Water Resistive Barriers on Building Exterior Walls
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This article explains the need for water resistant barriers on building exterior walls and explains the concept of a rain screen.
This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
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Water Resistive Barriers on Building Exterior Walls
Water leakage through building exteriors has been the
source of numerous callbacks and lawsuits across the
United States. In nearly every case, the problems have
been traced back to missing or poorly designed flashings
or to weather barriers that inadvertently directed large
amounts of water into building cavities or interiors.
of these leaks occur at window and door openings or at intersections
between building components. In some cases,
caulks and sealants forestalled leakage at these poorly designed
joints for the first few years. But eventually most
caulk joints fail, allowing water to enter.
All residential cladding systems are more or less
porous to water, particularly during wind-driven rain when
high air pressures on the windward side of a building force
water to flow toward lower-pressure areas behind the siding.
Under pressure, the water exploits butt joints, lap
joints, nail holes, and other openings to flow inside (Figure
1-1 at left ). Even without wind, some water will migrate
through tiny gaps to the back of siding through capillary
action, the way water is siphoned up a stalk of celery. This
is true of brick, wood, and stucco, as well as the newest
In older construction, water that penetrated the outer
cladding had ample opportunity to dry both to the interior
and to the exterior as wind washed through the wall
cavities, which were kept warm by heat leaking from the
In modern construction, however,
with high levels of insulation, continuous air and vapor
barriers, and low-perm sheathing panels, when water
gets in, it is much slower to dry and more likely to cause
Our photo (left) shows water stains on the interior of a clapboard-sided building wall on an 1860's vintage home restored by the website editor (DF) in Wappingers Falls, NY.
This structure relied on diagonal bracing for stiffness rather than an exterior sheathing board. Later an insulation improvement included blowing cellulose into the building walls - which was fine.
But now, water that used to leak into the wall cavity during windy rainy weather soaked the wall interior and was more of a problem.
Luckily cellulose insulation, probably because of the chemistry of its fire-retardant treatment, is rather mold-resistant. But that doesn't necessarily prevent an attack by termites or carpenter ants.
In freezing climates ice can also show where water leaks or moisture problems are occurring
While the exterior finish should be detailed to repel
and shed water, a backup system is needed for the times
when the primary system fails. The backup system needs to
catch any water that penetrates the cladding and to drain it
safely to daylight at the bottom of the wall.
Our photograph (left) of ice hanging from drainage openings in the building wall siding demonstrates how freezing weather can sometimes prove that a lot of water is running behind the building siding. The source of this leakage needs to be found and cured to avoid costly problems such as structural rot, insect damage, and even a wall cavity mold contamination issue.
layer in an exterior wall, called a water-resistive barrier by the International Residential Code (IRC), typically consists of properly lapped
building paper or plastic housewrap integrated with all flashings
to safely drain water away.
It is also called the drainage
layer or drainage plane. In this approach, the outer cladding
functions as a decorative “rain screen,” slowing down wind
and water, but it is not expected to be 100% waterproof.
As we report at WET BASEMENT PREVENTION, true foundation waterproofing, such as heavy textured plastic or rubber membranes placed against the foundation wall form a drainage layer to conduct roof spillage or ground water down the exterior foundation wall and into a drain system to carry water safely away from the building.
Carson Dunlop Associates' sketch (left) shows the use of a plastic membrane, protected by a geotextile to combine good water drainage down the foundation wall (and into the footing drains) with gravel backfill to nearly the top of grade (photo above left).
This basement waterproofing system was installed on a home that had suffered recurrent basement flooding due to a combination of in-slope grade at the rear and right side of the home combined with improperly installed and non-working footing drains, aggravated by wet soils in the area.
Bentonite clay waterproofing: bentonite clay can be pumped into soils around the building foundation wall - an old basement waterproofing method that in some installations works quite well to slow or stop foundation leaks. Watch out - by leaving water in soils near the foundation wall, the risk of foundation collapse may remain.
Use a basement waterproofing paint on the interior (or a damp proof or waterproofing coating on the exterior) of porous masonry block foundation walls - we have had excellent results with Thoroseal™ but don't expect an indoor foundation waterproofing paint to hold back a flood.
The optimal way to protect the structure, siding, and exterior
finishes from moisture damage is to design the outer
layer of the house as a decorative “rain screen” that is solid
enough to shed rain, block wind, and protect the sheathing
wrap, but porous enough to dry to the exterior when wet.
This is accomplished by separating the outer cladding from
the building’s water-resistive barrier by using an air space.
This system takes advantage of the fact that no siding system
is entirely waterproof and relies, instead, on the
drainage layer for waterproofing (see Figure 1-2at left).
The rain-screen system has four components: an exterior
cladding, an air space, a drainage plane, and weep holes.
Wall Cladding as a rain screen. While the main function of the exterior
finish material in a rain-screen wall is aesthetic, its
durability can have a big impact on the costs of home
ownership. Frequent repair, repainting, or replacement
can be very costly.
The cladding also protects the
sheathing wrap from wind and ultraviolet (UV) radiation,
and sheds most of the water that strikes the side of
the building. While some exterior claddings are more
porous to water than others—for example, brick, vinyl,
and vertical-wood sidings are particularly leak prone—
all can function well with a proper drainage plane.
Air space behind wall cladding. The air space serves several functions.
First, it provides a space for any water that has penetrated
the cladding to drain safely away.
provides a capillary break between the cladding and
the building paper. Wet wood siding or stucco has
been shown to degrade both building paper and plastic
housewrap if it is in direct contact with the wet
cladding. Cedar and redwood sidings can leach out
tannins that are particularly corrosive to building papers.
Third, the air space helps promote drying from
the back of wood siding or from the framing and
sheathing in the event of a leak.
With stained or
painted wood sidings, the air space will significantly
extend the life of the finish.
Some siding materials, such as vinyl, aluminum,
and wood shakes and shingles, are self-ventilating.
For others, an air space can be created by installing
vertical furring strips over the building paper.
furring out the siding provides optimal protection
for the siding and structure beneath, it also adds
significant cost and complication to the job, so it is
not commonly done. However, manufacturers are responding
to this need with a variety of thin drainage
materials that either install over the sheathing wrap or
replace it (see “Draining Housewraps,” page 5).
Drainage plane behind wall cladding. The drainage plane typically
consists of asphalt-impregnated building felt or a
plastic housewrap that is fully integrated with all
door, window, and wall flashings.
The system must
provide a clear drainage path out the bottom of the
building. In general, the housewrap must be cut to
lap over window and door cap flashings and under
window and door sill flashings.
In addition, the housewrap
should lap over step flashings, the upper leg of
abutting roof flashings, and deck ledger flashings.
Upper courses of sheathing wrap should lap lower
courses by at least 6 inches and vertical seams should
lap 6 to 12 inches.
Weep holes in building walls. Any trapped water must freely drain to
daylight at the bottom of the wall either through weep
holes, as in brick veneer, through a weep screed in
stucco, or out the bottom of vertical furring strips
installed beneath wood siding.
See STUCCO WALL WEEP SCREED DRAINAGE
If furring strips are
used, the openings at the bottom should be screened
against insects. Short sections of corrugated plastic
ridge vent material placed between furring strips
work well to provide solid backing for the bottom
course of siding.
Vinyl and aluminum siding products include weep holes in the bottom edge of each siding course.
Watch out: Siding vents such as the round spot vent shown in our photo (left) are in our experience (DF) ineffective. Some painting contractors who are repainting older painted wood clapboard-sided buildings drive wedges between overlapping clapboards to try to break the paint seal to improve wall venting and to reduce moisture problems in the wall.
We prefer to reduce indoor moisture to proper levels (see HUMIDITY LEVEL TARGET), and to provide a vapor barrier at the proper location in the wall structure.
Although a rain-screen wall design will improve the
longevity of any siding and finish, it is particularly critical
when installing wood siding over foam sheathing. Research
has shown that wood sidings installed directly over foam
sheathings are more prone to cupping, cracking, and paint
problems than when installed over wood sheathings.
Wood sheathing acts as a reservoir for moisture that penetrates
the siding.With foam, the moisture tends to build up on the
back of the siding and cause problems. An air space, even a
shallow space of 1/4 to 1/2 inch, between the siding and foam
sheathing has been shown to reduce these problems.
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ice at weep holes in northern Alberta buildings
(Mar 25, 2012) Tom Dang said:
We live in Northern Alberta (Canada) and I also find icicles coming out from weep holes of several sidings during warm spells near the end of winter (March). Most of the icicles are found on the outside wall of the bathroom area. But a few icicles are also found at the bottom corner of the front window, and at the lower portion of an exterior wall of the 2nd bedroom (an electrical box is nearby inside). It is a 2-story house and all icicles (except one) seem to start from the 2nd storey wall (1 to a few feet above the floor of the 2nd storey). Could you please advise how to find out the sources of these condensation, the contractors I talked said having seen thee before. Is it possible there may be a problem with the vapor barrier ? Can I remove a sheet of OSB sheathing to check the vapor barrier inside (it is not possible to check from inside of the bathroom without removing bathtub, cupboard, etc)? (the house has poly vapor barrier on the inside of insulated exterior walls, built in 2006). I appreciate greatly your help. Thanks.
I'd start inside by improving the exchaust fan installation: properly-sloped fan duct, insulation, and routing; then I'd be sure I didn't have air leaks into the walls from inside.
Question: insdtalling a true rock veneer on a building
(Sept 30, 2012) Libby said:
I live in San Diego and hired a contractor to install true rock veneer on the front of the house. I would like to have the veneer go all the way to the ground because it would look better. However, the folks that sold me the rock veneer said not to cover the weep screen located at the foundation of the house. Does covering the 1/2" weep screen located at the foundation of the house with veneer rock cause concern later? We don't get much rain, but the vote is now 50/50. Thought I'd ask you.
Yes you may trap water in the wall cavity leading to rot, insect damage, mold; but the installer ought to be able to continue weep openings in the veneer wall.
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