The Dollars And Common Sense Of Air Barriers

May 15, 2006

Air barriers are a relatively new
term to most individuals in
the U.S. market, even though
the materials and technology
have been around for
decades. Actually, air barriers
have been used since the very first building
was constructed.
The purpose of the building
enclosure (commonly known as the
building envelope) is to separate two
different environments. Whether
they are at work, at home, or anywhere
else, people want the environmental
conditions around them at
levels where they feel comfortable
and can be productive. They want it
warm enough but not too warm; cool
enough but not too cool. Also, the
environment should not be too
humid or too dry.
Air barriers and building airtightness
must be viewed in a “building
as a system” approach and in
consideration with the other functions
of the building enclosure. This
includes the use and function of liquid
water barriers, water vapor barriers,
and heat barriers (thermal
insulation). Liquid water must be
the design professional’s first concern.
In order to facilitate this, we
must install roofs, eaves, flashings,
water-resistive barriers, etc. All
these components are designed to
keep liquid water out of the building.
Once liquid water is addressed, the next
concern for the design professional should
be air barriers. Air barriers are more important
than vapor barriers or heat barriers.
Common sense calls for designers to
address water, water vapor, air, and heat
transfer all at once.
Design professionals who incorporate
air barriers into their building must keep in
mind that many materials provide more
than one function. You could have a material
that provides an air barrier function, a
vapor retarder function, a radiant heat barrier,
a water-resistive barrier, or combinations
of these functions.
The exterior enclosure (envelope) separates the exterior environment from the internal environment.
J A N U A RY 2006 I N T E R FA C E • 1 3
The Building Science of Air
Nature wants everything
to be balanced. Heat flows
from hot to cold, moisture
from wet to dry, and air from
high pressure to low pressure,
until everything is balanced or
has equalized. When we put
up a barrier in the building
enclosure to create a different
environment, nature exerts a
force on the building enclosure.
We then put mechanical
equipment and occupants
into the building. This adds
an additional level of force or
pressure exerted to the building
The building enclosure
must be designed to resist the
forces generated on it from
both the exterior and the interior. Wind,
solar radiation, night sky radiation, and
rain all exert forces against the building
enclosure from the outside. People,
mechanical equipment, stack effect, flue
effect, and ventilation effect all exert forces
from the interior. The interior forces can
either act against or act with the exterior
forces. These forces vary from day to day,
hour to hour, and in some cases, minute by
There currently is a lot of confusion
relating to the function of air and vapor barriers.
Simply put, a vapor barrier is intended
to stop moisture transport by diffusion;
that is, moisture movement of water molecules
physically working their way through
a material. For the most part, this is a slow
and tedious process. An air barrier, on the
other hand, is intended to stop or retard air
flow. Where some of the confusion may lie is
in the fact that air has the ability to carry
water vapor and transport it through holes,
cracks, and so forth in the building.
Estimates on the relationship of water
transport by diffusion vs. air transport have
indicated that air transport can carry
between 60 to 100 times more moisture
than by diffusion alone.
Many times, individuals will discuss the
need for a vapor barrier, but as soon as
someone starts to talk about holes, sealing,
or anything along that line, typically they
are no longer talking about a vapor barrier,
but rather an air barrier. When a vapor barrier
is damaged, repairs are needed, but
typically this is not as much of a concern as
having a hole or defect in the air barrier. As
one increases or decreases the size of a hole
in a vapor barrier, one simply has a different
water vapor transmission
rate for the area of the
hole. Conversely, in an air
barrier, as the size of the
hole is decreased, the velocity
of the air passing through
that hole increases. This is
due to the fact that the air is
moving the same amount of
volume through a now smaller
hole. Therefore, having a
hole in a vapor barrier does
not have the same impact as
a hole in an air barrier.
Also adding to the confusion
is that many materials
can provide more than one
function in the building
enclosure. There are materials that are air
barriers; air and vapor barriers; and air,
vapor, and heat barriers. Sometimes, materials
used with the building enclosure may
possess these characteristics but actually
are not intended nor designed to provide
more than one function. For example, drywall
is an air barrier and can be used within
a wall assembly. Although it is an air barrier,
it may serve no other purpose than to
provide a substrate for other materials on
the exterior or a surface that can be used to
provide interior finishing on the inside of a
building. The designer needs to identify
which materials provide the air barrier
function in their wall assembly and ensure
that he or she can trace a line of airtightness
through the assembly.
It is recommended that when a building
is designed, the design professional should
first go back to the basics of understanding
heat flow, water flow, and air flow in their
simplest form. Once that is achieved, a
designer can move onto looking at the composite
effect of these dynamic flows. If the
designer understands the basics, then combining
the functions becomes much easier
and much more practical.
The physics and science of buildings
and building enclosures do not change, no
matter where the building is constructed.
The climate may be dry and cold, hot and
humid, or anywhere in between. The details
of what materials to use, how to install
them, and where they are installed within
the building enclosure need to be carefully
reviewed based on the environment in
which the building will ultimately operate.
The building enclosure must resist the pressures exerted by both the exterior and interior environment.
(Courtesy of the National Research Council.)
Example of moisture transport by air vs. moisture
transport by diffusion over a heating season.
14 • I N T E R FA C E J A N U A RY 2006
Energy Savings
It takes energy to condition
the air in buildings,
whether heating or cooling
is taking place. Most
individuals are aware of
the increase in the cost of
oil. Although oil is not a
major energy source for
the heating or cooling of
buildings, the cost of natural
gas and electricity
has historically followed
the cost of oil. Most electricity
comes from coal,
but the majority of the
new generation of electricity
is using natural
gas as the fuel source. No
matter what the energy
source, most people agree
that energy costs will be
increasing and will
remain high.
The National Institute
of Standards and Technology
(NIST) has completed
a study to determine
the cost effectiveness
of installing air barriers
in buildings. The study can be found
at build05/
PDF/b05007.pdf. The report states,
“Despite common assumptions that envelope
air leakage is not significant in office
and other commercial buildings, measurements
have shown that these buildings are
subject to larger infiltration rates than commonly
believed. Infiltration in commercial
buildings can have
many negative
including reduced
thermal comfort,
interference with
the proper operation
of mechanical
ventilation systems,
indoor air quality,
moisture damage
of building envelope
and increased energy
The report
goes on to say that
the reduction in
energy consumption
and operating
costs has a potential gas savings of greater
than 40% and potential electrical savings of
greater than 25% compared to a baseline
Very simply, for the energy savings
alone, it is cost
effective to install
an air barrier into
a building.
Complete Building Performance
The benefits of including an air barrier
in a building go beyond simply saving
money. By having an airtight building
enclosure, the sizing of the mechanical
equipment can be reduced, as the equipment
will not have to make up for air that
has infiltrated or exfiltrated the building. In
some cases, the cost savings on the equipment
side alone can offset the cost of
adding an air barrier system to a building.
With an airtight building, the mechanical
equipment can work as intended and
deliver the conditioned air to where the
occupants are, resulting in making the
occupants more comfortable. Indoor air
quality will also be improved as the
exchange of air in the building will be more
precise. The bottom line is that an air barrier
helps the building to perform as intended.
If the thermal insulation used in a
building is not in itself an air barrier, then
the air barrier will help the thermal insulation
perform better. When air leakage
occurs through an air permeable insulation,
the value of the insulation may be
severely reduced.
Moisture and Durability Issues
In climates where there is a difference in
temperature between the two environments,
there is the possibility of moist warm air
moving by air transport through the building
enclosure, which would cause the moist
Cost/benefit analysis study on the cost effectiveness of
including air barriers in building enclosure systems.
Building façade distress due to
uncontrolled air leakage from the interior of
a building in a cold climate.
Exterior sheathing damage from excess moisture in wall assembly.
J A N U A RY 2006 I N T E R FA C E • 1 5
air temperature to drop to its dewpoint.
This can be warm, humid air from the
outside infiltrating into the building from
the exterior. It could also be warm, moist air
inside the building exfiltrating from the
interior to the exterior.
In either case, when water vapor turns
to liquid, there is a potential for problems to
develop. Building materials have varying
ability to absorb moisture and then release
it. This is referred to as the “wetting” and
“drying” of the building enclosure. When the
“wetting” of the building enclosure lasts for
a limited time and then the materials dry
out, it may not produce problems or degradation
to the materials. Most buildings are
constructed to withstand this as a temporary
occurrence. When the “wetting” continues
for a period of time or when there is limited
drying potential, air leakage through
the building enclosure can cause considerable
Installed Performance is Critical
It is important that materials used for
airtightness in a building enclosure have
low air permeance. Materials considered to
be air barriers have a maximum air permeance
of 0.02 L/(s·m2) when tested in accordance
with ASTM E-2178.
The installer takes air barrier materials
and components and installs them in a
building to form an air barrier assembly.
These assemblies (wall assembly, roof
assembly, door/window assembly) all have
to be connected together to form an air barrier
system that completely wraps the entire
building on all sides.
However, if the material has a very low
air permeance and then holes are subsequently
put through the material, then the
air will travel through the path of least
resistance, which in this case will be
through the holes. As no material can cover
all six sides of a building without any penetrations,
it is very important how these penetrations,
terminations, and the connections
are made. Whether it is from one
material to another material or assembly to
assembly, these are critical areas that must
be addressed in both the design and installation
of the air barrier assembly.
There is no easy way around this. The
skill level of the installer of the air barrier
materials and components is extremely important.
This includes constructing the air
barrier assembly and then the connections
of the different air barrier assemblies
together. As air barriers are required on all
six sides of the building, the air barrier
installation will be done by a number of
trades. In addition to each installer having
the proper skills, the different groups of air
barrier installers, general contractors, and
others involved in the construction process
need to keep lines of communication open
to determine who is responsible for what
connections and so forth. For example,
which trade is responsible to connect from
the roof to the wall, from the wall to the window,
and so on. Continuity of the system is
critical, and individuals involved in the construction
process need to be aware of the
sequence of construction so proper and
durable connections can be achieved.
Quality assurance programs for the site
installation of air barrier assemblies have
been developed and are being delivered
across the U.S. Key principles of the program
include defining what a proper installation
should be; the training of the installer
on how to achieve a proper installation;
then verifying on-site that the installer is
actually implementing what he or she has
been trained to do. The biggest issue and a
key reason for improper installation can be
attributed to not defining, in advance, how
the installation should be completed. There
Roof Consultants Institute
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Phone: 800-828-1902 • Fax: 919-859-1328 •
A monthly* publication
from the industry’s commercial
roofing experts, featuring the
latest technical information on
today’s roofing issues:
• avoiding trapped moisture
• energy efficient roof designs
• sustainable building products
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• protected membrane systems
• steep roofs vs. low-slope roofs
• bituminous roof systems
• cold-applied roof systems
• insulating cellular concrete
• wind resistance
• and more…
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16 • I N T E R FA C E J A N U A RY 2006
should be no surprises, and there should be
no requirements that the installer does not
know in advance.
Air Barrier Standards
Air barrier standards are required to
provide a balanced approach and to remove
bias from the project. For standards to be
developed, agreement must be reached on
terminology. As an example, discussion
over the years has included the term air
barrier vs. air retarder.
Today the term “barrier” is accepted as
not meaning absolute, but providing the
function that is anticipated. Therefore, an
air barrier is not an air impermeable material,
but rather any material that has a
maximum air permeance based on a specific
test procedure.
New terminology has been developed for
air barrier materials, components, assemblies,
and systems. In short, air barrier materials
are the main material used – the
“big” pieces so to speak.
Air barrier components are any materials
used to join materials or assemblies
Air barrier assemblies are defined as a
combination of air barrier materials and air
barrier components joined together. Current
construction terminology uses the term
“assembly” when describing parts of the
building (wall assembly, roof assembly, door
assembly, etc). All of these assemblies come
together to provide the airtightness of a
building, so they must be considered by a
designer. There needs to be consideration
given to all of the assemblies, with the ultimate
goal an airtight building. Air moves
through the path of least resistance, so when
the wall assembly is extremely airtight, but
the windows have an extremely high air
leakage rate, the air will travel through the
window, bypassing the wall.
The air barrier system is the airtightness
plane defined throughout the complete
building enclosure.
There have been two ASTM test methods
published to date for the air barrier industry.
The first standard developed was ASTM
E-2178 – Standard Test Method for Air
Permeance of Building Materials. The second
standard developed was ASTM E-2357
– Standard Test Method for Determining Air
Leakage of Air Barrier Assemblies.
Work is continuing to develop a com-
Air barrier installers attending ABAA training in both the theory of building science and
hands-on application of various air barrier materials.
Test your knowledge of roofing with the following
metal roofing questions, developed by Donald E. Bush Sr.,
RRC, FRCI, chairman of the RRC Examination
Development Subcommittee.
The sources for this month’s column are
NRCA Roofing and Waterproofing Manual,
5th Edition,Volume 3
1. How is Waterproofing
defined by the NRCA
Roofing and
2. How much
hydrostatic pressure
does water exert?
3. What is a Hydrostatic
Pressure Relief
4. Built-up roofing
membrane is typically
defined by headlap,
endlap, and sidelap.
Where are each of
these laps located in
a BUR cross-section?
5. When a properly
shingled BUR
membrane is showing
a lap exposure of 9
inches, how many
plies of roofing are
Answers on page 18
J A N U A RY 2006 I N T E R FA C E • 1 7
plete family of standards, including such
things as components, installation, inspection,
and a standard for air barrier systems.
The Future of Air Barriers
Building airtightness, performance,
and more energy-efficient buildings are
here to stay. As we move into the future,
the issue of effective air leakage control will
continue to be at the forefront. The move to
energy efficiency and green buildings will
continue to embrace the common-sense
approach of using an air barrier to reduce
energy use while reducing the building
owner’s cost to operate the building.
Steven J. Emmerich, Tim McDowell,
and Wagdy Anis. “Investigation of
the Impact of Commercial Building
Envelope Airtightness on HVAC
Energy Use.” National Institute of
Standards and Technology. June
Ryan Dalgleish, vice president of bpc Building Professionals
Consortium, has been involved in the building envelope
industry for over 10 years in the areas of research, training,
and quality assurance. He earned a certificate in adult education
from the University of Manitoba and is working
towards a master’s degree in this field at the same university.
Ryan is currently involved in both the development and
delivery of training programs on air barriers and related
building envelope products for both the commercial and residential
industries. He is the current chair of the Manitoba Building Envelope Council
(MBEC), a director of the National Building Envelope Council (NBEC), serves on various
technical committees in regards to building performance/green building practice, and
is a frequent speaker across North America on air barriers. Ryan also forms part of the
quality assurance management and training team for the Air Barrier Association of
America’s on-site quality assurance and training programs on air barriers.
Ryan Dalgleish
Laverne Dalgleish is president 0f bpc Building Professionals
Consortium, which designs, develops, and delivers site quality
assurance programs for the construction industry. Laverne
has been actively involved in the construction industry for
over 32 years and has specialized in building envelopes, energy
efficiency, and building performance for over 20 years.
Laverne is a frequent presenter across North America on a
variety of topics as they relate to building envelopes, energy
efficiency, green building practices, standards, and quality of
construction. He is actively involved in the standards development process and is currently
the chairman of ULC’s Thermal Insulating Systems and Standards. He is also an
active member in ASTM, CSA, CGSB, and is secretary of two ISO standard development
committees. Laverne is the executive director of the Air Barrier Association of America.
Laverne Dalgleish
Answers to questions on page 17:
1. The treatment of a surface or
structure to prevent the
passage of water under
hydrostatic pressure.
2. Water exerts a pressure of
62.4 pounds per foot of
depth. Therefore, water lying
against a barrier exerts a
steadily increasing pressure
as the depth of water
3. A system of perimeter and/or
under-slab drains used to
regulate the hydrostatic
pressure in the earth
surrounding a below-grade
4. Endlap – the overlap distance
that is measured from where
one roll of felt ends to where
another begins.
Sidelap – the overlap distance
along the length of the felt
where one roll of felts
overlaps the adjacent
underlying felt.
Headlap – the distance of the
overlap that exists between
the lowermost and uppermost
plies of a shingled portion of
a roof membrane when
measured perpendicular to
the long dimension of the
5. FOUR (4)
NRCA Roofing and Waterproofing Manual,
5th Edition,Volume 3
18 • I N T E R FA C E J A N U A RY 2006
The NRCA’s political action committee, ROOFPAC, will host a silent and live auction
to raise money during NRCA’s annual convention in Las Vegas on February 14.
All proceeds from the auction will go to support pro-business candidates for the U.S.
Senate and House of Representatives.
For more information, contact Leah McKnight, NRCA’s manager of public affairs,
at 847-299-9070, Ext. 7599, or e-mail