.1 Precipitation has the potential for delivering excep-
8.2.5 Strategies for control of rainwater that is deposited on
tionally large moisture loads to buildings, and is usually the
building walls can be broadly categorized as follows: (1)
largest potential moisture source (see Chapter 8 of
strategies to prevent water penetration of the outermost face of
ASTM MNL 18) (1). It is imperative that this source be
the wall system, (2) strategies to dissipate water that penetrates
controlled, specifically that precipitation be excluded from the
the outermost face of the wall system. Strategies in these two
building envelope. In some cases, entry of limited mounts of
general categories often are effectively used in combination.
precipitation into the envelope can be tolerated provided that it
Strategies for control of rainwater deposited on building walls
is rapidly dissipated by drainage, or (typically more slowly) by
are discussed in Chapter 2 of The Moisture Control Handbook
evaporation.
(Lstiburek and Carmody, 1991) (3). Further discussion on the
8.2.1.1 Moisture from precipitation enters building enve-
subject, as well as recommendations concerning design details
lopesalmostexclusivelyinliquidform,eitherasrainorasmelt
are found in “Nail-On Windows” (Bateman, 1995) (5). It is
water from ice or snow.
importantthatthestrategyorstrategiesselectedbythedesigner
8.2.2 The water exposure of horizontal or sloped surfaces
be clearly understood by construction contractors and those
(that is, roofs) is almost always greater than that of walls.
responsible for maintenance of the building.
Shedding and drainage of water from roof surfaces is impera-
8.2.5.1 Exterior Mechanicals—Penetrations of this type (for
tive. These surfaces must essentially be water tight (that is, not
example, electrical equipment) should be of a type suited for
leak).Penetrationsthroughwatersheddingmembranesofroofs
exterior service and be installed with adequate moisture seals.
are common leakage points; flashings are almost always
required at such penetrations. Design, installation and mainte-
8.2.5.2 Fenestration—Important consideration in selection
nance of roofs are very important.There is an entire volume of of fenestration units (windows and doors) are (1) the ability of
theAnnual Book ofASTM Standards (Vol 04.04) that contains
the units themselves to shed water, and (2) the ability with
standards concerning roofing and waterproofing. Therefore, a which the units can be integrated into the building’s water-
comprehensive treatment of these subjects is not attempted in shedding system.
this guide.
(1) A unit’s resistance to water penetration can be
8.2.3 Water intrusion through building facades (in low rise identified, in part, by laboratory tests such as Test Methods
construction, this primarily means walls) can be of substantial E331 and E547. Third party certification of a product’s water
consequence. There are two broad strategies for controlling resistance is highly recommended to help identify whether the
rainwater intrusion into walls: (1) reduce the amount of product is appropriate for its intended application (anticipated
rainwater deposited on building walls, and (2) control rainwa- in-service exposure of the unit to wind and rain).
ter that is deposited on building walls. (2) Proper installation and integration of the product with
8.2.3.1 Reducing rainwater deposition on wall assemblies the building’s water-shedding system are essential. Practice
has traditionally been a function of siting and architectural E2112 provides guidance for proper installation and water-
design. The following measures have historically proven ef- shedding system integration for simple fenestration products.
fective: (1) site buildings so they are sheltered from wind- Formorecomplexsystems(suchasmulledunits,stackedunits,
driven rain, (2) provide roof overhangs and gutters or other or new designs), pre-construction mock-up testing and field
piped roof drainage systems to shelter walls from direct rain testing early in the building project can be valuable for
exposure or roof runoff. purposes of risk reduction and quality assurance. Field testing
E241 − 20
isespeciallyvaluablewherewatermanagementandintegration climates, particular attention during design and construction is
details are unclear or are not provided. Test Method E1105 necessary so that the building will tolerate such levels.
outlines a useful field testing technique.
8.3.4 In most heating climates during cool or cold weather,
(3) Cladding termination accessories, window installation
air exchange with the exterior can significantly reduce indoor
accessories, or site-fabricated trim may provide a transition
humidity (Chapter 15 ofASTM MNL 18 (1) and Chapter 24 of
between fenestration units and the surrounding cladding sys-
the ASHRAE Handbook of Fundamentals). Chapter 24 of the
tem. Water penetration can occur at the interfaces between
ASHRAE Handbook of Fundamentals suggests that at normal
theseentitiesandeitherthefenestrationunitorthesurrounding
rates for residential occupancy and moisture generation and in
claddingsystem.Adequatebuildingdesignandadequatework-
all but mild humid climates, ventilation to a level of 0.35 air
manship during construction are both essential to reducing the
changes per hour (as recommended in ASHRAE Standard 62)
potential for water intrusion and water-induced damage to the
will usually be sufficient to prevent excessive indoor humidity.
building.
Mechanicaldehumidificationisrarelyusedforindoorhumidity
(4) Appropriate maintenance of the fenestration product
control during cold weather. In mild humid climates, air
and its interfaces with the wall system will help ensure
exchange with the exterior may be of limited effectiveness for
long-term delivery of the desired water penetration resistance.
control of indoor humidity. In these climates, dehumidification
Ifthewater-sheddingcapabilitiesofaunitarecompromisedby
may be more effective than ventilation for controlling indoor
mechanical damage or deferred maintenance, water intrusion
RH, but as indicated in 8.3.1 is more likely to be deemed
into the wall can occur.
necessary for reasons other than that of durability of the
building structure.
NOTE 1—The considerations mentioned previously in this section as
8.3.4.1 In designing for provision of air exchange between
applicabletofenestrationunitsinwalls(doorsandwindows)alsoapplyto
skylights. Skylights, which are installed on roofs, can be expected to have
the living space and the exterior, energy efficiency and air
greater weather exposure than fenestration units in walls.
quality considerations as well as durability considerations are
usually important.
8.2.5.3 Sealant Joints—Incontrasttohigh-riseconstruction,
8.3.4.2 In buildings constructed prior to 1970, air exchange
design of sealant joints in low-rise construction has generally
between building interiors and the exterior during winter in
not become a well-developed discipline. Design of reliable
temperate and cold climates has occurred primarily by a
sealant joints can include many factors such as: sealant-
combination of infiltration (much of which occurred through
substrate compatibility, avoidance of 3-sided adhesion, joint
fenestration units) and escape of air up chimneys (a combina-
geometry and anticipated movements in joints (see Guide
tion of air movement through furnaces, draft hoods, and
C1193). Workmanship, including conditions under which seal-
barometric draft dampers). The effect of chimney draft has
ant joints are installed, is also important. Maintenance of
often been sufficiently great that the buildings have operated at
sealant joints must not be overlooked, since anticipated life of
a negative air pressure relative to the exterior, causing air
sealant joints will almost certainly be substantially less than
leakage through the building envelope to be predominantly
design life of the building.
infiltrative.Infiltrativeairleakageisnotcapableoftransporting
8.3 Control of Indoor Humidity:
interior moisture into the envelope. Air exchange rates have
8.3.1 From the standpoint of building durability, indoor
been uncontrolled, responding to air temperature differences
humidity control is primarily of concern during winter in
and wind effects. During cold windy weather, air exchange
temperate or cold climates. It may also be of concern however
rates have often been well in excess of the amounts recom-
in air conditioned buildings in hot humid climates, particularly
mended as necessary byASHRAE Standard 62. In some cases,
if the building is designed to dry toward the interior. In mild
the air exchange rates during cold weather have been overly
weatherinanyclimate,humiditycontrolmaybeofimportance
effective at reducing indoor humidity levels (sometimes to
from the standpoint of preservation of property within the
levelsbelow25 %).Althoughsubstantiallylessthanidealfrom
structure or from the standpoint of indoor air quality (for
an energy use perspective, buildings that operate in this
example, preventing mold growth that releases spores and
traditional mode generally have not suffered significant
musty odors or inhibiting the propagation of dust mites), but
moisture-induced durability problems. Many existing
generally is not of great concern to durability of the building
buildings, perhaps a majority of existing buildings, operate in
structure.
this traditional mode during cold weather.
8.3.2 Indoor humidity can be limited by controlling mois-
8.3.4.3 Since the 1970s, buildings have generally been built
ture sources or by removing humidity by air exchange with the
so that they can be heated with less energy. For a building of
exterior or by dehumidification.
a given size, the increased energy efficiency has resulted in
8.3.3 As indicated in 5.1.1 and 8.3.1, the indoor humidity furnacesofsmallersizeorfurnacesthatrunalowerpercentage
(RH) level that a given building will tolerate is climate- of the time during the heating season, or both. The result has
dependent. Experience and computer simulation models sug- generally been a greatly reduced rate of furnace-induced
gest that damaging moisture accumulations can be expected in exhaustofinteriorairviathechimney.Inaddition,thebuilding
many buildings of customary design in cold climates if winter envelope, including fenestration units, have become more
indoor RH in heated buildings is maintained at levels in excess resistant to air leakage. The result is that some buildings now
of 35 to 40 %. When indoor humidity levels above 35 to 40 % operate at much lower air exchange rates than recommended
are necessary or desired during the heating season in cold byASHRAE Standard 62, and the low air exchange rates have
E241 − 20
in some cases resulted in excessive indoor humidity levels. moisture, but require that adequate provision be made for
With reduced rates of furnace-induced exhaust of indoor air, make-up air. If adequate provision for make up air is not made,
the probability for negative pressurization of the building is they may not vent effectively, and may cause dangerous levels
reduced. This in turn means that a greater proportion of the of depressurization within the building. If depressurization in
building’s air leakage (than had previously been the case) is the location of natural-draft combustion appliances exceeds
likely to be exfiltration. approximately5Pa,back-draftingoftheappliancesmayresult.
(1) Building scientists generally agree that modem build- Back-drafting has also been observed at negative pressures of
ings in heating climates that are reasonably energy efficient smaller than 5 Pa. In some cases, installation of safety
will in many cases have insufficient air exchange rates (from interlocks,whichpreventoperationofnaturaldraftcombustion
appliances when high capacity kitchen exhaust fans are in
the perspective of either indoor air quality, as outlined in
ASHRAE Standard 62, or from the perspective of indoor operation, may be justified.
humidity control) unless they are provided with some type of (3) Exhaust from powered ventilating equipment should be
ventilation system. From a prescriptive standpoint, there is not ducted all the way to the outdoors. Ducting may pass through
full consensus concerning what constitutes an adequate venti- locations that are cool or cold. When this occurs, precautions
lation system. Powered ventilatio
...