During a hurricane, homes are subjected to flying debris that can
easily shatter standard windows. Once the glass is broken, wind can
blow into the home and pressurize the structure enough to literally
blow the roof off. And once the roof is compromised, the building
and its contents are at severe risk of loss.
For a building to have a chance of surviving such storms, the
windows must be fortified to withstand hits of flying debris.
That's why, beginning with the 2003 model building code, a growing
number of state and local codes have required impact zone, or IZ,
windows in high-wind zones. Made with shatterproof laminated glass,
these windows are now offered by most of the major window
IZ windows come with different ratings that determine where they
can be installed, with "where" defined by the code's wind-speed
zones. The closer to the coast, the stronger the window needs to
be. Window manufacturers provide detailed labeling and test results
to help designers specify appropriate windows for their customer's
zone. (For more information, see "Speccing Windows in High-Wind
Zones," Winter 2005; at coastalcontractor.net.)
A carpenter installs a StormBreaker impact zone window on the
New Jersey shore. While these windows are engineered to withstand
the impact of flying debris without shattering, if not correctly
installed, they can be blown out of the opening.
But how an IZ window performs depends as much on proper
installation as on window construction, and installation details
can vary by zone as well.
As a consultant to the building industry, I teach job foremen,
project managers, and lead carpenters how to properly install IZ
windows. That includes how to follow code-mandated structural
requirements as well as weatherproofing details in published
While it's important to carefully read and follow codes and
standards, a basic grasp of a few key points will make these
documents easier to decipher and will help the installer understand
which details are most appropriate.
Codes and manufacturers alike base their structural requirements on
standardized testing procedures. The aim of these requirements is
to ensure that IZ windows get installed so that they transfer the
stresses of impact and pressure-cycling from high winds to the
rough framing around the window. This will lessen the forces acting
on the window frame and hardware. Otherwise, these forces would be
likely to blow the sash out of the jamb or the window out of the
opening, even while the glass itself remains undamaged.
Structural installation details can generally be found on the
window manufacturer's website. Manufacturers provide installation
sheets for each different window they make, with specifications for
fastener type and fastening schedule.
It's also important to compare the manufacturers' details to state
and local codes, as the codes may have additional requirements.
These requirements can vary in part by the wind zone where the
window is being installed.
The problem is that these details are often presented in a way that
is hard to decipher. Manufacturers invest millions in devising
state-of-the-art windows, but the installation details provided are
so cryptic, vague, or hard to find that they are rarely followed.
It's a serious problem: in nearly all post-incident assessments,
incorrect installation was a significant factor in causing building
The details will be easier to understand if you remember that they
are variations of two basic themes. In the first, the windows are
secured in place with metal hurricane clips fastened to the window
and the framing (Figures 1 and 2). In the second, approved
fasteners are driven through the window jamb into the structure
(Figure 3). In the case of masonry walls, that usually means
driving Tapcon screws through the window jamb and wood buck, and
into the masonry. In general, the higher the wind zone, the larger
the clips or fasteners, and the closer together they must be
Figure 1. Many IZ windows come with hurricane
clips that are designed to transfer the force of impact, and of
pressure-cycling from high winds, to the surrounding structure.
Some windows use screws instead of clips (see Figure 3).
There are no rules of thumb for fastening IZ windows. Each window
will have a set of fastening schedules that depend on the design
pressure where the window will be installed. For example in Zone 4
(closest to the beach), windows in one building may have to meet a
30 psf design pressure requirement, while the windows in a nearby
building in the same zone may have to meet a 60 psf design pressure
A good generalization would be that as the design pressures for the
opening go up, the clips or screws get larger and are spaced more
closely together, but there's no hard and fast rule here, either.
In Zone 4, a window from one manufacturer might require a spacing
of 8 inches on center, while similar windows from different
manufacturers, installed in the same opening, would have a spacing
requirement of 12 inches or even 15 inches on center. It depends on
the size of the window, as well as when it was constructed and
tested. The only way to find the correct spacing is to study the
manufacturer's installation instructions.
Keep in mind that the structural fastening system is designed to
transfer the force of impact to the surrounding wall. Even the best
window, installed perfectly, will do little good in a weak wall.
This fact was driven home to me on a recent visit to the University
of Florida's testing facility in Gainesville, where I observed
tests of how various wood and masonry wall systems stood up to
hurricane-force winds. During these tests, I could actually see the
glass bulge in and out as the wind speed was varied. The walls and
windows held firm because they were built to the highest standards
by proficient craftsmen. Installers in the field need to be capable
of meeting these high standards as well.
Figure 2. As these detail drawings from
Andersen show, code-compliant clips can be supplied to meet the
needs of different window types installed in different
Figure 3. In lieu of clips, some manufacturers
specify that screws be driven through the window jamb into the
framing. Masonry walls call for Tapcon screws to be driven through
the jamb and window buck into the framing.
Fortunately, building inspectors are becoming more knowledgeable on
the structural requirements for IZ windows — and they are
getting better at checking the installation details and calling for
corrections when the installation doesn't meet code requirements.
Understanding these details is the first step toward avoiding
do-overs, but it's also a good idea to consult with the inspector
or plan department before installing the window, or even before
starting construction. (Links to the Florida code are available
online. See sidebar on page 6.) Should you and the inspector
disagree on the window installation details, it helps to have
construction documents on hand and a good working relationship with
the building department to support your case.
While manufacturers are required by code to provide structural
installation details, the code does not put much emphasis on
managing moisture. The result is that moisture management details
often get left to the installer. The few companies that do address
the issue do so in a way that's generic enough to be nearly
useless. In my trainings along the Southeast and Gulf coasts, I
find poorly weatherproofed windows in hurricane zones a common
occurrence. And even without a hurricane, coastal areas have
sufficient day-to-day wind and water exposure to cause premature
failure if good moisture management details are not followed.
The three systems the installer needs to get right when
weatherproofing a window are the sealant, the housewrap or
water-resistant barrier (WRB), and the flashing. Within each of
these systems are two basic approaches to choose from. Each of
these approaches is laid out in published industry standards (see
sidebar on page 6). In order to protect themselves from liability,
I recommend builders and remodelers reference those standards in
their job specifications.
Sealant system. Sealant must be applied around the window perimeter
to keep water out of the framing, and can consist of a tube-applied
caulking or a liquid waterproofing membrane (Figure 4).
Figure 4. Exterior sealant is important for
keeping wind-driven rain out of the window assembly. The seal can
be provided by a traditional, tube-applied sealant or a liquid
membrane, as shown in this drawing. Liquid membranes include W.R.
Grace's Bututhene Liquid Membrane (www.graceconstruction.com) or
Protecto Wrap's Liquid Waterproofing Membrane
Sealant can be applied using the barrier approach or the drainage
approach. The barrier approach has as its premise that a window
should be installed as a barrier before any flashing is put in
place, and the entire perimeter of the window sealed from the
exterior to prevent moisture intrusion. This isn't my favorite
approach because it doesn't provide a way for the assembly to dry.
If water does get driven into the framing around the window, there
is nowhere for it to go except into the wall cavity.
The drainage approach, as its name implies, solves this problem. In
the drainage approach, a sill pan is installed in the opening
before the window gets put in place (Figure 5). Water that sneaks
by the sealant can drain down to the sill pan and then to the
outside. The seal at the sill in the drainage application is at the
back of the window where the window contacts the upturned leg of
the sill pan. This seal needs to be secure enough to withstand the
pressure of wind-driven rain. And the sealant must be compatible
with the pan material.
Figure 5. The drainage approach requires that
a sill pan be installed in the opening before the window, and that
sealant be placed between the pan and the windowsill. If the
fastening schedule requires that a fastener be driven through the
pan into the framing, then sealant will also be needed where the
fastener penetrates the pan.
Some manufacturers' structural requirements specify that fasteners
be driven through the windowsill and sill pan to the framing. While
moisture must be able to drain from the sill pan, these
penetrations have to be sealed to prevent leakage of incidental
water into the wall cavity while still letting moisture drain from
the sill pan to the outside. One way to address this is to place a
strip of peel- and-stick, self-sealing membrane under the pan.
Another is to predrill the hole and inject sealant into the
WRB details. The installer also has two basic approaches to choose
from when installing the WRB. The basic difference is whether the
WRB is put on the house before or after the windows (Figure
Figure 6. The WRB can be installed before or
after the window, depending on contractor preference. If put on
afterward, the sill must be lined with a water-resistant barrier
before the window gets hung. Although few manufacturers provide
clear guidance on flashing, appropriate systems are documented in
The first approach is probably the most common one. The WRB is
rolled out over the entire face of the house before window
installation, modified I-shaped cuts are made in each window
opening, and the pieces are folded back and stapled in place. The
windows are then installed, followed by the sill, jamb, and head
In the second approach, a skirt of WRB is placed at the base of
each window opening, and left loose at the bottom. The windows are
then installed and the sill, jamb, and head flashings put in place.
Finally, the WRB is installed on the entire house.
Industry standards have tested and certified both of these
approaches, so the builder can direct installers to use either one.
The first approach is more common along the eastern seaboard and
Gulf Coast, while the second tends to be more common in California.
There's actually a rational reason for the difference in techniques
used: installers are more likely to put the WRB on first in places
where they are worried about keeping the house dry during
construction, which is often the case on the shore from Texas and
Louisiana all the way up to Rhode Island and Maine. California is
dry most of the time, so carpenters install the windows and leave
the WRB for the stucco guy to worry about.
However, both approaches are used along both coasts. The important
point is that either approach, if done correctly, will perform
equally well in the face of a storm. ~
Bill Robinson is an Installation Masters
trainer and building consultant currently working in New Orleans
and the Gulf Coast region to help with rebuilding efforts. For more
information, contact him at www.train2build.com or