Frustration with Fenestrations

CURTAINWALL, STOREFRONT, AND other
window systems are installed as building
enclosures comprised of glass and some type
of metal frame (most commonly aluminum)
or wood and vinyl intended to protect
the building interior from the elements.
Curtainwall and storefront systems are typically
designed to carry only their own weight. The
exterior wall transfers wind loads to the main
building structure, also known as the main
wind-force-resisting system, at connection
points at floors or columns of the building. This
type of system is designed to resist air and
water infiltration, movement created by wind
and seismic forces, its own weight, and thermal
expansion and contraction.
Approaches to the installation of curtainwall
systems and storefront systems in strip, ribbon,
multispan, and/or punched-opening situations
vary significantly, as installation teams must
cope with changes to the building code (such
as requirements for continuous insulation on
the exterior face of walls), the introduction of
various and new building cladding systems,
and designers’ efforts to create commercial
buildings with distinctive aesthetics. The
ever-evolving design expectations have
contributed to the frequently observed technical
issues related to window systems installed on
small- and large-scale commercial, institutional,
multifamily, and similar residential-style
construction projects. Even traditional
installations may encounter problems.
I have been performing quality assurance
observations, water-spray testing, and chamber
testing of window systems for the past 15-plus
years and have taken part in subsequent
“forensic” investigations to determine the
source(s) of water infiltration during testing.
Repetitive conditions have been observed
that have been found to be primary sources of
leakage and substandard performance of the
subject systems. While there are multiple and
varied systems and conditions, there are “top
10” conditions that continue to be observed and
documented even in today’s construction, which
might be perceived as a more knowledgeable
and technically advanced industry trade.
The conditions described in this article are
considered to be common and pervasive in this
industry. These conditions are highlighted with
the desire to identify obvious and potential
Feature
By Karl A. Schaack, PE, RRC
Frustration with Fenestrations
issues and options for corrective and/or
preventative action(s).
SEALANT CONTINUITY
Maintaining continuity of primary sealant at
perimeters of “traditional” curtainwall frames is
a common issue. Curtainwall frames are typically
secured to the structure with either F-shaped or
T-shaped anchors where the flange or base plate
of the anchor is secured to the structure at sill
and head conditions. The primary sealant joint is
created by the installation of the backer rod and
elastomeric joint sealant applied in a conventional
hourglass joint profile between the window
frame and substrate. Horizontal flanges/plates of
anchors commonly extend into this joint (Fig. 1).
The plate of the anchors can impede
the insertion of a backer rod and affect the
continuity of the hourglass configuration of the
primary sealant joint and the bond area of the
curtainwall frame at the ends of vertical mullions.
Consequently, to continue and maintain the
primary sealant joint, the profile has to be altered
to more resemble a fillet-shaped profile that is
applied along the edge of the anchor plate. Note
that in this type of application, a bond breaker
tape should be applied over the edge of the
anchor plate to prevent three-sided adhesion of
the sealant. Traditionally, there are two anchors
located at each edge of the opening or vertical
mullion and any intermediate vertical mullions.
The profile of the sealant can change multiple
times depending on the size of the opening
and therefore can become tricky to maintain a
weathertight seal. During chamber testing of
these systems, leakage has been commonly
observed at voids in the sealant that occurs at
interfaces of the different sealant profiles.
RANGE OF EXPECTED
MOVEMENT
As with all types of fenestrations, wind load is an
important structural consideration for curtainwall
Interface articles may cite trade, brand,
or product names to specify or describe
adequately materials, experimental
procedures, and/or equipment. In no
case does such identification imply
recommendation or endorsement by
the International Institute of Building
Enclosure Consultants (IIBEC).
28 • IIBEC Interface October 2024
systems. The more deflection occurring in
the frames due to the induced wind loads,
the more stress is placed on the assembly
and greater likelihood of system failure and/
or glass breakage. There are multiple factors
to consider when designing a curtainwall
system to accommodate expected movement,
including thermal expansion and contraction,
movement due to wind load and gravitational
forces, and movement caused by deformation or
displacement of the building. Movement must be
accommodated to limit stress on glass, framing,
and anchors, and without excessively reducing
the frame’s “bite” or capture of the glass.
Curtainwall installations commonly do not take
into account the thickness (typically ¼ in. [6 mm])
of the base plates of F or T anchors. Because the
base of the anchor extends beyond the end of the
horizontal mullion, when the frame experiences
anticipated vertical movement, it can come in
contact with the anchor plate, consequently
restricting full movement at wind-load anchors at
the head of the opening. Joints between frames
and substrates presented on shop drawings for
curtainwall systems typically do not depict or
accommodate the thickness of anchor plates
in the cross-section profile, and total allowable
movement often can be missed. As depicted
in Fig. 2, a ½ in. (13 mm) wide joint is noted
between frame and concrete slab. Assuming
a ¼ in.thick anchor plate and any variations
in the substrate, ¼ in. or less is available to
allow movement.
When curtainwalls span multiple floors,
they are anchored at floor lines, typically with
some type of “clip” (such as a steel angle or bent
plate). This anchorage can happen at a concrete
slab edge or at a structural member such as a
steel I-beam. To accommodate expansion of the
curtainwall frame, the attachment points at these
clips used to resist wind loads typically include
vertically oriented slotted holes. If the holes with
the clips are slotted, a through-bolt is allowed
to experience vertical movement in either
direction while still resisting wind loads (Fig. 3).
Additionally, the floors/structure can move
independently of the window system.
During installation, bolts have sometimes been
observed to be positioned either at the top or
bottom of the slotted holes (Fig. 4), consequently
Figure 4. Anchor at top of slotted hole in wind load anchor.
Figure 1. Base plate of anchor extending into joint between frame and substrate.
Figure 2. Curtainwall sill detail.
Figure 3. Diagram of wind load anchor with slotted holes.
October 2024 IIBEC Interface • 29
not allowing full range of movement, which
could result in excessive stresses on the system.
In most cases, the bolts should be positioned
near the middle of the slot to accommodate
future movement.
PRESSURE BARS
A traditional curtainwall system uses pressure
bars/plates to secure glass lites to frames.
Pressure bars are secured with screws installed
through predrilled holes. To provide suitable
compression of the glass to the gaskets and
against the frame, the screws should be installed
at the manufacturer’s recommended torque
(typically, 25 to 100 in.-lb [2.8 to 11.3 N-m]).
During installation, technicians commonly use
battery-powered drills to install these screws,
and after continual use, battery power becomes
diminished, and the screws most likely will
not be installed with proper torque to achieve
adequate compression. During installation, the
installed screws should be checked periodically
and monitored with an appropriate digital torque
wrench. If the pressure bars are not secured
appropriately, proper compression will not be
achieved between gasket and frame/glass,
and water leakage will likely be experienced,
especially during chamber testing and in service.
Proper positioning/placement of the pressure
bars in relation to the location of the weep holes
in the bar is also important (Fig. 5). Pressure
bars can be mistakenly installed “upside down,”
which makes weep holes improperly located
below the centerline of the pressure bar,
resulting in excessive accumulation of water and
subsequent leakage into the interior.
SETTING BLOCKS
Setting blocks are installed under glass lites
when the glass lites are placed within frames.
Proper size, hardness, and placement of setting
blocks within glazing pocket are essential to
any quality glazing installation. This ensures
full bearing of the glass while allowing water
passage to the weep system. Common criteria
for setting blocks are as follows:2
• Setting blocks should be made of neoprene,
ethylene propylene diene terpolymer (EPDM),
silicone, or another compatible elastomeric
material, and they should have a Shore A Scale
hardness of 85 ± 5.
• A minimum of two identical setting
blocks should be used with each glass lite
installation. The location of these setting
blocks depends on the size of the lite, with the
preferred location being equidistant from the
centerline of the glass at the quarter points of
the sill, but not less than 6 in. (150 mm) from
the corner edge.
• The width of the setting block should be
at least 1⁄8 in. (3 mm) wider than the glass
thickness and 1⁄16 in. (1.5 mm) less than the full
glazing channel width to allow for the passage
of water.
• The length of each block should be sized
to provide 0.1 in. (2.5 mm) of coverage per
square foot of glass area, with a minimum
length of 4 in. (100 mm).
• The height of the setting block should be
adequate to provide the recommended
glass bite and minimum glass-edge
clearance.
Figure 5. Diagram of typical curtainwall pressure plate. Diagram adapted from
reference 1.
Figure 6. Plastic shim between setting block and glass lite.
Figure 7. Diagram of typical sealant application on sill receiver.
Diagram adapted from reference 3.
Figure 8. Moisture detected at back leg of sill receiver during field spray
leakage test.
30 • IIBEC Interface October 2024
• Adequate edge and face clearances are
essential to allow cushioning of an insulated
glass unit (IGU) within the framing system
and to provide pressure to support the edge
of an IGU under deflection. If adequate
clearances are not present, glass breakage
may result from glass-to-frame contact.
Improper materials are sometimes used for
setting blocks (improper material and size),
such as sections of hard plastic shims, which
have been frequently observed to be used
to support glass (Fig. 6). The typical plastic
horseshoe-shaped shim has a Shore A Scale
hardness greater than 100, which exceeds
industry recommendations.
SEALANT SILL PANS OR
RECEIVERS
Typical storefront systems are composed of a
sill receiver that is secured to the substrate. The
common profile of the sill receiver is L-shaped,
with a horizontal leg and a vertical back leg.
After the sill receiver is secured in place, the
frame is set into the sill receiver. In addition to
fastener heads and end dams, a bead of sealant
is to be applied between the vertical back leg
and storefront frame to achieve watertight
performance (Fig. 7).
This sealant creates a seal to prevent water
intrusion into the assembly when the assembly
is subjected to rainwater with a pressure
differential. This sealant has been observed to
be omitted or improperly applied on a regular
basis and can readily result in water penetration
during water-spray testing (Fig. 8).
F AND T ANCHORS
F and T anchors are used to secure frames of
curtainwall systems to the structure at heads and
sills. F and T anchors are shaped like the letters
they are named for (Fig. 9).
The stem of the “T” and the legs of the “F”
extend up and slide into the open ends of the
vertical mullions, and the remainder of the clip
or plate is intended to bear completely on the
substrate, with the fasteners installed through
pre-drilled holes in the plate and into the
substrate. F anchors have only one anchoring
leg and are installed at jambs, while T anchors
have two anchoring legs and are installed at
intermediate vertical mullions (Fig. 10).
During installation of window assemblies
into rough openings, installers sometimes place
plastic horseshoe-shaped shims under these
anchors in an apparent attempt to make up
tolerances in openings to achieve the desired
installation. Common plastic horseshoe-shaped
shims are often placed in stacked formation
between elements and substrate to fill gaps/
spaces (Fig. 11). However, the placement of
shims between the base plates of these anchors/
clips and the substrate can create improper load
distribution, resulting in frame distortion and
possible glass breakage.
GASKETS
EPDM rubber or silicone gaskets are installed
between glass lites and aluminum frames
to hold the glass in place and to function as
a weather seal. The gaskets should typically
be 1⁄8 to ¼ in. (3 to 6 mm) per foot (300 mm)
of horizontal dimension longer than daylight
opening of frame. For example, for a gasket
for a 4 ft (1.2 m) long window opening, the
length of the gasket would need to be ½ to
1 in. (13 to 25 mm) longer than the opening.
Additionally, the ends of glazing gaskets should
be cut on an angle to form a mitered corner and
then properly cleaned, with silicone sealant
applied between abutting ends of the gaskets
(Fig. 12).
If the gaskets are not installed at the proper
length or are stretched during installation,
shrinkage of the rubber gaskets after installation
would most likely result in gaps occurring
between the ends of the gaskets located at the
corners of the frames (Fig. 13). These gaps can
result in excessive amounts of water entering
Figure 9. Photos of F and T anchors.
Figure 10. Diagram of F and T anchors.
Diagram adapted from reference 4.
October 2024 IIBEC Interface • 31
the frame and overcoming the weep system or
allowing an introduction and accumulation of
dirt within the frame that can possibly impact
proper weeping, causing water infiltration into
the building interior.
PERIMETER TRIM EXTENSIONS
Current enclosure designs often depict window
frames projecting out from openings in a
wall to integrate with the selected cladding
system/finish. This practice results in the
primary sealant joint being located beyond
or “out-board” of the opening in the wall.
In this detail, a sheet metal L-shaped angle
or weathering flange is commonly installed
around the perimeter of the opening to
provide a substrate beyond the opening
to receive the sealant (Fig. 14). The sheet
metal angle is typically installed on top of
the weather-resistant barrier (WRB) that is
applied around the opening and secured to
the framing or substrate around the opening.
Then the flanges are stripped into the WRB,
typically with a self-adhering transition
membrane.
One concern with this type of detail is
achieving a proper watertight seal at laps and
corners of the sheet metal angles. The use of pop
rivets and sealant sandwiched in laps of sheet
metal may not provide an adequate seal when
the subject area is tested with a spray wand or
when the window system is tested in a pressure
test chamber (Fig. 15). Using stainless steel
sheet metal for the weathering flange, securing
laps with pop rivets, and soldering the joints can
result in watertight joints. However, experience
has revealed that highly skilled technicians
are required to properly achieve an effective
end result.
An additional area of concern is the proper
attachment of the flange. The use of flat-/
wafer-head and/or pancake-head screws is
essential because it is quite difficult to adhere
a self-adhering membrane over fasteners with
protruding-style heads such as hex-head or
bugle-head styles of screws. Bridging, loose
areas, and/or fish mouths in the self-adhering
membrane can result when the membrane is
placed over large-head screws (Fig. 16). When
nails are used to secure a flanged item to a wood
substrate, the head should be driven flush to the
Figure 11. Photo of plastic shims placed under F anchor. Figure 12. Diagram of sealant application at ends of gaskets.
Diagram adapted from reference 5.
Figure 13. Photo of gap between ends of abutting gaskets at
interface of vertical and horizontal mullions.
Figure 14. Diagram of sheet metal weathering flange at window opening.
32 • IIBEC Interface October 2024
frame to allow proper installation and adhesion
of strip-in membrane.
FRAME CLOSURES/BACKER
PLATES
Aluminum extrusions used to construct frames
of window systems have open ends and backs
when assembled to form a complete frame for a
new window. The ends of vertical mullions that
terminate at heads of sills are “open” and do not
provide a suitable substrate dimension to receive
backer rod and sealant. Consequently, a sheet
metal cap/cover should be set in sealant on the
ends of the frames to create a substrate to bond
the sealant along the perimeter of the frame.
(Fig. 17).
The profile of common aluminum extruded
frames have open “backs.” When placed within
an opening in a wall, a proper sealant joint must
be applied between the frame and substrate
to achieve the desired weather-resistive
performance. Because a framing member has an
open back, an adequate substrate is not present
to receive the backer rod and sealant. The width
of the outer back leg is typically ¼ to 3⁄8 in. (6 to
9 mm), which does not provide ample surface
area for receiving the backer rod to achieve
compression and provide backup for the sealant.
Additionally, the surface area is not sufficient
to obtain adequate surface “bite” area for the
sealant. Backer plates should be installed on the
extrusions to provide this important substrate
(Fig. 18).
SEALANT INTERFACES
Although they are not an actual part of a
curtainwall, storefront, or other type of window
system, sealants installed in joints that occur
between window frames and adjacent cladding
are critical in maintaining watertight systems.
Proper preparation of substrates before sealants
are installed is important to achieve proper
adhesion. Adjacent substrate materials vary
significantly, with types including metal panels,
brick masonry, exterior insulation and finish
systems, precast concrete, and cut natural or
cast stone, among many other materials. Many
sealants are promoted as “primerless” or “may
require primer,” and the sealant manufacturer
may not require application of primer on
substrates for proper sealant adhesion. Not only
is primer recommended on substrates to achieve
the optimum performance, but proper cleaning
of the joint substates is also important. Utilizing
a two-cloth cleaning method is the best process
to achieve a clean joint due to amount of dirt,
dust, etc. that can accumulate on substrates on a
construction site. The two-cloth cleaning method
consists of an initial wipe with solvent on one
cloth, followed by a second wipe with a dry cloth
Figure 15. Photo of sheet metal weathering flange corner lap. Figure 16. Photo of fishmouth and loose area of self-adhering flashing
applied over fastener head.
Figure 17. Diagram of closure plate at end of vertical mullion. Diagram
adapted from reference 6.
Figure 18. Diagram of backer plate on vertical mullion. Diagram adapted
from reference 6.
October 2024 IIBEC Interface • 33
to lift and remove solvent and contaminants that
may be suspended in the solvent.
Another issue can be obtaining and achieving
joint dimensions between frames and adjoining
substrate materials that allow for the proper
sealant joint size and profile. Window systems
and other materials are installed by multiple
trades, and they are typically field measured and
fabricated/installed within certain tolerances.
However, improper joint sizing is commonly
encountered.
In conjunction with achieving proper joint
sizes, maintaining sealant compatibility is also
critical. On many projects, the glazing contractor
is given the responsibility for installing sealants
around perimeters of frames, and silicone sealant
is commonly used in this application. Sealants
to be installed in joints in precast concrete and
other similar claddings are often specified to be
polyurethane. When the joints in the cladding
intersect with the joints at the perimeters of
windows, the sealants in these joints need to be
compatible (Fig. 19). Silicone sealant is therefore
recommended for each of these joints.
CONCLUSION
Although installation of window systems is
considered to be somewhat of a high-tech
industry with trained technicians and installation
manuals, we continue to encounter common
issues associated with water infiltration that can
be traced back to installation problems. Quality
assurance implemented by the installation crew,
or third-party reviewers, can be instrumental
in achieving the optimum performance of
the system.
REFERENCES
1. EFCO Corp. n.d. EFCO Series 5900 Outside Glazed
Curtain Wall Installation Instructions. 4/29/2021
https://www.efcocorp.com/images/products/
remote/instructions/Y350%20-%205900%20OG.pdf
<accessed Aug. 26, 2024>.
2. National Glass Association (NGA). 2022. NGA’s GANA
Glazing Manual. Vienna, VA: NGA. https://members.
glass.org/cvweb/cgi-bin/msascartdll.dll/ProductIn
fo?productcd=GANAGLAZINGMANUAL <accessed
Aug. 26, 2024>.
3. Atlas Architectural Metals. 2020. Hurricane Impact
Installation Manual. Houston, TX: Atlas Architectural
Metals. www.atlasarchmetals.com <accessed
Aug. 26, 2024>.
4. Kawneer. 2007. Kawneer Series 1600 Installation
Manual. Norcross, GA: Kawneer. www.kawneer.us
<accessed Aug. 26, 2024>.
5. Tubelite Inc. 2021. 14000 Series Storefront
Installation Manual. Walker, MI: Tubelite Inc. https://
tubeliteusa.com/wp-content/uploads/2021/03/
Tubelite-14000-IM.pdf <accessed Aug. 26, 2024>.
6. CRL U.S. Aluminum. 2019. Flush Front Storefront
Installation Manual. Waxahachie, TX: CRL U.S.
Aluminum. https://www.usalum.com/techdocs/
usalum/install/11M0203-FF450_FF451_FT451_
FF600_FF601_FT601.pdf <accessed Aug. 26, 2024>.
ABOUT THE AUTHOR
Karl A. Schaack is
president of Price
Consulting Inc.,
located in Houston,
Texas. Price Consulting
is a professional
engineering consulting
firm that has specialized
in building enclosure
technology, including
roofing, waterproofing,
and exterior wall
construction and cladding for more than 25
years. Schaack has been employed in the
consulting industry since 1983 with emphasis
on building enclosure technology. He performs
condition assessments; prepares design
documents; performs constructability reviews;
performs material and system testing; and
performs quality assurance monitoring. He is a
registered Professional Engineer in Texas and
North Carolina.
Figure 19. Photo of sealant joints in wall panels intersecting joint around window frame.
KARL A. SCHAACK
Please address reader comments to
chamaker@iibec.org, including
“Letter to Editor” in the subject line, or
IIBEC, IIBEC Interface,
434 Fayetteville St., Suite 2400,
Raleigh, NC 27601.
34 • IIBEC Interface October 2024