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Water Management - Control moisture problems, and you can
by Steve Andrews 06/01/2002 - Taken from www.housingzone.com
The first line of defense against mold is preventing the
entry of bulk water into building assemblies. If you don’t
win that battle, you’ll lose the war. Yes, if you want
a mold-free and durable building, controlling indoor relative
humidity requires attention as well. But first things first:
It all starts with draining water down and out from all roofs,
walls and foundations.
“Mold is a water problem,” says forensic engineer
Joe Lstiburek (Building Science Corp., Westford, Mass.). “To
beat water problems, the idea is to shed water by layering
materials in such a way that water is directed downward and
outward, out of and away from the building.”
This might seem like a keen eye for the obvious, but drive
around many new subdivisions today, and it isn’t hard
to spot problems such as dubious surface drainage and essential
flashings that are installed improperly if not missing.
To help upgrade and fine-tune current practice, the Energy
& Environmental Building Association is publishing the
Water Management Guide, a 36-page document written by Lstiburek.
It’s loaded with drawings and simple text that describe
and detail how to do this essential job. Typically you’ll
need to tailor some details to your specific application.
The guide states that all homes in all climates need drainage
planes — surfaces that shed water rather than absorb
it. Establishing a drainage plane is a snap with simple roof
assemblies but takes more thinking and careful detailing for
complex wall and roof intersections. To keep it simpler here,
we’ll focus on walls. Note that because oriented strand
board wall sheathing absorbs water, it can’t serve as
a drainage plane.
Exterior wall cladding often is supposed to double as the
drainage plane. The guide calls this a face-sealed “barrier”
approach. A barrier system relies on the exterior cladding
to shed water for the life of the building. That means penetrations
in the exterior cladding, such as doors and windows, require
perfectly installed and ageless caulking. With no backup drainage
plane behind the barrier, any water that leaks past the face-sealed
cladding might cause trouble.
In many wall assemblies, the drainage plane lies inboard
of the exterior cladding. In the case of traditional masonry
stucco over wood frame, the stucco sheds some water but also
absorbs a substantial amount. Single or doubled felt paper
located behind the stucco acts as the drainage plane.
Less common but frequently recommended “screen assemblies”
include a secondary line of defense. Consider the case of
brick veneer outside a framed wall, or wood siding attached
to furring strips over exterior sheathing. Any water absorbed
by the brick or wood can be driven inward by wind and solar
heat. But the air space behind the exterior finish promotes
drainage and ventilation, and a drainage plane applied over
the exterior sheathing provides final protection against water
movement. “A leak is not truly a leak if it drains back
to the exterior without wetting a water-sensitive material,”
Windows and doors
Lstiburek cites studies showing that 20% of all windows leak
water. If that’s hard to believe, remember that windows
are just components assembled from individual elements that
are susceptible to leakage at joints. And if the window itself
doesn’t leak, most exterior sealants around windows
eventually fail. So the key water-management principle —
drain water down and out — must be applied to all windows.
Exterior window flashings are showing up increasingly on
job sites. The trick is proper detailing (see Figure 2). Those
details might vary with different products and designs. The
critical step is to integrate flashings (installed shingle
style) with a drainage plane (installed shingle style) so
that any water penetrating the window won’t wet the
Do you flash your door openings, especially the sill, to
prevent leakage? If not, you should. Among the recommended
options, the Water Management Guide lists pan flashings, membrane
liners, formable flashings and precast sills in concrete slabs.
Dampproofing will slow but not stop the wetting of basement
walls. Dampproofing applied to basement walls offers moisture-control
benefits by reducing water absorption by those concrete walls.
However, it doesn’t provide a waterproofing function.
Waterproofing implies a watertight seal without holes, and
that’s not feasible. Good-draining materials and good
de-sign reduce the need for dampproofing.
Strategies that support dry basements start with use of proper
surface slopes, overhangs and gutters. Next comes placement
of fast-draining backfill against the wall, capped by a layer
of impermeable clay-type soil that reduces penetration by
surface water. But downpours that hit sidewalls hard, even
beneath well-designed overhangs, occur several times a year
in many climates. So any substantial amount of water that
drains down next to the wall and through the fast-draining
backfill should be collected by a fabric-covered, perforated
drainpipe with positive drainage to daylight or a sump pit.
Also, a capillary break is needed over the footing so that
any water absorbed by the footing doesn’t wick up and
into basement walls.
Worst case, some water gets absorbed. The guide recommends:
“Interior basement insulation systems and finishes should
be vapor-permeable in order to allow this penetrating water
to pass into the interior space and be removed by the mechanical
system.” This means no vinyl-covered blankets or polyethylene
vapor barriers in below-grade applications be-cause they prevent
wetted surfaces and insulation from drying to the interior.
To prevent water problems with slabs, place a layer of polyethylene
vapor retarder directly beneath the slab and above a coarse-gravel
bed. The guide specifies avoiding any sand layer between the
poly and the slab, as this would allow water to be trapped
in the sand layer by capillary action; where this method is
used, water can enter in minutes but take years to dry to
the interior. Further, to prevent moldy flooring, the guide
points out that slabs shouldn’t be covered with vinyl
or carpet until the slab is giving off less than 0.3 grams
of water per square foot per 24 hours of drying time.
While the specific solutions for different building systems
might vary a bit from what’s shown here, the general
drainage principles undoubtedly will apply.