Choosing The Appropriate Air Barrier Membrane System

May 15, 2006

6 • IN T E R FA C E J A N U A RY 2006
Massachusetts, Minnesota,
Michigan, and Wisconsin
all have codes that
require the use of an air
barrier system that prevents
air leakage. Massachusetts
was the first state to require air
barriers as part of a broad-reaching energy
code adopted in July 2001. The code standard
essentially mirrors similar code that
has been in effect in Canada since 1985.
Even without mandatory code requirements,
many architects and owners are
designing air barrier systems for buildings
across the U.S. The choice of an appropriate
air barrier system for a building will depend
on a number of factors and variables. The
best time for a designer to make this decision
is before or during design development.
Key variables that go into this decision
process are:
• Exterior climate,
• Interior climate,
• Building design,
• Back-up wall construction,
• Exterior cladding or rain screen system,
• Construction budget,
• Expected building longevity,
• Type and placement of exterior
cladding anchors or brick ties,
• Wall design load,
• Insulation placement, and
• Insulation R-value.
Air barrier systems generally fall into
several broad categories. In this article, we
will focus on the two predominant types of
air barrier systems: self-adhering sheet
membranes, and cold, fluid-applied membranes.
Fluid-applied membranes are differentiated
from thin-coating type systems intended
primarily for adhered EIFS systems.
True membrane air barriers cure to a minimum
of 40-mil thickness and are often
specified to be thicker than that.
Membranes are much better at bridging
cracks and joints due to settlement or wind
loading because extensibility is dependent
on elongation and, more importantly, membrane
thickness.
Self-adhered sheet membrane air barriers
are always air and vapor barriers.
These details show typical cold climate permeable (left) and non-permeable (right) assemblies.
Cold Climate Permeable Assembly Cold Climate Non-Permeable Assembly
Typically, these are
40 mils thick and are
comprised of 36 mils
of rubberized asphalt
and a 4-mil crosslaminated,
HDPE
film. These membranes
always have a
permeance of .01
perms or less.
Cold, fluidapplied
air barriers
can be air and vapor
barriers with a low
permeance of less
than .01 perms. However,
there are also
thick, cold, fluidapplied
air barriers
that have a relatively
high permeance to
moisture vapor. These
materials can have a
perm rating from 7 to
over 12 perms. Why would a permeable air
barrier be preferable to a impermeable air
barrier? Actually, there are several answers.
Let’s look at some of the key variables mentioned
above.
Permeable or Non-permeable – Climate
In Canada, Massachusetts, and other
northern climates, the “perfect” cavity wall
system design is considered to consist of
exterior cladding or brick, an air gap, exterior
grade insulation, an impermeable air
and vapor barrier, CMU or steel studs with
weather-resistant sheathing, and no interior
batt insulation. This design is considered
“perfect” for cold climates because
the dewpoint, exterior moisture, and
water vapor-laden air are all prevented
from entering the building or interior conditioned
space.
A similar argument can be made for hot,
humid climates like Miami, Florida, with
the difference being insulation placement.
In Miami, the “perfect” cavity wall system
design might be exterior cladding, air gap,
non-permeable air and vapor barrier, and
CMU with interior insulation or steel stud
back-up with interior batt insulation. In the
Isometric drawing of a fluid-applied
system on a CMU back-up.
Is sheet the best solution? Sometimes there is difficulty when using a sheet membrane system with multiple brick
ties and penetrations. Fluid-applied systems can be less costly to install and may produce better results in some
cases.
J A N U A RY 2006 I N T E R FA C E • 7
deep South, the primary concern is vapor
infiltration though diffusion and air movement.
Placing the insulation behind the air
and vapor barrier usually will prevent condensation
within the wall because the dewpoint
will never reach the cooler, conditioned
interior environment.
Another climate zone in the U.S. is
“mixed-humid climate.” Think of the
Carolinas or Tennessee. In these tricky climate
areas, heating degree-days and cooling
degree-days are nearly equal. Many
experts agree that the best air barrier system
for mixed-humid climates are permeable
air barriers. These are vapor permeable
so that when the dewpoint occasionally is
reached on the interior or conditioned side,
water vapor is able to escape via diffusion.
In fact, many experts also advocate eliminating
any vapor barrier within these wall
designs because part of the year the vapor
barrier will be on the “cold side” or wrong
side of the insulation, exacerbating dewpoint
and condensation problems. Several
cold-applied, permeable air barrier membrane
systems are available on the market
in the U.S.
Permeable or Impermeable – Insulation
Placement and R-Value
Choosing an air barrier system will
depend on insulation placement. This is
also related to climate. In northern climates,
sometimes it is necessary to place
insulation within steel stud framing. In this
situation, the air barrier needs to be permeable
with a separate vapor barrier on the
warm-in-winter side of the interior insulation.
There are numerous examples of this
type of system in code-driven markets like
Massachusetts. Condominiums often opt
for this design to increase interior dimensions.
Another reason to select a permeable
membrane is for projects with high R-value.
In order to achieve R-values in excess of 20,
sometimes architects will place R-10 insulation
in the exterior cavity side and also
use R-19 batt insulation on the interior
side. Running a dewpoint analysis or a
hydrothermal analysis program like “WUFI”
will show that an impermeable air barrier
should never be used in the middle of two
layers of insulation. Experts also suggest
that for borderline, hot, humid climates
with exterior insulation, permeable membranes
are the best choice.
Sheet Membrane or Fluid-applied?
Once the decision is made whether the
air barrier system should be permeable or
impermeable, then there are other criteria
8 • IN T E R FA C E J A N U A RY 2006
During (below) and after (right)
construction shots of the cancer
research center at SUNY
Albany in Rensselaer, NY,
show a system Air-Bloc 33
permeable, fluid-applied air
barrier, UV-resistant coating,
and a Trespa, METEON composite
panel system outboard
air barrier. The architect was
Einhorn, Yaffe and Prescott of
White Plains, NY, and the air
barrier subcontractor was
Cornerstone Waterproofing of
Cooperstown, NY.
that would influence a choice between cold
fluid applied systems and sheet membrane
systems.
Some cold, fluid-applied systems have
other specific physical properties that will
affect a choice of materials. One product
available in the marketplace today is a permeable
and UV-resistant product. In openjointed-
panel, rain screen systems such as
METEON® by Trespa, UV resistance is often
required because the membrane will be permanently
exposed to some UV light that
would deteriorate other membranes.
Because UV-resistant membranes are
almost always applied outboard of insulation,
these products also must be permeable.
Sheet membranes are always impermeable
and are not generally UV resistant
and, therefore, are inappropriate in this
application.
Other cold, fluid-applied products are
fire resistant. Fire resistance with low flame
spread and smoke developed ratings are
often requirements for interior applied air
and vapor barrier systems. In cities like
New York or Chicago, there is some thinking
that even exterior applied membranes
should be fire resistant to prevent a possible
chimney affect in multi-story cavity wall
conditions. Rubberized, asphalt-based
sheet membrane systems inherently have
high flame spread and smoke-developed
ratings and should not be specified when
fire resistance is a concern.
Lastly, some impermeable fluid systems
also serve as insulation adhesive in addition
to being air and vapor barriers. This is particularly
beneficial in preventing cold air
from migrating behind the exterior insulation,
resulting in thermal shorts. This
approach is also cost effective in eliminating
additional mechanical fasteners or another
separate adhesive. The New York City
School Construction Authority has chosen
to specify an impermeable, adhesive-type
system on CMU as part of an energy upgrade
over previously specified damproofing.
Brick Ties and Other Penetrations
In non-permeable designs, the choice
between a cold, fluid-applied system and a
sheet membrane system usually comes
down to installed cost. Sheet membranes,
in general, cost less per square foot of material
only – often as much as $0.50 per
square foot less. In very straightforward
applications with few penetrations and
post-applied brick ties or anchors, sheet
membrane systems are often less expensive
than fluid-applied systems on an installed
The National Museum of the American
Indian (NMAI) is a new Smithsonian
Museum in Washington, DC. The Smith
Group was the architect for the
building, which used a Blueskin SA
sheet membrane air/vapor barrier with
BASF sprayed urethane foam
insulation behind sandstone.
J A N U A RY 2006 I N T E R FA C E • 9
cost basis. In high labor cost markets, this
price differential tends to skew back toward
fluid-applied membranes because of the
speed of application.
It’s important to understand that all
membrane systems – whether sheet or
fluid-applied – universally use sheets or cut,
tape-size sheets for critical details, cold
joints, and other membrane transitions. So
for designers who are concerned about uniform
thickness and workmanship, sheet
systems and fluid-applied systems should
perform equally well.
When doing projects with brick ties
already in place, the least costly system and
perhaps better-sealed air barrier system
will invariably be a fluid-applied system.
Cutting sheets
around brick
ties requires
m e t i c u l o u s
workmanship
to prevent fishmouths
and
holes. Even
when done
properly, a fluid sealant is prescribed
around penetrations. Needless to say, it is
easy to see why using a fluid system on a
wall full of ties makes the most sense.
At Hawk Mountain Sanctuary, Orwigsburg, PA, the walls
on either end of the building leaked water and air. Stone
was removed and Air-Bloc 32 added on previously bare
CMU block back-up walls. The leaks stopped. Assembly
included exterior Foamular insulation, Mortar Net, and
reapplication of the stones. Architect: Spillman Farmer
Architects, Bethlehem, PA; Contractor: Schelgel Contracting.
Product: Air-Bloc 32 non-permeable, fluid-applied; retrofit
vapor barrier, air barrier, and waterproofing; Grace Perm-ABarrier
compatible with through-wall flashing.
The Marina Bay Tower in Quincy, MA, used Georgia Pacific’s DensGlass Gold Sheathing (yellow in photo), Henry’s Air-bloc 31 membrane
(a liquid emulsion, vapor-permeable air barrier), Dow 1″ extruded polystyrene insulation (light blue), and an Alucobond panel system. It is
also shown on the cover of this publication.
10 • I N T E R FA C E J A N U A RY 2006
Conclusions
The choice to use a true membrane air
barrier system for a building is excellent for
obvious reasons. Choosing the appropriate
product starts with getting the best advice
possible.
Design professionals – including architects
and consultants – rely upon qualified
manufacturers’ product representatives.
During the design process, manufacturers
might offer a hydrothermal analysis or dewpoint
analysis that can help determine
whether a permeable or impermeable material
is best suited.
A knowledgeable product representative
will not try to force fit a impermeable system
that clearly should have a permeable
membrane. Likewise, a helpful product representative
will listen to the architect’s
design criteria, including budget and complementary
building components, to help
select the appropriate product.
Most building envelope consultants
have been working with air barrier systems
and moisture control through wall assemblies
for many years and often decades.
Architects and building owners may employ
a consultant directly to assist in their wall
assembly design. Consultants will often
continue their contracted involvement
through the bid and submittal review
process and construction administration.
Once a suitable design is chosen, specified,
and bid, the best air barrier manufacturers
will join the consultant by attending
pre-construction meetings as well as providing
separate on-site observations. Air
barriers as systems depend upon proper
installation. Contracting with a qualified
consulting firm will ensure the entire building
envelope, including the air barrier, is
successfully installed. The most effective
air barrier system will be the one that is
supplied by a recognized air barrier manufacturer,
properly installed by a qualified
air barrier installation contractor, and
overseen by a qualified building envelope
consultant.
William F. Foley, CCPR, has been active in commercial construction
for more than 25 years. For the last 18 years, Bill
has worked in waterproofing, roofing, and air barrier sales
and product management. Since early 2001, Foley has been
involved in the advancement of air barrier technology in the
U.S. As of December 2003, he joined the Henry Company as
a regional manager for the Eastern U.S. for building envelope
systems.
William F. Foley, CCPR
J A N U A RY 2006 I N T E R FA C E • 1 1
An interpretation of the guidelines for the U.S. Green Building
Council’s LEED™ Rating System as applied to roofing standards is
now available on the EPDM Roofing Association’s (ERA) Web site at
www.epdmroofs.org.
The bulletin explains how roofing systems can impact the types
of voluntary green building standards for commercial applications
that are available from the USGBC, including: LEED-NC,Version 2.1
for New Construction and Major Renovations; LEED-EB for Existing
Buildings; and LEED-CS for Core and Shell Development.Templates
for reporting the attributes of an EPDM roofing system also can be
found online at the site’s LEED information area.
ERA PUBLISHES LEED
ROOFING BULLETIN