The Performance of Weather-Resistant Barriers in Stucco Assemblies

S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 6 A l l a n a • 1
The Performance of Weather-Resistant
Barriers in Stucco Assemblies
Karim Allana, RRC, RWC, PE
Allana Buick & Bers, Inc.
Abstract
This paper will focus on the performance of traditional three-coat cement plaster with
two layers of Grade-D 60-minute paper. The speaker will discuss observations of moisture
movement through the plaster, building paper, and plywood/OSB sheathing into the interior.
Plaster performance was judged through nail pullout strength testing (ASTM D1037).
The results were mapped to determine the level of damage and loss of structural strength.
Rilem tube tests were conducted to determine the porosity, moisture absorption, humidity
levels, and interior and exterior temperatures. The presenter will demonstrate how to better
design plaster mix and use drain mats and rainscreens to prevent wood rot and damage.
Speaker
Karim Allana, RRC, RWC, PE — Allana Buick & Bers, Inc.
Kar im Allana is the CEO and principal of an A/E firm specializing in the building envelope
and sustainable construction. Allana earned a BS in civil engineering from Santa Clara
University and is a licensed professional engineer. He has been in the A/E and construction
fields for over 30 years, specializing in forensic analysis of roofing, waterproofing, and the
building envelope. Allana has acted as an expert witness in more than 250 construction
defect projects and is a frequent speaker and presenter at professional forums.
2 • A l l a n a S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 6
INTRODUCTION
The most consistent feedback I get from
all cement plaster contractors and plaster
institutes is that the “proven” methods of
applying weather-resistive barriers, lath,
and plaster have been working for almost a
century, so why change it? The fact of the
matter is that while not much has changed
in the plaster industry, the building “assembly”
has changed completely.
When lath and building paper were first
introduced as building materials—albeit at
different times—their application was over
open framing with wood space boards acting
as the lath. Later construction included line
wire over wood studs to support building
paper, and lath installed with furring nails.
Wall cavities did not contain insulation or
incorporate air barriers. In contrast, today’s
buildings incorporate airtight construction,
full exterior sheathing (like plywood or gypsum),
stud bays filled with insulation and
interior paper-backed sheetrock, and interiors
finished with vapor-impermeable materials
(like vinyl wallpaper). Furthermore,
when cement plaster or stucco was first
popularized, it was used in single-family
one- and two-story residential structures.
Today we find cement plaster often used in
high-rise construction, commercial buildings,
public and institutional buildings, and
large multifamily structures. These larger
commercial structures often have little or
no roof overhang, so they are less protected
from direct rain and experience much
higher wind pressures. Shingling, lapping
flexible flashings, and building paper with
fin or flange-style windows have given way
to finless punched windows, storefronts,
and curtainwalls. We often see architects
integrating traditional cement plaster with
barrier-type metal panels and other traditional
siding. Further complicating matters,
the International Building Code (IBC)
is moving to have continuous insulation
behind claddings without first testing wall
assemblies that have truly continuous insulation
to determine how to secure lath over
continuous insulation and their moisture
removal viability behind traditional cement
plaster or stucco.
We all learn from failures, and in that
sense, I have been extraordinarily lucky.
Over the past ten years, I have been fortunate
enough to study failed cement plaster
assemblies in millions of square feet of large
commercial-type structures ranging from
multifamily buildings to office spaces, education
facilities, and residential structures.
This paper will focus on what we can
learn from these forensic studies, understanding
the forces at play that can cause
assembly failure and how to avoid failures,
and will conclude with how to modify the
standard building assembly to perform successfully
with commercial-type applications.
BACKGROUND
To understand the evolution of the
cement plaster system, we have to start
at the beginning. We will start with “openstud”
construction (Figure 1), as it was the
most widely used construction method in
the western United States 50 years ago
and continues to be used in many states.
Historically, contractors have used Portland
cement plaster or stucco as the outer covering
for most construction projects because
it is easy to apply and use, has good waterresistive
properties, is durable and fireresistive,
and can be modified for color and
finish relatively easily. For more than 100
years, it has been touted as the product of
choice.
A century ago, buildings were made
using lath and plaster construction with a
layer of boards covered with cementitious
plaster over building studs. This typical
assembly did not use building paper, and
under wet conditions, it would allow some
level of water to soak through the wood lath
and collect in the wall cavity. The lath held
the plaster in place, and when the plaster
dried, it was a strong, durable assembly
with open space between studs and did not
have any insulation in the stud bay. The
lack of insulation in the wall cavity allowed
it to dry quickly during dry spells or dry to
the warm interiors. Interior walls were often
constructed similarly with gypsum-based
plaster on wood lath. Heat from the interior
or exterior surfaces would dry the moisture
and readily permeate it out of the wall to
either the interior or exterior. While humidity
can build up to over 90% inside walls
during wet cycles, it did not result in mold
or rot due to the lack of organic paper-faced
sheetrock, old-growth wood framing, and
constant air movement.
“Sackett Board” was invented in 1894
in the UK, and in 1910, the United States
The Performance of Weather-Resistant
Barriers in Stucco Assemblies
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Figure 1 – “Open-stud” construction components.
Gypsum Corporation bought the Sackett
Plaster Board Company and wrapped the
gypsum plaster board with paper-based facings
instead of the felt sheathing; they called
the product “sheetrock.” As construction
boomed in the ’40s and ’50s, drywall was
easier to install, could be mass-produced,
added a measure of fire safety, and reduced
the process from a weeklong application to
a one- or two-day project. Consequently,
cement plaster cladding incorporated closed
cavity construction in lieu of the traditional
open-stud wall assembly.
As construction became more widespread,
governments became more involved
in the process and began to standardize
requirements for buildings and their
construction. The Uniform Building Code
(UBC), and now the International Building
Code (IBC), established standards for the
construction industry that addressed fire
codes, structural performance, weatherand
water-resistive construction, as well as
establishing life safety criteria, accessibility
standards, and other construction and
design standards. As new regulations are
developed, they apply to new construction
primarily, but if you make changes or renovate
an older building, you may be subject
to compliance with all the new regulations
established since the original construction
date. For us, this update requirement has
created new challenges for the industry.
Green buildings are more energy efficient,
better insulated, more
airtight, and enclosed
with wood or gypsum
sheathing. As the
industry itself pushes
forward to make these
changes for energy,
fire, and structural
reasons, how these
assemblies manage
water and moisture
has changed, sometimes
taking on unintended
consequences
and not performing as
intended.
In the 1940s,
asphalt-coated “felt”
was often used under
exterior siding, and
cement plaster with
poultry netting was
used for reinforcing
instead of wood lath.
Original felt papers
(15- and 30-lb. organic
felts) used behind
exterior cement plaster
were more “waterresistant”
but less
permeable. I imagine
this construction may
have caused some
condensation, but it
would have self-dried
due to a lack of insulation
that allowed the
free flow of heat and
air through the walls.
In 1964, the establishment
of UU-B-790
(a federal specification) required minimum
water-resistance rates that correspond to
a permeance rating of about 5 perms for
water-resistive barriers.
In 1968, the code (UU-B-790a) started
requiring “Grade-D” type paper as a
weather-resistive, breathable paper, likely
as a reaction to condensation issues, which
was the next significant change in exterior
stucco and siding. This breathable Grade-D
type paper with a minimum 15 minutes of
water resistance would allow more moisture
to flow into the wall cavity, but again, the
moisture dried steadily due to higher permeability
and the lack of insulation being
crowded into the wall cavities. The addition
of various types of batt insulation was
another change to the hygrothermal performance
of the wall assembly, and the time
it took to dry out the wall cavity increased.
Depending on the interior use of paperfaced
gypsum boards and the level of incidental
water intrusion through the exterior
wall, some assemblies would undoubtedly
have developed some level of mold growth
and rot as a result of the fully insulated,
closed-cavity exterior wall construction.
Full plywood or OSB-type exterior
sheathing was the next significant shift
in siding/stucco substrates. Dry plywoodtype
sheathing has a permeability ranging
from 0.5 to 1.5 perms—technically a vapor
retarder. However, when plywood and OSB
get damp (moisture content in excess of
20%), the permeability increases to over 20
perms, making them much more permeable.
Gypsum-based sheathing boards are
often used in construction of noncombustible
types of sheathing product to reduce
and retard fires. Interior wall and exterior
sheathing boards are highly permeable, in
excess of 30 perms, and can readily move
moisture across the sheathing.
As buildings have become more energyefficient
with the use of insulation that
reduces heat flow, moisture barriers that
reduce air and water movement, and solid
sheathing to improve structural and fireresistive
building performance, we have also
created a new challenge: how to remove the
moisture that gets trapped inside a wall
cavity.
A common assembly with 60-minute
Grade-D paper over full exterior gypsum
sheathing, plywood, or OSB sheathing is
very sensitive to the level of “incidental”
water that passes through the stucco. While
fluid-applied weather-resistive barriers and
4 • A l l a n a S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 6
Figure 2 – Stucco or cement plaster, moisture-drained
system.
Figure 3 – Rainscreen moisture management system.
polymetric housewraps have
become more popular in the
past decade and are considered
more cutting-edge,
they also have similarly high
permeability ranging from
15 perms to over 70 perms.
In order to understand
weather-resistive barrier
performance and how walls
behave with full sheathing
and insulation, it is important
to understand how
water and moisture move
through these assemblies.
Poor performance of
traditional stucco assemblies
is giving way to rainscreen-
type assemblies
(Figures 2 and 3) with wall
drainage boards or furring
strips to improve the wall’s
ability to remove water
as well as introduce airflow
to dry these assemblies.
Manufacturers have
introduced better-designed
building paper to improve
drainage and not deteriorate
under repeated wet and dry
cycles.
To ensure a building’s
sheathing and structural
components are not
impacted by normal wet/
dry cycles, it is vital to the
long-term performance and
integrity of wall systems to
remove any incidental or
excess moisture. The damaging
effect of repeated
wet/dry cycles can eventually
result in degradation
of organic building paper
and sheathing and cause metal flashings to
rust, as well as nails to rust, lose strength,
or back out. Additionally, excess moisture
retained within a wall can promote biological
growth such as mold and mildew.
TRADITIONAL MO ISTURE
MANAG EMENT
Today we are building elaborate, complex
structures that utilize all sorts of technology
to provide us comfort and protection,
while at the same time being efficient
and safe. Keeping moisture or rain out
of an inner wall space has been going on
for decades, but the manner in which the
building envelope has evolved has taken us
to new dimensions in design and construction.
All of this advancement continues to
evolve with new and better products, techniques,
and skills.
Just as the exterior surfaces are meant
to keep water from entering the wall system,
the permeability of many siding materials—
including stucco—allows water and
moisture from rain, fog, or dew to intrude.
In West Coast climates like California and
Washington, during rain events the interior
is not only much drier, but the temperatures
are often higher than the exterior, which
promotes drying to the interior (Figures
4A and 4B). Permeance rates of damp or
wet materials such as wood sheathing
can be much higher, which leads to moisture
movement inwards where humidity is
lower and where most of the drying occurs.
Cracks or other wall defects can raise this
level even higher as shown in the figures.
As water found inside the wall permeates
through exterior assemblies and dries
to the inside, it raises interior humidity
levels, often causing condensation on the
inside surfaces of nonthermally broken
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Figure 4A – Moisture diffusion study results with occupant internal loads.
Figure 4B – Moisture diffusion study results with internal sources and wall diffusion due to
excessive cracking and defects.
aluminum window frames. Where cabinets,
bedside tables, and beds press against exterior
walls, elevated moisture levels in the
gypsum wallboard lead to mold growth on
the interior surface of the sheetrock. Tightly
packed closets up against exterior walls
that do not have air circulation can cause
clothes, books, and shoes to get moldy.
Interior weather-resistive elements such
as Grade D paper or water-resistive barriers
(WRB) work in tandem with the exterior
skin to manage and keep out “incidental”
water. The level of moisture movement to
the inside and wood rot or metal rusting
damage caused by such moisture movement
is directly proportional to the increase
in humidity level inside occupied space.
The solution to improving the performance
of the wall assembly is dependent on two
primary factors:
• Reducing the level of incidental
water on the WRB
• Improving drainage and drying
behind the siding/stucco
It is commonly believed that incidental
water landing on the WRB will easily drain
down the weeps and flow out of the assembly,
but that is not entirely true. Trace amounts
of water trapped behind siding material can
move laterally within the assembly. When left
standing, it permeates the assembly, leaving
it hanging like a wet blanket. Moisture gradually
works its way through the WRB and solid
sheathing if present and dries to the inside.
High levels of moisture permeating through
the sheeting finds its way into the wall cavity,
raising humidity levels not only in the wall
cavity, but also in the interior living space,
causing damage.
Water diffusing
through the weather-
resistive barrier
raises moisture levels
in the OSB or
plywood to above
20%, promoting
wood decay and the
rusting of nails and
staples. Humidity
rates in the wall cavity
can often exceed
90%, promoting
biologic growth in
paper-faced gypsum
boards.
Without drainage
mat or rainscreen
construction, water does not flow
down freely to the weeps or “air” dry. Much
of the water entering the system onto the
WRB intrudes into the interior space.
The decades of the 1980s and 1990s
saw an increase in construction failures
and defect litigation related
to water-induced damage to frame
buildings with notable hotspots in
such places as California, British
Columbia, and North Carolina. In
the last three years, mold became
the focus of attention. Although
there are a myriad of reasons for the
apparent increase in water-related
building damage, the increased airtightness
of buildings to achieve
energy conservation is generally
accepted as a major contributing
factor. The historic ability of wall
components wetted by precipitation
or condensed water vapor to dry
through air movement is no longer
as effective as it was before the
advent of energy-efficient new construction.
Thomas K. Butt
Journal of ASTM International
November/December 2005
Therefore, as we have become more
energy-efficient using wall insulation and
impermeable moisture barriers that reduce
air and water movement, we have also created
a new set of problems: how to remove
moisture that becomes trapped within a
wall cavity. Rilem tubes (Figure 5) enable
the assessment of water absorption properties
of walls or other substrates. Higher
absorption of the cement plaster or cement
board-type siding can greatly increase the
moisture permeation through the wall.
These tools work with a variety of materials
and coatings, and the results are
repeatable and reliable. Cement plaster
and fiber cement siding and/or materials
are unique, and their use in combination
with coatings and paints can create different
results, so each building should be
tested independently to provide accurate
and repeatable result measurements.
While there is no clear standard for
absorption testing with a Rilem tube for
many cladding materials, it does provide
a great relative scale to measure absorption.
In cement plaster assemblies, we find
that absorption rates can vary greatly in a
20-minute test, ranging from no loss to several
inches. In many cases, we found that a
traditional moisture-drained assembly without
a drain mat or rainscreen (more porous
assemblies) results in greater and uniform
damage to the substrate and higher interior
humidity levels.
Electronic moisture measurement devices
that rely on transduction to measure
the change in conductivity are used to
measure moisture content of softer assemblies
such as gypsum or wood sheathing.
Levels of moisture in plywood sheathing
can vary from the exterior face of the plywood/
OSB to the interior face of plywood/
OSB, depending on the cycle of wetting
and drying. The amount of moisture in a
building sheathing or wall cavity will vary
with airflow, exterior and interior humidity,
and temperature. Moisture meters—either
surface or pin-type—are only qualitative
measurement devices, measuring specific
relative moisture levels, and not the relative
humidity within an environment.
We had the opportunity to conduct an
extensive study of dozens of large multifamily
buildings and some large single-family
homes with traditional building paper and
either cement board siding or stucco. In
both, siding types of assembly and stucco
assembly, the siding/stucco was completely
removed to expose the condition of the OSB
or plywood or gypsum sheathing. While
some of the damage to the sheathing was
definitely due to leaks from wall penetrations
or improperly flashed horizontal waterproofing
systems, we surprisingly documented a
lot of moisture movement and damage due
to moisture soaking through the building
6 • A l l a n a S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 6
Figure 5 – Rilem tube level dropped in 15 minutes.
paper and damaging the
wood sheathing (Figures
6A and 6B).
Depending on the
elevation, we observed
and documented damage
to the wood sheathing
and graphed it visually to
depict damage. We realized
that other than poking
the sheathing with an
awl, most experts were
not using any scientific
means for analyzing the
damage to the sheathing.
Surprisingly, even
though the sheathing
was discolored due to
different levels of water
damage, slight, moderate,
and some severe
areas passed the “awl”
poke test.
To make the assessment
process more scientific,
we went on to create
a testing protocol using
ASTM D1037, which is
a nail pull-through test
(Figure 6C). The nail pullthrough
tests were more
telling and representative
of loss of structural value
such as shear value.
These types of tests can
provide a detailed profile
of force exerted over time
and give a more realistic
assessment of the
loss in structural capacity
(Figure 6D). Using the
standard measurements,
we were able to demonstrate
a significant difference
in nail pullout
strength between slight,
moderate, and severely
damaged areas.
Although a building envelope may be designed to prevent moisture and
water intrusion into the interior spaces, care must be taken to prevent damage
to the wall framing and allowing unintended humidity inside. Construction
itself may allow a measurable amount of moisture absorption and diffusion
to occur over an extended time period. Unintended and higher levels of moisture
diffusing through the assembly can lead to not only decay and loss of
strength in materials, it can cause mold on interior surfaces due to elevated
humidity levels. The question is, how do we deal with the incidental levels
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Figure 6A – Partial damage at south elevation.
Figure 6B – North elevation – partial.
Figure 6C – ASTM D1037 nail pull tests – loss of strength.
of moisture? How do we design to prevent
unintended levels of moisture? Moreover,
how do we make it more foolproof to prevent
against both?
UNDERSTANDING INCIDENTAL
WATER PENETRATION
The typical source of incidental water
intrusion is the perimeter of windows,
reveals, control joints, wall penetrations,
inside and outside corners of buildings,
joints in siding panels, and small 1/64-in.
shrinkage cracks.
Typical sources of unintended water
intrusion include open or unsealed sealant
joints, cracks 1/32 in. or larger, shrinkage
cracks around reveals, and unsealed butt
joints in reveals, corner molds, and control
and expansion joints. The typical source of
excessive water behind stucco and other
siding are items such as water run-off from
horizontal waterproofed elements (roofs
or balcony decks and walkways) directing
water behind the stucco assembly. Water
managed on “horizontal waterproofing” elements
should never be drained onto WRB.
Traditional Grade-D 60 papers wrinkle
as a result of being left exposed to moisture
and sun during installation. Wrinkling of
the paper can sometimes promote drainage
but often obstructs water flow and
creates pockets for water to collect. Stucco
also tends to adhere to traditional paper,
effectively cutting off the water flow on top
of the paper. In those instances, water will
penetrate to the second layer of paper, if
present. Since traditional weather-resistive
barriers like building paper depend on
water shedding, any buildup of water can
lead to lateral migration of water at the laps
in the paper. While fluid-applied barriers do
not have laps, and therefore, water cannot
effectively get behind them, water standing
on highly permeable fluid-applied barriers
can permeate behind the barrier just the
same.
Tightly fastened horizontal control
joints, expansion joints, or reveals create
water cut-offs, and water tends to collect
above them. Generally, we see fastener
rusting and moderate to severe damage
both above and below horizontal control/
reveal joints. Unsealed joints in control,
expansion, and reveal joints, as well as
screeds and corners, can be a source of
unintended water on WRB. Applying flexible
flashings behind these joints, reveals, and
screeds—both on the horizontal and on the
vertical—helps mitigate some of the damage.
Examples of this type of assembly and
the results of damage are in Figures 7 and 8.
Standing or trapped water behind the
siding or stucco can create a hydrostatic
head. Under hydrostatic head, water can
penetrate through fasteners, absorb through
the WRB, and permeate right through to the
interior. In the absence of a drainage mat or
rainscreen, I see no advantage of WRB that
has a permeability of greater than 10.
Another common element that allows
unintended moisture is the porosity of the
cement stucco material. Where we measured
high levels of water penetrations
using a Rilem tube test, we also observed
high levels of sheathing damage. Synthetic
siding or stucco that is porous is another
source of unintended water/moisture on the
WRB (Figures 9A and 9B). We have observed
the “uniform”-type damage in many projects
clad with cement board siding. Cement
board siding can be just as porous, if not
more so, than cement plaster. Top edges of
lap siding can create a lip where water can
accumulate. Joints in the lap siding, corners,
trim, and around window and other
openings allow similar, if not more, water
intrusion as cement plaster.
Large quantities of water can soak
through the stucco/siding material itself,
depending on its level of porosity. Our
measurements of high humidity levels in
the interior living spaces of buildings with
“cement board siding” were just as high as
those we experienced in stucco assemblies,
even though the materials are very different
in manufacture and installation.
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Figure 6D – Nail pull-through strength test: ASTM D1037.
IMPROVING DRAINAG E
AND DRYING
In the process of conducting forensic
building studies, the reason for
failures is most evident with moisture
intrusion into the building where there
is no avenue to remove that moisture
or allow it to dry out. Once moisture
infiltrates behind the tightly designed
cladding with little or no air movement,
problems are going to occur.
To alleviate such problems, there
are a few measures that—when designing
or constructing a building—can
ensure that water intrusion is dealt
with before it becomes an assembly
failure. These will greatly reduce moisture
intrusion, allow the building to last
longer, and save owners from future,
highly expensive (and often unnecessary)
repairs due to water damage.
Improve Drainage. Furring horizontal
control joints or reveals will allow water to
travel behind them unimpeded. Fluid-applied
WRB used in conjunction with traditional
building paper can improve drainage. Use of
engineered and tested weather-resistive paper
that has drainage grooves or built-in channels
has been proven in testing to improve drainage
and drying. In certain dryer climates like
southern California and Arizona, this level of
improvement may be enough.
Improve Air Movement. Rainscreens
are typically created by installing drain
mats or vertical furring strips behind siding
or stucco. The air gap and cavity allow
for effective drainage and promote drying.
In addition to an engineered drainage
layer, through-the-wall flashings are
typically installed at each floor to drain
the water, equalize pressure, and promote
airflow. Rainscreen systems not only allow
unimpeded drainage, they also create positive
airflow and promote active drying.
Effectively moving water off the weatherresistive
barrier and promoting air movement
are the keys to getting better performance
from the weather-resistive barriers.
This type of a system is much more foolproof
against defective construction and
unintended water behind the cladding. The
system is designed to prevent water from
becoming a standing element.
Western “one-coat” type cement plaster
systems with 1-in. rigid insulation board
are rainscreen systems. While the cement
plaster system is actually two coats, it is
only ½ in. thick, and allows more incidental
water behind it. The drainage channels in
the ridged insulation effectively drain water
away and allow the assembly to dry. Despite
being half as thick as traditional cement
plaster, it manages water on traditional
WRB better than most other siding or traditional
cement plaster material.
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Figure 7 – Traditional fiber
cement siding assembly.
Figure 8 – Uniform damage
behind fiber cement siding.
CONCLUSION
Today’s building envelopes are designed
to be energy-efficient, technologically
advanced, and to provide a safe, comfortable,
and manageable environment. Though
many of the current exterior cladding materials
have been in use for over a hundred
years, the wall assembly has changed,
and the manner in which these cladding
systems perform has changed. Siding and
stucco directly applied over highly permeable
weather-resistive barriers can cause
a high level of damage, depending on the
sources of incidental and unintended water
penetration.
Since Grade-D WRB became a code
requirement behind nonbarrier-type assemblies,
a lot has changed. Traditional WRB
do not have rainscreen-type cavities or
drainage boards, and the cladding is built
tight up against the WRB. With air barriers
and insulation in the wall cavities, this tight
assembly allows for very inefficient drying,
mostly to the interior, and results in issues.
The consequence of this tight assembly
construction is like having a “wet blanket”
up against the exterior wall sheeting. While
I have documented many such projects
and their impacts with traditional two layers
of Grade-D building paper, I have not
performed a similar study with fluid-applied
WRB in a tight assembly. Traditional WRB
can be saturated over time and has the
capacity to hold water and permeate it
through the sheathing.
In case of a cement plaster assembly
with a primary fluid-applied WRB, often a
layer of building paper is used to separate
the plaster from the WRB. In an assembly
that has a combination of fluid- and paperbased
WRB, I believe that the amount of
moisture permeation through the assembly
will depend on the permeability of the fluidapplied
barrier. Some of the available fluidapplied
WRBs are much more permeable
than 60-minute Grade-D papers; therefore,
I do not believe that the fluid-applied WRB
will fare better than two layers of Grade-D
papers. I do believe that in a tight assembly,
it is better to choose a lower permeability
fluid-applied barrier.
In a case where stucco or siding is
porous, consider penetrating sealers or
elastomeric-type coating to reduce the water
absorption through the skin. Provide functional
sealant joints around openings to
reduce incidental water. Design and install
plaster with properly constructed control
and expansion joints to reduce cracking.
The best solution is rainscreen-type
assemblies. Rainscreen-type assemblies manage
excess water and dry so efficiently that
they are not dependent on the quality of the
WRB. In such an assembly, high permeability
WRB can perform well and will allow drying
from the inside out just as effectively as from
the outside. It is my opinion that rainscreen
should be the standard of care in commercial
high-rise type buildings. Rainscreen is
already a standard of care in the Pacific
Northwest areas, although not required by
code. Rainscreens offer such amazing redundancy
and are so forgiving that every building
envelope consultant should make it their
number-one recommendation. The problem
with this assembly is the higher cost, which
owners tend not to approve.
Figure 9A – Control joint
corner unsealed.
Figure 9B – Control joint
impedes water drainage.
1 0 • A l l a n a S y m p o s i u m o n B u i l d i n g E n v e l o p e T e c h n o l o g y • Oc t o be r 2 0 1 6