A I A / A RC H I T E C T U R A L R E C O R D
CONTINUING EDUCATION Series
Moisture Management in Wall Assemblies:
Air, Wa t e r, and Vapor Barriers
Selecting the appropriate protective barrier
based on climate, codes, and design criteria
Photograph © 2006 David S. Allee
Provided by DuPont Tyvek
egardless of project location or building type, the goal of a successful building design
is to keep water out and provide thermal control within the interior spaces.
Understanding the basics of moisture control and the role of air, water, and vapor
barriers in design of wall assembly systems is important in order to avert potential
problems that can arise from air and water infiltration.
R
The building envelope, or building enclosure, separates indoor and outdoor environments. A
building enclosure controls heat flow, airflow, water vapor flow, rain, groundwater, light and
solar radiation, noise and vibrations, contaminants, environmental hazards, odors, insects,
and fire.
When m o i s t u re infi l t rates the building envelope, seve ral undesirable conditions can
o c c u r, including mold and mildew, structural steel corrosion, and rotting wood. These
conditions can result in high energy costs, ongoing maintenance pro b l e m s ,
compromised indoor air quality, and failure of one or more architectural and
engineering building systems. If they are not adequately addressed, the pro b l e m s
caused by moisture infiltration can potentially increase risk and liability concerns for
architects, design professionals, building ow n e rs, and building occupants.
M o i s t u re is the ge n e ral term used for water in its diffe rent physical states, and
includes solid (ice), liquid (water), and gas (water vapor). For all pra c t i c a l
purposes, moisture moves through the building enclosure as liquid water and as
water va p o r. Moisture problems are ge n e rally the result of liquid wa t e r
accumulation, which could be from liquid water infi l t ration or condensation of
excess water vapor transported by air currents or by vapor diffusion. Condensation
CONTINUING
EDUCATION
Use the learning obj ectives bel ow to focus your stu dy as you
re ad Moi s t u re Ma n a gem ent in Wall As s em blies: Ai r, Water,
and Vapor Barriers. To earn one AIA/CES Le a rning Unit,
including one hour of health safety wel f a re credit, a n s wer the qu e s ti on s
on page 7, t h en fo ll ow the reporting instructi ons on page 8 or go to the
Con ti nuing Edu c a ti on secti on on a rch re cord.construction.com and follow
the reporting instructi on s .
LEARNING
OBJECTIVES
After reading this article, you will be able to:
Uncontrolled air leakage occurs from higher to lower pressure and could
transport significant amounts of water vapor
of excess water vapor occurs on surfaces with tempera t u res below the dew point
t e m p e ra t u re, which is the onset of condensation.
Some moisture problems cannot be avoided. Proper design can help reduce risk and make
a building more tolerant to moisture. This article will describe moisture control strategies,
and the use of protective barriers, including water-resistant barriers, air barriers, and vapor
barriers. Requirements related to the position of each barrier in the building envelope for
moisture management and condensation control will also be addressed.
Bulk Water Transport Through The Bulding Enclosure
Rain is the main water source for above grade walls. Rain can penetrate behind cladding
and into the building enclosure through any openings, cra c ks and gaps in the assembly, and
accumulate inside the wall cavity. The main mechanisms for bulk water intrusion include
g ravity, capillary action, kinetic energy of rain driven droplets, and pre s s u re differential.
• Identify moisture management strategies for buildings
• Analyze design criteria for air, water, and vapor barriers
• Determine the impact of climate, heating, and cooling cycles on selection and
location of protective barriers within wall assemblies
• Discuss code requirements for air, water, and vapor barriers
• Design wall assemblies using protective barriers
Reprinted from Architectu ral Record, December 2006
Architectural
Record
Version: 01
Issue
Date: 11/06
Page
No: 208
Filename: DuPont Tyvek_CE_11_06
Writer/ Designer: Paul
Carroll
Last
Revision:
11/21/2006
There are two basic approaches for rain penetration control: control the driving forces
across the openings or eliminate the openings. The first approach includes proper sloping
to the outside (gravity drainage to outside), capillary break (to control capillary action),
shielding the openings (to control rain penetration
Rain is the main water
due to kinetic energy), and pressure equalized rain
source for above grade
screen walls (to control pre s s u re diffe re n t i a l
walls.
across the enve l o p e ) .
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Section 1404.2. Water-resistive barrier: A minimum of one layer of No. 15 asphalt felt,
complying with American Society for Standards and Materials (ASTM) D226 for Type 1 felt
or other approved materials, shall be attached to the studs or sheathing, with flashing as
described in Section 1405.3, so as to provide a continuous water-resistive barrier behind the
exterior wall veneer.
Section 1405.3 Flashing: Flashing shall be installed in such a manner so as to prevent
moisture from entering the wall or to redirect it to the exterior.
Water intrusion mechanisms through above grade wall assemblies
The second approach consists of eliminating the openings. This can be achieved through a
face-seal design, which is difficult to achieve, due to sealants exposure and extensive
maintenance requirements, or by using a secondary line of defense, such as a waterresistant barrier.
Water-Resistant Barriers
Water-Resistant Barriers are materials specifically designed to protect against rain
penetration and prevent water leakage into the building interior. Water-resistant barriers,
also referred to as the drainage plane, are typically installed behind cladding for the two
main types of wall assemblies, cavity insulation and exterior insulation.
In wall assemblies with insulation inside stud cavity, the wa t e r- resistant barrier is typically
installed on the exterior face of the exterior sheathing. In exterior insulation walls, the wa t e rresistant barrier can be either sandwiched between the exterior sheathing and the exterior rigid
insulation(1), or outside of the exterior rigid insulation(2). The former is the most common
installation, while the latter should be used for insulation materials that are sensitive to water.
Water-Resistant Barrier
Stud Cavity insulation
Water-Resistant Barrier
Exterior rigid insulation
Typical position within the building envelope for water-resistant barriers
Codes and Performance Requirements
The 2006 International Building Code (IBC) requires that water-resistant barriers be used and
installed to provide a continuous drainage plane and drainage pathways to the outside of the
building envelope, including weep holes and through-wall flashing. The following excerpts apply:
Section 1403.2. Weather protection: The exterior wall envelope shall be designed and
constructed so as to prevent the accumulation of water within the wall assembly by providing a waterresistive barrier behind the exterior veneer, as described in Section 1404.2, and a means for draining
water that enters the assembly to the exterior.
2
Materials Requirements
The minimum requirement for water-resistant barriers is for the material to withstand a
fixed hydrostatic pressure of 55 centimeters (cm) for five hours without leaking, as per the
American Association of Textile Chemists and Colorists test, known as AATCC-127 or the
Hydrostatic Head test. This standard is based on a No. 15 asphalt felt and is the minimum
re q u i rement. There are many wa t e r- resistant barriers that meet and exceed this
requirement, such as fluid applied membranes, self-adhering membranes, and many nonperforated building wraps. Perforated wraps have virtually no hydro-head and do not qualify
as water-resistant barriers.
Water Vapor Transport Through The Building Enclosure
Water vapor can be transported into the building enclosure in two different ways, via air
transported moisture and vapor diffusion.
Air transported moisture is the main source of water vapor in the building enclosure. Air
leakage occurs through porous materials and unintentional openings in building assemblies,
such as cra c ks and channels, due to the diffe rence in total air pre s s u re across the assembly.
The air fl ows from higher to lower pre s s u re and it could transport significant amounts of wa t e r
vapor into the building enclosure. The amount of water vapor contained in the air depends on
temperature and re l a t i ve humidity, with warm air
being able to hold more mo i s t u re than cold air. The location of air and
Even though air leaka ge has consequences water-resistant barriers in
b eyond moisture transport, including energ y
the wall assembly depends
efficiency, indoor air quality (IAQ), and comfort,
the air transported moisture is one of the most on continuity, durability,
damaging consequences of uncontrolled airfl ow. and maintainability.
Air Barriers
Materials that resist airflow are called air barriers. Many building materials can resist
airflow and therefore could function as air barrier components. However, for effective
envelope airtightness, these materials must be joined together into airtight assemblies,
which are further joined into a continuous air barrier system for an airtight building
enclosure. Air barrier membranes are materials designed to resist air infiltration and provide
a more practical way to achieve continuity. A continuous air barrier controls airflow, hence
the moisture migration through air currents as well as other undesirable consequences of
unplanned airflow.
The location of air barriers within the building envelope is not important from the standpoint
of controlling air leakage, as it can be located anywhere in the wall assembly. However, the
location is very important for durability and constructability. For durability, it is preferable
to have the air barrier behind the exterior cladding, to protect it from direct weather
exposure. For constructability, it is preferable to have the air barrier outward of the
structural frame, for maintaining continuity at penetrations associated with structural
elements. Air barriers are ideally located towards the exterior of the building envelope,
because it is easier to achieve and maintain continuity.
Moistu re Ma n a gement in Wa ll Assemblies: Ai r, Wa ter, and Va por Ba rriers
Architectural
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Date: 11/06
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No: 208
Filename: DuPont Tyvek_CE_11_06
Writer /Designer: Paul
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Last
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The installation requirements for air barriers are similar to those four water-resistant
barriers. In wall assemblies with insulation inside stud cavity, the air barrier is typically
installed on the exterior face of the exterior sheathing. In exterior insulation walls, the air
barrier can be either sandwiched between the exterior sheathing and the exterior rigid
insulation or outside of the exterior rigid insulation. The former installation is most
common; the latter should be used when thermal insulation performance can be affected
by wind-washing. According to the Building Science Corporation, wind-washing is the
phenomenon of air movement that occurs due to wind entering building enclosures,
typically at the outside corners and roof eaves of buildings. Wind-washing can have
significant impact on thermal and moisture movement, and hence thermal and moisture
performance of exterior wall assemblies.
The location of both air and water-resistant barriers in the wall assembly depends on
continuity, durability, and maintainability. A single membrane could perform as an air and
water barrier, if proper materials are chosen. An air barrier membrane is often waterresistant; however, not all materials that meet the minimum requirement for water
resistance, such as asphalt impregnated felts, and papers, are effective air barriers. Other
materials, such as perforated wraps, are not effective as water or air barriers.
Air Barrier Codes and Performance Requirements
Four main performance requirements for air barrier materials and systems include air
i n fi l t ration resistance, continuity, structural integrity, and durability.
These criteria are described in Chapter 26 of the American Society of Heating, Re f r i ge ration and
Air-Conditioning Engineers (ASHRAE) Fundamentals Handbook and summarized by the Air
Barrier Association of America at www.airbarrier.org.
Air Barrier Materials
There is no national standard for acceptable air infiltration rates of air barrier materials in the U.S.
The National Building Code of Canada, which has required air barriers since 1995, specifies that air
infiltration rates of air barrier materials must not exceed 0.004 cfm/sq. ft. (cubic feet per minute per
square ft.) at 0.30 inch water pressure differential. This standard was adopted by Massachusetts in
2001, and is proposed for adoption by Minnesota in 2007.
Examples of materials that qualify as air barriers include self-adhered membranes, fluid applied
membranes, non-perforated building wraps, and closed cell polyurethane foams. Examples of
building materials which do not qualify as air barriers
There is no national
include asphalt impregnated papers and felts, perforated
standard for acceptable
housewraps, expanded polystyrene (EPS), plain and
air infiltration rates of
asphalt impregnated fiberboard, uncoated concrete block,
batt and semi-rigid fibrous insulation, and cellulose
air barrier materials in
spray-on insulation.
the U.S.
cfm/sq. ft. at 0.30 inch water. No trends towards increased airtightness in more recent
buildings were found.
The impact of building envelope airtightness on building performance is ge n e ra l l y
recognized. However, while existing codes require sealing of the building enclosure, there
is no quantifiable measure for an acceptable air-tightness of the opaque building envelope,
unlike windows, which require some measure of airtightness for compliance.
An excerpt from the International Code Council (ICC) requirements for building envelope
sealing is as follows:
502.4.3 Sealing of the building envelope. Openings and penetrations in the building envelope
shall be sealed with caulking materials or closed with gasketing systems compatible with the
construction materials and location. Joints and seams shall be sealed in the same manner or
taped or cove red with a moisture vapor-permeable wrapping material.
It should be noted that the code specifies the use of a moisture vapor-permeable wrapping
material, as opposed to a vapor non-permeable material, or vapor barrier, which could
interfere with condensation control strategies.
The code description on the sealing of the building envelope provides no guidelines on the
recommended airtightness, and has been compared to requiring that care be taken when
installing insulation without specifying a minimum R-value. Some states and code
organizations have adopted compliance criteria for airtightness of materials or assemblies
airtightness, including the National Building Code of Canada (NBCC) (1995), Massachusetts
(2001), Wisconsin (2003), and Minnesota (2007). In 2006, ASHRAE approved an amendment
to the ASHRAE 90.1 model energy code that would require air barriers for most commercial
buildings, and provide three quantifiable compliance options: air barrier materials, not to
exceed 0.004 cfm/sq. ft. at 0.30 inch water; air barrier assemblies, not to exceed 0.04 cfm/sq.
ft. at 0.30 inch water; and whole building, not to exceed 0.4 cfm/sq. ft. at 0.30 inch wa t e r.
Vapor diffusion is the second water vapor transport mechanism through the building
enclosure, in addition to air-transported moisture. Vapor diffusion is the movement of water
vapor molecules through vapor permeable materials, from regions of higher vapor pressure
to lower vapor pressure, or higher vapor concentration to lower vapor concentration.
The rate of water vapor diffusion depends on vapor pressure difference, or concentration
difference, across the material or assembly, as well as the water vapor permeance, which
is expressed in perms. Based on water vapor permeability, materials are classified as vapor
permeable or vapor non-permeable.
Air Barrier Assemblies
The standard specification, ASTM E1677, requires that air infiltration rates for air barrier
assemblies must not exceed 0.06 cfm/sq. ft. at 0.30 inch water pre s s u re diffe rential. These
assembly requirements we re adopted by the Wisconsin energy code in 2003. ASTM E 2357 is
a new test method which was approved in 2006 and can be used as an alternative test method
for air barrier assemblies.
Whole Building Airtightness
There are no accepted criteria for what constitutes an airtight building. A study by the
National Institute of Standards and Technology (NIST) entitled, “Investigation of the Impact
of Commercial Building Envelope Airtightness on HVAC Energy Use,” summarizes envelope
airtightness of 166 buildings, 144 in North America and 22 in the U.K. The air leakage rates
measured by various standard methods and normalized by area of the above-ground portion
of the building envelope were found to vary over a broad range, from less than 0.5 to 2.7
Water vapor diffusion is the transport of water vapor molecules through
vapor permeable materials
Moistu re Ma n a gement in Wa ll Assemblies: Ai r, Wa ter, and Va por Ba rriers
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Date: 11/06
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No: 208
Filename: DuPont Tyvek_CE_11_06
Writer/ Designer: Paul
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Vapor permeable materials allow vapor diffusion. Vapor permeability can va ry over a wide range. The
higher the perm rating, the more permeable the materials. According to the 2006 International
Building Code (IBC), materials must have a minimum of fi ve perms to be considered vapor permeable.
Vapor non-permeable materials do not allow vapor diffusion. These materials are called
vapor barriers or vapor retarders. According to the 2006 IBC, materials with vapor
permeability of 1.0 perm or less are vapor barriers.
Vapor Barriers
Vapor diffusion is another source of moisture for the building enclosure, and a vapor barrier
(or vapor retarder) is sometimes used to control diffusion and potential condensation.
However, the amount of water vapor transported through vapor diffusion is significantly
lower than the amount transported by air currents. It is estimated that less than 2% of all
water vapor movement through the building enclosure is due to diffusion, while over 98%
is due to air transported moisture. Consequently, in order to avoid conditions which could
lead to interstitial condensation it is critical to firstly protect against air leakage (using air
b a r r i e rs), and protect against vapor diffusion when appropriate. The installation
requirements for air barriers and vapor barriers are quite different, and using a single
membrane to perform both functions (e.g. air and vapor barrier) could lead to condensation
problems if not properly understood. For example, while the location of air barriers within
the building envelope is not important from the standpoint of controlling air leaka ge, and the
air barrier can be located anywhere in the wall assembly, the vapor barrier location is critical
for condensation control, and it is climate specific.
understanding of different climate needs and the consequences of the impact of vapor
barriers on diffusion drying was reached, the code was changed. The 2006 IBC no longer has
prescriptive requirements for vapor barrier use in mixed climate zones 1, 2, 3, and 4, which
cover southern and coastal zones of the U.S.
Dry (B)
Moist (A)
Marine (c)
Warm-Humid
Below White Line
All of Alaska in Zone 7
except for the fo l lowing
Boroughs in Zone 8
Bethel
Dellingham
Fairbanks N. Sta r
Nome
North Slope
Northwest Arctic
Southeast Fairbanks
Wade Hampton
Yukon-Koyukuk
Zone 1 includes
Hawaii, Guam,
P u e r to Rico ,
and the Virgin Islands
US Climate zone map
The 2006 International Building Code does not mandate vapor barrier
use in climate zones 1, 2, 3, and 4 (below the black line)
Climate Issues
The reason climate is important when considering diffusion drying is because the climate
determines the exterior temperature and relative humidity. This determines the exterior
vapor pressure, hence the vapor pressure difference between the exterior and the interior
conditioned space, therefore the diffusion direction. The climate determines how buildings
dry, whether towards the inside or towards the outside, depending upon the sign of the
vapor pressure difference, whether positive or negative.
Typical vapor barrier location in the wall assembly
Heating Climates
Cooling Climates
Vapor barrier inside
Vapor barrier outside
The vapor barrier must be located on the side of the building envelope with higher vapor
pressure in order to prevent diffusion into the envelope, known as diffusion wetting, and
not interfere with diffusion of incidental moisture out of the envelope, or diffusion drying.
As a general rule, for walls with insulation inside the stud cavity, the vapor retarder should
be located toward the inside in climates dominated by heating, and toward the outside in
cooling dominated climates.
While these general rules are useful guidelines, it is still appropriate to conduct a
condensation analysis for specific climates, building envelope systems, and intended
building use. Many U.S. climates have both heating and cooling cycles, and simple rules
may not apply. In such cases, the vapor barrier may end up on the wrong side during one of
the two cycles.
The vapor barrier codes were first introduced in Canada, with predominantly heating
climates, where vapor barriers were installed on the interior (warm) side of the wall. The
IBC then adopted similar requirements across all U.S. climate zones, without a proper
understanding of the impact of vapor barriers in different climates. As a greater
4
Diffusion direction: from higher to lower water vapor concentration (or
higher to lower vapor pressure) and it is climate specific
Material Choices
The choice of materials is critical for providing a diffusion open pathway in order to promote
diffusion drying. As a general rule, condensation control requires that the building envelope
materials increase in permeance in the direction of vapor diffusion. This means that in
predominantly heating climates, where diffusion is typically from the inside to the outside,
the wall assembly must have vapor permeable materials towards the outside. In cooling
climates, where diffusion is typically from the outside to the inside, the wall assembly must
have vapor permeable materials towards the inside. In mixed climates, proper moisture
management requires diffusion open pathways in both directions, to the inside during
summer and to the outside during winter.
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Moisture Management Principles And The
Choice Of Barriers
Moisture problems occur if buildings get wet and stay wet, because they are unable to dry.
Ac c o rding to building scientist Joe Lstibure k, Ph.D., P.E., “moisture problems are
fundamentally rate issues: moisture accumulation occurs if the wetting rate exceeds the
drying rate.” Consequently, moisture management must include strategies to manage the
balance between wetting and drying, that is, to prevent wetting and promote drying.
The main moisture sources, bulk water, air transport, and diffusion, do not have an equal
contribution to wetting. In order of their contributions, these sources are bulk or rain water
infiltration, which is usually the largest moisture source for above-grade walls; air
transported moisture, which could contribute up to 200 times more moisture than water
vapor diffusion; and water vapor diffusion, which has the smallest contribution to wetting,
when compared to the other two sources.
Wetting
1.Bulk Water
2.Air Transport
3. Diffusion
Drying
1. Drainage
2. Ventilation
3.Diffusion
The balance of wetting and drying, main sources and mechanisms
In order to prevent wetting, it is essential to protect against the main moisture sources,
bulk water (with a water-resistant barrier) and water vapor transported by air currents (with
an air barrier). In addition, it is recommended to protect against moisture transported by
diffusion (with a vapor barrier), as long as the use of a vapor barrier does not interfere with
the wall drying ability.
The potential drying mechanisms in buildings include drainage of incidental water to the
outside, controlled ventilation (not to be confused with uncontrolled air leakage through the
building envelope), and vapor diffusion.
For example, a continuous drainage plane is an effective strategy for removing incidental
bulk water, and a ventilated air space behind the cladding can reduce the moisture content
in the air space outside the sheathing and facilitate drying. However, in order for ventilation
drying to be effective, it is still necessary for water vapor to be able to diffuse from the
envelope into the ventilated space. Given the importance of vapor diffusion as a drying
mechanism, it is critical that a diffusion pathway is considered.
Wall design is important because the arra n gement of building envelope elements
influences the conditions within the wall, and therefore the environment under which the
materials must function.
wetting, the wall must have diffusion open pathways, to allow diffusion drying. This wall
design relies on diffusion drying for incidental moisture management.
For the exterior rigid insulation wall design, the main temperature gradient occurs across
the exterior insulation, outside the interior wall cavity. As a result, the dew point
temperature is moved outside the wall cavity, reducing the potential for condensation. This
wall design is more robust and less sensitive to condensation, and does not rely on vapor
diffusion for drying.
In order to choose the appropriate barriers for building projects, the fo l l ow i n g
guidelines apply:
Water-Resistant Barriers are critical for most climates to prevent bulk water intrusion.
They are typically installed behind the cladding, towards the exterior side of the building
envelope. If the water-resistant barrier is vapor permeable, it will not interfere with the
condensation control strategies.
Air Barriers are critical for most climates to prevent heating and cooling energy loss, as well
as prevent moisture transported by air currents. The location of an air barrier within the
building envelope is not important from the standpoint of controlling air leakage, because
an air barrier can be located anywhere in the wall assembly. Air barriers are located
towards the exterior of the building envelope for ease of constructability and continuity. If
the air barrier is vapor permeable it will not interfere with the condensation control
strategies. A single membrane is generally used to perform both air barrier and water
barrier functions.
Vapor Barriers should only be used in those climates where diffusion into the wall cav i t y
occurs predominantly in one direction. The vapor barrier should be located on the high va p o r
p re s s u re side of the envelope. In climates dominated by heating, this means that the va p o r
barrier should be located towa rd the inside, and in cooling climates towa rd the outside.
A single membrane can perform all three functions, and serve as an air, water, and vapor
barrier. When a membrane is intended to be used as a vapor barrier, in addition to an air and
water barrier, it must be determined if the typical location of air and water barrier, such as
on the exterior of the building envelope, also meets the condensation control criteria for a
vapor barrier, that is, if the vapor barrier is on the high vapor pressure side of the building
envelope.
Every time that a vapor non-permeable membrane (such as vapor non-permeable peel and
stick or fluid applied membrane) is used with insulated stud cavity wall design, conditions
for condensation are created in most North American climates. In predominantly heating
climates, a vapor non-permeable air and water barrier installed on the exterior side of the
building envelope (the preferred installation for air and water barriers) creates conditions
for condensation all year round. In U.S. climates with both heating and cooling seasons,
this creates conditions for condensation during the heating season. Section 502.4.3 of the
ICC, Sealing of the Building Envelope, requires a moisture vapor-permeable wrapping
material, in order to avoid potential condensation. The requirements and properties of vapor
barriers must not be confused with those of air and water barriers.
Factors to consider when selecting barriers for projects include the following:
For walls with insulation inside the stud cavity, the main temperature gradient occurs
across the interior wall cavity, over the thermal insulation. As a result, in heating climates,
temperatures towards the exterior of the insulated cavity could drop below the dew point
temperature creating conditions that could lead to interstitial condensation. In a cooling
climate, the cooler surfaces would be towards the inside. In order to avoid persistent
1. Water and air barriers are critical for controlling the most important moisture sources in
buildings and should be used in most climates. As long as these membranes are vapor
permeable, their typical location, at the exterior of the building enclosure, and under the
cladding, will not interfere with the condensation control strategies.
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2. Diffusion is essential for drying, and a diffusion pathway must be allowed. The preferred
diffusion direction is climate specific. Many U.S. climates require diffusion pathways in
both directions, toward the inside and toward the outside.
3. Vapor barriers should only be used in climates where they do not interfere with diffusion
drying, such as predominant heating or cooling climates. In many U.S. climates, the vapor
barrier would be located on the wrong side of the wall assembly during the heating or
cooling season.
4. If an air and water barrier is also a vapor barrier, its location in the building envelope is
determined by the vapor barrier function and must follow the vapor barrier requirements
for condensation control. In many U.S. climates (except for predominantly heating or
cooling climates) such membranes should be avoided. Air and water barriers should be
vapor permeable.
building enclosure to provide drainage of water to the exterior of the building. The
materials that form the drainage plane overlap each other in shingle fashion or are sealed
so that water drains down and out of the assembly. The drainage plane is also referred to
as the water-resistant barrier, or WRB.
Hydrostatic Head Test: measures the static pressure at which water will penetrate a
material. Building codes, such as the International Conference of Building Officials (ICBO),
require the time to failure method, whereby the sample must keep a fixed hydrostatic
pressure (55 cm) for 5 hours without leaking.
Permeance: The physical property that defines the ease at which water molecules diffuse
through a material. It is to vapor diffusion what conductance is to heat transfer. The units
are measured in perms.
Water-Resistant Barrier: A wa t e r-resistant barrier (WRB) is also re fe r red to as a
dra i n a ge plane.
Water Vapor Diffusion: The movement of individual water vapor molecules through a
material. The movement occurs because of concentration gradients and thermal gradients,
independent of airflow.
Wind-Washing: The phenomenon of air movement that occurs due to wind entering
building enclosures, typically at the outside corners and roof eaves of buildings. Windwashing can have significant impact on thermal and moisture movement and hence
thermal and moisture performance of exterior wall assemblies.
References
Anis, Wagdy A.Y., AIA, The Impact of Airtightness on System Design, December 2001,
ASHRAE Journal, pages 31-35.
Vapor barrier located on the cold side of the wall assembly leading to
potential condensation
Persily, Andrew K., Envelope Design Guidelines for Federal Office Buildings: Thermal
Integrity and Airtightness, National Institute of Standards and Technology (NIST), NISTR
4821 U.S. Department of Commerce, 1993.
Glossary
Air Barrier: Air barriers are systems of materials designed and constructed to control
airflow between a conditioned space and an unconditioned space. The air barrier system
is the primary air enclosure boundary that separates indoor (conditioned) air and outdoor
(unconditioned) air. In multi-unit, townhouse, and apartment construction, the air barrier
system also separates the conditioned air from any given unit and adjacent units. Air
barrier systems also typically define the location of the pre s s u re boundary of the
building enclosure.
Building Enclosure or Building Envelope separates the interior environment from the
exterior environment. A building enclosure controls heat flow, airflow, water vapor flow,
rain, gro u n d wa t e r, light and solar radiation, noise and vibrations, contaminants,
environmental hazards, odors, insects, and fire.
Dew Point Temperature: the temperature at which condensation begins to occur.
Drainage Plane: Drainage planes are water repellent materials (such as building paper,
housewrap, and foam insulation) that are typically located behind the cladding and are
designed and constructed to drain water that passes through the cladding. They are
interconnected with flashings, window and door openings, and other penetrations of the
6
Emmerich, Steven J., Tim McDowell, W agdy Anis, Investigation of the Impact of
Commercial Building Envelope Airtightness on HVAC Energy Use, National Institute of
Standards and Technology (NIST) Report 7238, 2005.
Persily, Andrew K., Airtightness of Commercial and Institutional Buildings, Proceedings of
ASHRAE Thermal Envelope VII Conference, 1998.
Quirouette, R. L., The Difference Between a Vapor Barrier and an Air Barrier, Building
Practice Note No. 54, July 1985, National Research Council Canada. (http://irc.nrccnrc.gc.ca/pubs/bpn/54_e.pdf)
Quirouette, R. L., “The Air Barrier Defined,” in An Barrier for the Building Envelope,
Proceedings of Building Science Insight ’86, National Research Council Canada, 1985.
S t raube, John, “Moisture Management,” Chapter 9, Building Science for Building
Enclosure Design, Building Science Press, 2005, p.185.
Trechse, Heinz R., Moisture Control in Buildings, American Society of Testing and
Materials, 1994.
Moistu re Ma n a gement in Wa ll Assemblies: Ai r, Wa ter, and Va por Ba rriers
Architectural
Record
Version: 01
Issue
Date: 12/06
Page
No: 6
Filename: DupontTyvek_CE_12_06_reprint
Writer /Designer: Paul
Carroll
Last Revision: 11/28/06
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A I A / A RC H I T E C T U R A L R E C O R D
CONTINUING EDUCATION Series
CLICK FOR ADDITIONAL SUPPLEMENTAL READING
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A I A / A RC H I T E C T U R A L R E C O R D
CONTINUING EDUCATION Series
LEARNING
OBJECTIVES
After reading this article, you should be able to:
• Identify moisture management strategies for buildings
• Analyze design criteria for air, water, and vapor barriers
• Determine the impact of climate, heating, and cooling cycles on selection and
location of protective barriers within wall assemblies
• Discuss code requirements for air, water, and vapor barriers
• Design wall assemblies using protective barriers
INSTRUCTIONS
Refer to the learning obj ectives above . Com p l ete the qu e s ti ons bel ow.
Go to the sel f - report form on page 8. Fo ll ow the reporting instru ctions,
a n s wer the test qu e s ti ons, and su bmit the form. Or use
the Con ti nuing Edu c a ti on sel f - report form on Re co rd ’s web site —
a rch re cord.construction.com — to receive one AIA/CES Le a rning Un i t
i n cluding one hour of health safety wel f a re credit.
QUESTIONS
1
.
Which statement describes characteristics of water-resistant barriers?
a
. They are generally installed on the north face of the building envelope,
depending on climate zones
b
. For walls with cavity insulation, the water-resistant barrier is generally
installed on the interior face of the exterior wall.
c
. For exterior insulation walls, the water-resistant barrier should sandwich
both sides of the exterior rigid insulation.
d
. They are designed to protect against rain penetration
2
. What is the main source of water vapor in the building enclosure?
a
. Porous materials
b
. Relative humidity
c
. Air transported moisture
d
. Mold
3
. Which term describes materials that resist airflow?
a
. Ventilating membrane
b
. Air barrier
c
. Vapor barrier
d
. Wind screen
4
. Wind-washing impacts moisture performance, and does not impact thermal
movement or performance.
a
. True
b
. False
5
. Which of the following is not a performance requirement for air barrier
systems?
a
. Structural integrity
b
. Durability
c
. Thermal resistance
d
. Continuity
6
. Which of the following materials qualify as air barriers?
a
. Non-perforated building wraps
b
. Expanded polystyrene
c
. Batt and semi-rigid fibrous insulation
d
. Cellulose spray-on insulation
7
.
As a general rule, where should a vapor barrier be located?
a
. On the side of the building envelope with higher vapor pressure
b
. Toward the inside in climates dominated by cooling
c
. Wherever it will prevent diffusion of incidental moisture out of the
envelope, or diffusion drying
d
. On the side of the building that will encourage vapor diffusion into the
envelope
8
. Which direction does diffusion drying occur for buildings in mixed climates
(with heating and cooling seasons)?
a
. From the inside to the outside
b
. From the outside to the inside
c
. Both directions (to outside in winter, to inside in summer)
d
. None of the above
9
. If an air and water barrier is also a vapor barrier, which of the following
determines the appropriate location in the building envelope?
a
. On the face of the building with the highest relative humidity
b
. Anywhere in the wall assembly
c
. The vapor barrier function and requirements for condensation control
d
. In climates dominated by heating, toward the outside
1
0
. Which of the following states has not adopted compliance criteria for
airtightness of materials or assemblies?
a
. Minnesota
b
. Massachusetts
c
. Wisconsin
d
. Washington
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• High air infiltration resistance to stop unwanted airflow into the building envelope (e.g. wind)
• Excellent water resistance to prevent bulk-water penetration
• Optimum water vapor permeability for drying (by water vapor diffusion) to help prevent the growth of mold and mildew and costly moisture-related damage.
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tyvek.com
Moisture Ma n a gement in Wa ll Assemblies: Air, Wa ter, and Vapor Ba rri ers
Architectural
Record
Version: 01
Issue
Date: 11/06
Page
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Filename: DupontTyvek_CE_12_06_reprint
Writer /Designer: Paul
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Last Revision: 11/28/06
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