Weatherproof Walls Keep Water Out
Mark Robins, Senior Editor,
Posted
12/07/2011
Water damage in buildings can
destroy wood, steel and plywood. Short-term exposure to water won't
harm most building materials, but over the long term it's a major
contributor to property loss. Water intrusion via rainwater,
snowmelt and wind-driven moisture can occur through air leaks in
wall assemblies. Without moisture-, air- and water-protective wall
barriers, mold, mildew and fungus growth can occur, causing
expensive building repairs and replacements, occupant sickness and
even structural deficiencies.
The correct weatherproof wall can prevent or at least
minimize water and moisture migration into buildings. "The best
definition of a weatherproof wall in metal construction is an
assembly of metal and accessory components that work together to
provide air, moisture and temperature barriers designed
specifically for the project's local climate and the intended use
of the building," says John Pierson, PE engineering services
manager, The Garland Co.
Inc., Cleveland.
Local climates and regional considerations influence
weatherproof wall use, as do site conditions, exposure and
overhangs. "Some groups, such as the Building Science Corp. and
National Research Council of Canada, define certain regions that
are more prone to excessive rainfall or that have a certain
moisture index and vital areas in need of something like a
rainscreen or a drainage system," says Ted Kerwood-Winslow, product
development specialist, Benjamin Obdyke Inc.,
Horsham, Pa. "Other areas have adopted building codes that require
the use of providing drainage, like Oregon and
Minnesota."
Weatherproof wall realities
While many try, the reality is no wall or building is
fully weatherproof. "As long as there are doors, windows, vents,
and you have transitions from the roof above or floor slab below,
weather and moisture will always be a potential issue," says Warren
Barber, Dens brand product manager, Georgia-Pacific Gypsum, Atlanta.
"The better term-and goal-is a wall that is 'weather resistive.'
They are also referred to as an air barrier, or air- and
water-resistive barriers. A weather-resistive exterior wall
provides air- and water-movement control, creating an ideal
scenario for protection and durability."
To understand weather elements' effects on metal
buildings, it's important to understand how building science has
evolved over the past 50 to 100 years. "In the past, the primary
building materials were wood and stone; both of which had the
ability to hold the moisture that naturally influenced it, and then
let it go and dry out," Barber says. "These walls were thick,
containing a lot of mass, giving the wall the ability to absorb the
moisture as it moved through the wall, allowing it to dry back out
to the interior or the exterior of the wall. Today, building
science is all about energy conservation. We are building our
buildings airtight to reduce energy loss and air loss across the
exterior walls. And, we are using highly engineered building
materials. Many of these new materials are not able to withstand
moisture and today's walls are thinner. This new way of building
brings new concerns."
Many times in today's new buildings, when air enters
buildings it brings moisture with it, which can accumulate. "The
moisture has nowhere to go and it cannot dry out because the air
movement that would carry the moisture out of the building has been
restricted," Barber says. "Water can cause rust and produce
corrosion in metal buildings. The water-affected metal can oxidize
over time, causing weakness within the building's framework and
ultimately shortening the life expectancy of the
building."
Rainscreen walls
A rainscreen wall is actually more than a wall. It is an
entire cladding system designed to keep moisture out of the
building. "The rainscreen can be brick, stucco, terra cotta, metal
or even glass," says Barber. "It contributes to the building's
appearance, but also acts as the first line of defense, providing a
drainage plane to keep moisture out of a building."
Rainscreen walls neutralize wind currents on the
cladding's inside and outside surfaces so that pressure differences
do not drive water through gaps or flaws in an assembly and into a
building interior. "Walls designed with these pressure differences
in mind are said to conform to the so-called 'rain-screen
principle' and are sometimes referred to as 'pressure-equalized
rainscreens,'" says Doug Todd, market manager at Dow Building Solutions, Midland,
Mich. "Such systems are composed of an airtight plane protected by
an open-jointed or vented cladding. Separating these two layers is
a cavity or air chamber. The joints are sized so that air, but
little or no water, can pass through as gusts buffet the cladding,
equalizing the pressure on the exterior and within the
cavity."
Because of this air chamber or ventilation space, any
wind-driven rain or moisture that does get past the exterior siding
can dry out or escape, avoiding damage to interior walls. "It
doesn't get trapped and is able to free-drain out of the system,"
says Kerwood-Winslow. "Any residual moisture is able to dry out due
to convective air movement. One way of looking at these systems is
that although they may cost more upfront, they can prevent some
very costly callbacks-it is like a 'moisture security
system.'"
A rainscreen is not a watertight assembly. "Typically on
the rainscreen wall assembly, the portion of the wall I would
consider to be weatherproof is the substrate," says Brain Nelson,
general manager, Knight
Wall Systems, Deer Park, Wash. "And usually the substrate that
the façade, or panels are to be attached to should be completely
wrapped with a weather-resistant barrier, an air barrier, and
sealed at all service openings and/or penetrations prior to the
façade installation. This is what ultimately stops, or blocks any
water from infiltrating the interior wall assembly. Think of a
rainscreen as a building envelope support mechanism. This is
because the sole purpose of a rainscreen is to significantly limit
the forces that act on the wall, therefore limiting the amount of
water that actually contacts the weather-resistant
barrier."
Vapor retarders
A vapor retarder is any element that controls and limits
water flow in its vapor form across exterior wall systems. "The
primary line of defense against condensation damage is a good vapor
barrier," says Robert A. Zabcik, P.E., LEED AP, director of
research and development/Green Building Initiative, NCI Group Inc., Houston. "But it
doesn't stop there. Even the strongest vapor barrier is a detriment
to the building and occupants if it is not installed properly. This
obviously includes sealing and boundary detailing but also
physically where the vapor barrier is installed relative to the dew
plane in the envelope."
Vapor barriers' position and usage in a wall assembly
depend on climate conditions, location of insulation relative to
the subcavity and cladding absorptiveness. "Many air barrier
products also act as a vapor barrier," says Jane Wu, product
marketing manager, W.R. Grace &
Co., Cambridge, Mass. "Grace offers Perm-A-View Modeling
Service that generates critical information to help optimize wall
design and guide selection of proper air barrier and vapor barrier
material." Grace Perm-A-Barrier air barriers provide continuous
water and air resistance to ensure buildings are protected from
water and air intrusion. "They are continuous and fully bonded to
construction surfaces to resist positive and negative air forces,"
Wu adds. "Their elongation can bridge cracks and fissures even in
cold temperature."
Vapor retarders are rated for their permeability (perm
rating) and the lower the perm, the better the product retards the
migration of moisture. When insulation is left exposed and the
vapor retarder on the insulation becomes the exposed surface, the
surface integrity becomes critical. In most applications where the
vapor retarder is exposed, it serves a dual purpose as an air
barrier enhancing its importance.
"Recent testing by the Metal Building Manufacturers
Association has shown that a properly sealed vapor retarder
covering the insulation does indeed meet most air infiltration
requirements for an assembly," says Mark Engebretson, director of
marketing and business development, Therm-All Inc., North Olmsted,
Ohio. "All seams, joints and any tears should be treated. When
exposed to the interior, appearance and light reflectivity of the
vapor retarder should also be considered. Ultraviolet rays from the
sun and interior lighting can degrade vapor retarders, so it is
important to choose the appropriate facing based on the conditions
it will be exposed to."
New technologies minimize
moisture
Continuous insulation is currently receiving a lot of
attention and many are seeing it as a growing trend to manage
moisture. "With continuous exterior insulation, the risk of
condensation is dramatically reduced," says Nelson. "This is
because the dew point is pushed to the outside of the wall
assembly. With the stud cavity and interior wall assembly kept warm
and dry, the risk of mold and other fungal growth is heavily
reduced. Continuous insulation will also dramatically increase a
building's energy efficiency as well due to increased levels of
insulation and higher R-values."
There are new software programs to help manage moisture
with weatherproof walls. "WUFI (Wärme und. Feuchte instationär or
Transient Heat and Moisture), offered by Oak Ridge National
Laboratory and the U.S. Department of Energy's Energy Plus analysis
software can actually help remotely analyze wall systems to
ascertain their efficiency and durability," says Barber. "Oak Ridge
National Laboratory is a national multi-program research and
development facility managed by UT-Battelle LLC for the U.S.
Department of Energy. The program inputs the climate in the area of
the building to be reviewed. Users just enter a zip code and the
assembly type of the wall, and the program puts that data and the
wall through a 12-month analysis based on weather."
Enhanced housewraps are gaining attention in the fight
against moisture. They feature additional functions beyond the
traditional "drainage plane" that allow the moisture that gets
behind the cladding to drain even more freely. "We have combined a
rainscreen ventilation matting product to a highly regarded
housewrap to combine the benefits of both products in one simple
installation step," says Kerwood-Winslow. "We are also in the
process of launching a one-of-a-kind housewrap that features a 3-D
pattern that is incompressible and will retain its drainage space
in any wall installation. This ensures protection from the damaging
effects of mold due to bulk water penetration. The continuous gap
allows the water to drain freely. This ensures the system meets the
code requirements to provide the wall assembly the ability to drain
out the water that will get in. Many companies are constantly
innovating in this space and new products are introduced monthly if
not weekly."
Barber predicts many code changes impacting the
construction world in the next years, such as the International
Green Construction Code and LEED changes related to weather
resistance and indoor environmental quality. All these code and
program changes will dictate the use of weather-resistant barriers
and help drive new technology like improved weatherproof walls, he
says.
[sidebar]
Minimizing mold
Mold has been in the news a lot lately. "Liquid water
collecting in the wrong places can cause mold development, which is
usually left unseen and therefore can grow to be a very expensive
problem to correct," says Jane Wu, product marketing manager, W.R.
Grace & Co., Cambridge, Mass.
Mold is a living organism requiring food in order to be
sustained. It feeds on organic material such as wood and paper
found in homes and buildings. It originates from two separate
problem areas. "It is not enough to look at toxic mold as a problem
coming from the exterior of the building," says John Pierson, PE
engineering services manager, The Garland Co. Inc., Cleveland.
"Moisture problems that lead to mold are not only generated from
leaks, but also result from condensation inside the building. Metal
wall assemblies need to be designed to address moisture from both
inside and out."
Hundreds of species of mold are found in the United States
that are not harmful to people. However, certain toxic molds such
as aspergillus, penicillium and toxic mold atra (also known as
stachybotrus chartarum) can cause illness. Toxic mold or black mold
can be deadly, especially to children with mold allergies. Toxic
mold has become almost epidemic in some parts of the United States
where foam board was used as wall insulation with no air space to
let walls breathe. After black mold begins to grow, microscopic
mold spores quickly become airborne and travel throughout air
conditioning and heating systems.
Weatherproof wall manufacturers are trying to stop mold.
"The expandable polystyrene (EPS) in our accel-E Steel Thermal
Efficient Panel (S.T.E.P.) does not hold moisture and is not a food
source," says Bob Ross Sr., construction technician, SYNTHEON Inc., Pittsburgh.
"This is also true of the steel framing that's embedded in the EPS
of the STEP panel."
Georgia-Pacific Gypsum manufactures substrates that act as
weather-resistant barriers in response to today's tight buildings
that don't allow for moisture that enters a building to dry. "Our
gypsum product's fiberglass mat technology allows for some moisture
to be let in and also for it to dry back out, which helps reduce
the potential for mold growth," says Warren Barber, Dens brand
product manager, Georgia-Pacific Gypsum, Atlanta. "The fiberglass
facings are mechanically bound to the gypsum core, eliminating the
starch bond and paper facings that are potential food sources for
mold growth."