On the coast, builders battle wind-driven rain
and high humidity levels to avoid weather-related callbacks and
lawsuitsby Clayton DeKorne
"There's no such thing as a waterproof siding," insists Robert
Criner, owner of Criner Construction of Yorktown, Va. "We
assume that every joint in every piece of trim and in every
type of siding will leak, no matter where the house is located.
But when we build on the coast, we check it three times."
Coastal climates are more troublesome than most due to
prevailing winds and high humidity levels. It's not just the
driving force on the windward side of a house that pushes water
into walls. As wind rises over the top and around the sides of
a building, suction pressure can pull water upwards, drawing it
underneath lapped siding materials and through even the
tightest trim and siding joints. Sea spray — moisture
atomized by the crashing surf — accumulates on the
exteriors of beach homes, and some of this water wicks to the
back of siding and trim through tiny openings by capillary
action. When the sun comes out to warm a wet wall, surface
water begins to evaporate and vapor pressure forces some of
this moisture through porous siding materials where it can
condense on the cooler inner-wall surfaces. All of these
mechanisms move water through the exterior cladding and trim,
but the trouble doesn't end there.
MAPPING THE PROBLEM
"We've always thought wind-driven rain was the biggest issue in
coastal environments," explains Sylvio Plescia, a building
scientist with the Canadian Mortage and Housing Corp. and
project leader for the CMHC's Best Practice Guide to
Wood-Frame Envelopes in the Coastal Climate of British
Columbia (available at
"Certainly wind pressure contributes to problems, but the time
of wetness — how often a wall gets wet and then has a
chance to dry out — determines how quickly problems
show up." According to Plescia, if water soaks a wall assembly
and it never has a chance to dry out, problems such as peeling
paint and mold growth can show up in just a few months, and rot
can take hold in only a few years. In areas subject to strong
winds and rain but also long dry spells, problems take much
longer to manifest. Mold, in particular, will flourish when
water is available but will go dormant when it dries up
— only to grow again the next time it gets wet enough.
Regions where the relative humidity is high and the drying
potential is low are at greatest risk.
In the United States, the high hazard zone extends along the
coast from Virginia Beach to Corpus Christi, Texas. Here, not
only do frequent weather squalls force water into walls, but
according to data from the National Weather Service, this is
where average relative humidity levels reach above 70% all year
round. Under these conditions, walls rarely have a chance to
dry, and the best way to avoid problems is a
belt-and-suspenders approach to protecting wall assemblies.
Criner, like many builders along the eastern seaboard and in
Gulf regions, relies on a combination of housewrap or asphalt
building paper and a variety of flashing materials, layering
these together to ensure that any water which does leak through
has a chance to drain down and out.
WRAPPING THE WALLS
There has been a great deal of debate in the last decade over
whether plastic housewraps perform better than black paper. In
general, plastic housewrap stays more flexible in cold weather
and resists tearing far better than asphalt felt, making it
easier to install. Non-perforated polyolefin products, such as
Typar and Tyvek, are much more water-resistant than cross-woven
polyethylene or needle-punched products. However, all plastic
housewraps are susceptible to damage from extractives that can
leach out of wood sidings. Builders who regularly use cedar or
redwood siding, which are rich in extractives, often gravitate
towards black paper to avoid problems. Certainly, all painted
wood siding and trim materials should be back-primed with a
primer or clear wood sealer. Back-priming not only reduces
leaching, but also helps block moisture from moving through the
siding and blistering the finish.
No housewrap will work very well if it's not properly
integrated with the flashings around openings. Therefore,
builders have developed different procedures for installing
housewrap so it is properly lapped. For example, Patty
McDaniel, owner of Boardwalk Builders in Rehoboth Beach, Del.,
prefers black paper as a housewrap. Her crew installs the paper
up to the bottom of first-floor windows, and then the flashing.
They install the windows before proceeding with the black paper
up to the next floor level. However, Tim Cross of Merrick
Construction in Monmouth, N.J., prefers to wrap the house as
soon as possible. This means his crew must later slit the
housewrap when the windows are installed to ensure that the
head flashing is overlapped by the housewrap.
Regardless of the preferred material, most experienced builders
now agree on this: It is not a good idea to X-cut the housewrap
across rough openings and fold the triangular flaps around the
wall framing. Instead, use the modified I-cut shown in
Figure 1. The critical detail here is the
straight cut across the top of the window. There should not be
a flap that gets folded around the window header.
FIGURE 1. WRAPPING IT RIGHT
Any sheathing wrap should be applied from the bottom up.
The wide overlaps shown here are recommended for coastal
regions where increased wind pressures can pull water uphill,
allowing it to leak past a narrow overlap.
FLASHING WINDOW AND DOOR OPENINGS
The advent of peel-and-stick flashing tapes has revolutionized
flashing methods. These materials come in widths ranging from 4
to 12 inches (9 inches is typical) and are made from either
modified bitumen (similar to eaves flashing membranes) or from
butyl rubber. Butyl-based flashing tapes are generally more
expensive, but will stay flexible in cold weather and remain
much more stable at high temperatures. Butyl products also bond
better to difficult substrates. Perhaps most important of all,
they can be peeled off and adjusted during installation.
Modified-bituthane products start to lose stickiness below
about 50°F and will not bond well below 40°F.
Problems may also arise with modified-bithuthane flashings in
high temperatures. Under a dark-colored metal flashing in
direct sun, for example, a modified-bituthane flashing will
soften and may begin to drip or ooze. Modified-bituthane
flashings should never come into contact with any caulk or
sealant unless the sealant is specifically formulated for that
While peel-and-stick flashing has gained in popularity, there
is still enormous confusion in the field about the correct
procedure for applying these materials. Most builders agree
that a head flashing must lap under the housewrap, but beyond
this, application practices vary for the other flashings. We
recommend the procedure shown in Figure 2,
which closely compares to the typical sequence used by builders
Criner, McDaniel, and Cross.
FIGURE 2. WINDOW FLASHING SEQUENCE
It's not enough to caulk window flashings when installing a
window. Instead housewrap and flexible flashing tape should be
interwoven with the flange, as shown in this installation
sequence. (Illustration courtesy APA - The Engineered Wood
Sill pan. The sill flashing may be
the most important flashing of all, for it allows water that
does seep though to drain to the outside (Figure 3 and
FIGURE 3. WINDOW SILL DETAILS
Water will leak through any window over time, so any sill
flashing should allow this water to drain to the exterior.
Using flexible flashing, this can be achieved by applying a dam
or a piece of bevel siding over the rough sill before the pan
flashing is put in place.
FIGURE 4. DOOR THRESHOLD DETAILS
As with windows, every door should include a pan beneath
the threshold to prevent wind-blown water from seeping
This piece goes in first, and the bottom flap must lap over the
top of the housewrap. A sill flashing can be formed easily in
the field from flexible flashing tape. Tyvek FlexWrap
(www.construction.tyvek.com), a butyl-based
flashing tape, has a wrinkled facing that allows it to be
molded to the rough opening without any cutting and folding at
the corners. This material is decidedly more expensive than
most other flexible flashing materials, but the labor savings
generally makes up for the higher material cost.
Side flashing. Side pieces should go
in after the window has been installed. Some builders dispute
this, arguing that these pieces should wrap the rough opening
and protect the framing. Any water that gets past the window
flange will presumably drain down to the sill flashing. While
this theory sounds about right, intense wind pressure in
coastal regions can pull water straight back to the interior
drywall, long before it reaches the sill pan. Instead, Criner,
McDaniel, and Cross all recommend applying the side pieces
after the window is installed, lapping the flashing over the
window flange to prevent water from getting into the rough
opening in the first place.
When positioning the side flashings, don't butt the tape hard
to the flange. Leave 1/4 to 1/2 inch of the flange exposed, so
the flashing tape won't show after siding or trim is installed.
Trim or siding should never be tightly butted to the window,
but must have at least 1/4 inch to accommodate the expansion of
window and cladding materials.
TO CAULK OR NOT
Most window and door manufacturers, as well as the makers of
housewraps, recommend bedding the window flange in caulk.
Exterior trim details routinely include exterior caulking, as
well. However, a 2002 test conducted by NAHB Research Center
demonstrated that caulk never lasts as long as the materials it
is sealing. The report concluded, "Over time, all exterior wall
sealant systems, including caulk, will leak. Caulks work from a
few days to a few years, which makes it impossible to predict
when and where maintenance will be required." In short,
caulking is largely a waste of time. Instead, the NAHB Research
Center advocates caulkless siding systems, which rely on a
well-detailed weather barrier and flashing system, similar to
those described in this article.
Some builders have argued that caulk helps to seal the window
against air leaks. However, air sealing is probably best done
from the inside using a spray foam such as Dow's Great Stuff
(www.dow.com/greatstuff). This type of foam
remains flexible and won't cause a window to jam as wall
materials expand with climate changes. If air sealing is
attempted on the outside, it would necessarily be incomplete
because the bottom window flange should never be caulked.
Windows will inevitably leak over time, and any water that
leaks through the sill must be allowed to drain back out. In
general, any horizontal caulking bead will create a dam that
holds water, and this dam may prevent water from draining
outside as it follows properly lapped membranes (Figure
FIGURE 5. LEDGER FLASHING
Any horizontal element will create a dam, which blocks
water that penetrates the siding from draining down and out. To
avoid problems, housewrap must overlap the flashing in order to
direct water to the exterior.
Many researchers (but only a handful of builders) push the
drainage concept to the extreme, advocating a "rain screen"
method of siding. This typically involves installing siding
over vertical strapping, or otherwise creating an air space
behind siding to allow the siding assembly to dry. Benjamin
Obdyke's HomeSlicker (www.benjaminobdyke.com), for example,
provides a dense synthetic mat that presumably provides this
air space, while adding only about 3/8 inch to the thickness of
the wall, thereby eliminating the need to pack out windows and
doors. Most EIFS products also have gone to a rain-screen-type
system to avoid moisture problems.
While the theory of a rain screen is sound, strapping is rarely
used under wood, vinyl, or fiber-cement siding in practice
— primarily because it is difficult to fur out windows
and doors. And while more and more builders are beginning to
experiment with HomeSlicker, especially now that it is
available pre-attached to Typar for a one-step weather-barrier
installation, the jury is still out on whether this dense
material allows moisture to drain or creates enough air flow to
actually allow the siding assembly to dry. A CMHC study to
measure the drying potential of various wall systems, including
a wall using HomeSlicker, is now underway, and results are
expected in 2005. In the meantime, successful coastal
contractors are putting their efforts into carefully lapping
housewrap and flexible flashings, and allowing clear pathways
beneath windows, doors, and other horizontal building elements
to allow water to drain to the outside.Clayton DeKorne is editor of Coastal Contractor.