Moisture
Management in EXTERIOR WALL ASSEMBLIES
While
much of the attention has focused on problems affecting
EIFS-clad homes, any exterior cladding represents
potential trouble if the right steps are not taken
to create a weather-tight barrier to moisture intrusion
in exterior wall assemblies.
During the past decade
there has been plenty of talk, study, and finger pointing
over how to best protect exterior wall assemblies
from potentially damaging water intrusion. With the
recent increase in mold litigation, the stakes for
builders have been raised considerably. Constructing
problem-free exterior wall assemblies is now more
important than ever.
Against this backdrop, it’s not surprising to
discover there is a great body of research being conducted
by building scientists and industry organizations
to determine how best to protect exterior wall assemblies
from moisture intrusion, while at the same time allowing
them to breathe so that moisture buildup on the inside
can escape.
Much of what they’re finding is that the materials
developed years ago, that have proven the test of
time in the field, oftentimes do the best job of managing
moisture in wall structure. In some cases, they do
a better job than newer materials that have come on
the scene in the last few years.
Building basics
To be effective, an exterior wall assembly must embody
the four Ds of moisture management: deflection, drainage,
drying and durability. We will explore each of the
four Ds in detail by examining a typical stucco wall
assembly.
But we also need to consider the often overlooked
fifth D of moisture management: do-ability. Watertight
wall assemblies need to be practical in both design
and execution. Regardless of how well an exterior
wall assembly or building product performs in a laboratory,
if those results can’t be achieved within the
context of real world construction practices, the
building component or assembly is of little practical
use.
Deflection: Deflection is the primary means by which
an exterior wall assembly protects the building envelope
from moisture intrusion. While stucco will resist
the majority of bulk water it encounters, under wet
conditions it does absorb moisture. However, liquid
water can only penetrate stucco assemblies through
defects in the wall assembly, such as through cracks
or failed sealant joints. The deflection of any wall
assembly can be improved by limiting the exposure
of the exterior to bulk water, namely through the
use of roof overhangs and eaves in the exterior design.
Unfortunately, current trends in construction have
all but eliminated these favorable design elements
from exterior wall assemblies.
Drainage: Presuming that a certain amount of bulk
water will penetrate the exterior cladding through
defects in design or construction, it is imperative
that the wall assembly provide a means of draining
away such incidental moisture intrusion. In stucco
assemblies, this is accomplished by means of building
paper, flashing and weep screeds. When properly integrated,
these materials provide both an effective and efficient
drainage plane to evacuate any water that gets behind
the stucco cladding.
Recent independent studies have noted that asphalt-saturated
kraft building paper is particularly well suited for
this task. In comparative testing conducted by Building
Science Corp of Westford, MA, stucco assemblies incorporating
two layers of Grade D building paper over wire and
open studs delivered the highest drainage performance
of tested assemblies. Among the poorest performers
were stucco assemblies employing 15-pound felt over
OSB sheathing, and creped plastic wrap over OSB sheathing.
The reason behind the strong performance of building
paper in stucco assemblies has been noted by building
organizations such as the Northwest Wall and Ceiling
Bureau. In testing conducted by the NWCB, it was observed
that, during the stucco curing process, building paper
forms natural drainage channels that provide a separation
from the back of the stucco cladding, creating an
optimal drainage plane. Joseph Lstiburek, principal
with Building Science Corp, has noted that this separation
does not occur with stucco applied directly over plastic
wraps, and cautions that stucco should never be installed
in direct contract with plastic-based housewrap.
Drying: Because situations can arise where moisture
finds its way into the wall cavity or in direct contact
with the exterior sheathing, most often due to defective
or improperly integrated building materials, the wall
assembly must provide an additional means for the
trapped moisture to escape. Building paper provides
this capability by virtue of its rate of moisture
vapor transmission, the rate at which moisture vapor
can pass through the material.
The MVT of building paper, combined with the natural
separation of building paper and stucco cladding,
is ideally suited to provide the optimum release of
any moisture that finds its way behind the drainage
plane. In fact, preliminary findings of a current
research study indicate that building paper responds
to changes in relative humidity in the most favorable
way possible. In high-humidity conditions, when drying
of the wall assembly is most critical, the MVT of
building paper is at its highest, providing the greatest
capacity for release of moisture trapped within the
wall assembly.
Asphalt-saturated kraft building paper also increases
the drying potential of the wall assembly. Because
today’s homes are built much tighter than in
decades past, exterior walls have less capacity to
deal with moisture that makes its way behind the exterior
cladding. If water gets into the wall assembly, it
stays in, ultimately leading to deterioration of building
materials. But because building paper has the ability
to intercept moisture and release it over time, it
is far less susceptible to moisture overload than
other types of weather-resistive barriers such as
plastic housewraps.
What this means is that building paper is unique in
its ability to adapt to changing environments-by providing
both a high resistance to moisture penetration and
the highest capacity for drying-making it the right
material for many climatic conditions.
Durability: The long-term success of any wall assembly
is dependent upon its component building materials
maintaining their integrity and performance characteristics
over the long haul. Although stucco may develop hairline
cracks over time, the cladding is remarkably durable,
and will maintain its water-resistive qualities for
tens if not hundreds of years.
The same durability can also be found in building
paper, which has been commonly used in stucco assemblies
for more than four decades with a proven track record
of success.
Do-Ability: An exterior wall assembly that properly
addresses all four desired performance criteria provides
the proper foundation for long-term weather protection.
However, if the design details can’t-or won’t-be
properly assembled in the field, the wall assembly
is likely to fail at some future point.
Consider flashings
One key consideration is the proper integration of
window and door flashings with the weather-resistive
barrier and other building components. All materials
must be installed in shingle fashion, with proper
overlaps, so that any moisture that makes its way
behind the cladding stays on top of the drainage plane
until it can be discharged from the wall assembly.
With some approaches to moisture protection, such
as liquid weather barriers applied directly to the
sheathing, proper sequencing can be difficult, if
not impossible, to achieve.
Compatibility of building components is also an important
consideration. Sealants with high solvent or plasticizer
content, for example, can attack bitumen flashing
products. Also, some rubberized asphalt products are
incompatible with single-ply flexible PVC roofing
materials.
When selecting individual components of exterior wall
assemblies, the operative word is caution. Before
relying on any material, check first with the manufacturer
to verify that the product is appropriate and compatible
with all other materials used in the wall assembly.
Or better yet, work with a manufacturer that provides
a complete system of compatible materials.
Given the heightened risk of litigation to contractors,
it is foolish to take chances with exterior assemblies,
particularly when there are already a number of proven
building materials and assemblies with an extensive
history of success.
While “new–fashioned” building materials
may have sex appeal-particularly to those manufacturers
who introduce them-what is most important is that
exterior wall assemblies meet the five essential criteria
for moisture management: deflection, drainage, drying,
durability and do-ability.
Asphalt saturated kraft building paper may not be
the latest fad in building materials, but it performs
the way it should-both in the lab and the field-and
has been delivering this superior protection far longer
than virtually every other weather-resistive barrier
or housewrap using in homebuilding today.
In an environment where the failure of a wall system
could mean a major lawsuit, how much do you really
want to experiment?
Courtesy
of EIFS Alliance