|Windows For The Soul
Protective Exterior Covering
STAINED GLASS WINDOWS in a church or temple are
intended to artistically illuminate, beautify and spiritually
uplift the interior space while also forming an integral
component of the architectural texture of the building's
exterior. Protective glazing is often added to the windows on
the exterior of a building with the intention of protecting the
stained glass from vandalism and storm damage and to add
some measure of insulation for energy conservation. The
problem is this protective glazing can drastically reduce or
even eliminate the visibility of significant architectural
features of a building's exterior facade, such as the intricate
traceries of a compound window frame and the leaded panes
of the stained glass windows.
Let's examine the issue of insulating value for energy
conservation. Churches that are heated intermittently, that is
heated occasionally for services then allowed to cool back,
will not experience a significant saving in heating expenses.
This conclusion was reached in a 1996 study of 'Protective
Glazing' that was commissioned by the National Center for
Technology and Training (conducted by Inspired Partnerships
Inc. Chicago, Illinois). This study examined data from 160
churches in the Chicago area and found that the energy saving
benefit derived from the installation of exterior glazing was
minimal for intermittently heated buildings. The report is
titled 'Protective Glazing Study' and is available on the NCPTT
However this study did not go so far as to say that exterior
glazing did not improve the quality of the heat inside the
building. It cannot be disputed that cold air will draft through
and around a stained glass window in addition to an increase
in humidity from condensation and leaks as a result of wind
driven rain; all these are inherent in any single glazed system.
Heating and cooling cycles promote expansion and
contraction of the stained glass window and this movement
will loosen the glazing cement that is packed between the
flanges of the lead came and the stained glass, eventually
producing leakage (see cementing page 54). The traditional
method to handle this condensation and leakage was to install
collection pans at the bottom of the stained glass windows.
Medieval gothic cathedrals with stone frames had these water
collection troughs carved directly into the stone sash. Some
frames even had these water collection gutters slope in from
both sides to the middle with a weep hole cut through the
frame to channel the collected water to the exterior of the
building, in some cases out through a gargoyle's mouth.
An exterior covering reduces air
infiltration, improves the
security of the building and
reduces the likelihood of vandal
or storm damage to the
A modern window with exterior
glass in an aluminum frame
that has built in ventilation.
A properly designed and installed exterior glazing system will create
an effective barrier to prevent cold air drafts and rain leakage. In
addition it will reduce the consequence of vandalism and storm damage.
It is imperative to specify a protective glazing system that is a proven
and effective barrier to ensure the precious stained glass heritage is
preserved for future generations.
Unfortunately follow up studies, conducted to measure the
effectiveness of protective glazing installed in the US, indicate that these
windows have suffered more damage to the stained glass and their
frames from improperly designed protective glazing systems than from
damage caused by storms, fires and vandalism combined. How could
this happen? The primary cause is the condensation that naturally forms
on the interior side of the exterior glazing. In a single glazed system the
stained window is the exterior glazing and this condensation moisture is
collected at the bottom of the window and allowed to evaporate into
the interior of the building (see previous paragraph). However, in an
unvented double glazed system, as is the case with a stained glass
window and a protective glazing, the condensation moisture is trapped
within the closed airspace. A continuously damp space such as this, is
conducive to the growth of microorganisms that secrete organic acids
that attack the stained glass, oxidize the lead and metal frames and rot
Additionally this unvented space is also a serious heat trap. The
'Protective Glazing' study (mentioned in the 2nd paragraph on page 60)
found air temperatures of up to 165°F (74°C) trapped in the air space,
exaggerating the expansion and contraction cycle. It is widely accepted
that expansion and contraction cycles deteriorate most building
materials, including stained glass windows, causing reinforcing systems
to fail, premature metal fatigue and deterioration of both the frame and
the lead in a stained glass window. The super heated air also creates
pressure on the stained glass window and protective glazing,
contributing to the deflection of the stained glass window. From our
observations while restoring stained glass windows with these types of
problems, the less the space between the stained glass window and the
unvented protective covering, the more severe the damage becomes -
the greater the space, the less severe the damage. A quick visual
inspection will give clear evidence if a moisture problem exists. From
the outside of the building look at the surface of the lead behind the
protective covering, if you detect a white powder you have found 'lead
oxide' (the equivalent of rust on steel) and this window may have a
problem (see photo above right). If the window frames are wood,
check for rot; if steel, check for rust; if stone, check for spalling. From
the interior, check the stained glass window for sagging, bulging and
cracks in the stained glass panes and the glass pulling out of the flanges
of the lead came in these areas. These indicators are evidence that the
stained glass windows are in need of expert care.
So what can be done? The solution, to eliminate both the moisture
and heat build up in the closed space between stained glass window and
a protective glazing is actually quite simple – adequate ventilation! One
square inch (6.5 SqCm) of ventilation at the top and another one at the
bottom of the stained glass window is the minimum ventilation
recommended for 16 SqFt (1.5 SqM) of stained glass within a 'closed'
double glazed system. If the protective covering is being placed over
previously installed stained glass windows the venting is produced by
drilling holes in the exterior glazing that are covered using screened
vent plugs with a rain guard feature. When a new stained glass window
is being created for a preinstalled frame that already has exterior glazing
(either single or insulated units) the venting is allocated to the interior
side, as part of the stained glass framing, allowing the equivalent of a
minimum of one square inch (6.5 SqCm) at the top and bottom per 16
SqFt (1.5 SqM) of stained glass.
Bovard Studio is resolute to the essential necessity for venting to
maintain and preserve stained glass that we dedicated our resources to
find the best possible solution. Our engineers and experienced field staff
researched and proposed several venting solutions before settling on a
method that could be built directly into the frame creating a seamless
installation that provided more than enough ventilation while
preventing water and insect infiltration. We refined and tested our
designs and finally arrived at our Precision Flow® ventilation system.
We received our official Patent Pending status in November 2003 and
are hoping to receive our final patent approvals at any time.
Our Precision Flow® ventilation system can be retro-fitted into in a
frame where the exterior glazing is pre-existing (the vents are placed on
the interior side of the installation) or it can be built into a frame to hold
the exterior glazing (whether in a single glazed or insulated unit
system). We have a Precision Flow® ventilation solution for all types of
frame applications including, single glazed interior installations,
protective exterior glazing (single and double glazed), thermal barrier
aluminum frames and traditional wood frames.
Specifications for the Precision Flow® ventilation system is a
minimum of one SqIn (6.5 SqCm) of ventilation (at both the bottom and
top of the unit) per 16 SqFt (1.5 SqM) of stained glass. The venting ports
are precisely positioned to promote optimum airflow and an easy escape
for the heat and condensation. Exterior venting solutions have a water
shield to prevent water from wind driven rain from entering into the
system. In addition, perforated aluminum screens are placed flush with
the exterior surface area to prevent insects from entering or nesting in
or around the vents, blocking the air flow and cause the ventilation
system to fail.
Adequate ventilation is essential but it is only one consideration for a
properly designed and installed exterior glazing system. The other
consideration is the choice of glazing material. There are several types of
materials available for protective glazing systems for stained glass
windows, they are: standard float glass, laminated glass, tempered glass,
laminated-tempered glass, polycarbonate (Lexan™), acrylic
(Plexiglass™), and extended life polycarbonate (polycarbonate with a
coating of acrylic).
|THERE ARE PROS AND CONS FOR EACH OF THESE CATEGORIES:
Standard float glass maintains a clear, colorless appearance, is
resistant to scratching and is less expensive than any of the other
material choices. Its disadvantage is its relative lack of strength and
when broken the shards become a safety hazard, especially during
windstorms or earthquakes.
Laminated glass has the same properties as standard float glass with
one important distinction; it holds together when broken and will
continue to protect the window from most hurled projectiles. This is an
invaluable safety feature in severe storm and earthquake zones.
Tempered glass maintains all of the attributes of float glass with the
added benefit of having 10 times more resistance to breakage from
impact. If tempered glass does break it shatters into countless small
shards, making it unusable in locations with hurricane and severe
weather codes. The broken bits of glass become high velocity
projectiles that can be fatal in fierce winds.
Laminated-Tempered glass combines all of the clarity and beauty of
float glass with the strength of tempered glass and the safety of
laminated glass. The only drawback is it is expensive.
Plastic polycarbonate (Lexan™) is virtually shatter proof.
Unfortunately it tends to yellow when exposed to ultraviolet (sun) light
and is susceptible to hazing from wind blown particulates.
Polycarbonate expands as the outside temperature rises, causing it to
flex during these expansion cycles creating an unattractive glare as light
reflects off of the concave or convex surfaces. This effect can be
minimized by using the more rigid 1/4" (6 mm) thick material and
compensated for in the framing system.
Acrylic (Plexiglas™) is hard and somewhat brittle and that means it
is shatter resistant (but can break). It does block most of the UV light
that causes the yellowing in polycarbonate however it will haze from
wind blown particulates. Acrylic has a similar coefficient of expansion
to that of polycarbonate and the same precautions due to flexing apply.
Acrylic coated polycarbonate (Extended Life Lexan - XL10™) is a
product that has been developed combining these 2 materials. The
acrylic coating is harder, providing more resistance to scratching, and it
protects the polycarbonate from ultraviolet (sun) light to reduce
yellowing. The polycarbonate is virtually shatter proof providing a
greater level of security.
This window has a white "lead
oxide" powder (the equivalent
of rust on steel) on the surface
of the lead indicating it had a
problem with moisture build-up
between the protective covering
and the stained glass window.
Vents were drilled (note upper
section) for this installation.
Venting produced by drilling
holes in the exterior glazing that
is covered using screened vent
plugs with a rain guard feature.
A new stained glass window
showing the ventilation system
built into an aluminum frame,
private chapel, Wichita, Kansas.
Don Berg is working on a new
mahogany wood frame being
fabricated in our shop. It will
have a Precision Flow®
ventilation system built directly
into the frame. Notice the vent
hole at lower right in the photo.
Tempered glass (1/4"– 6 mm)
protective glazing system with
ventilation built into a new
wood frame for St. Paul's
Episcopal Church, Missouri.
|LOW-E COATED FLOAT GLASS
When standard float glass is selected as the exterior glazing it's
important to be aware of a special type of glass called 'Low-E' that is fast
becoming a popular choice for many building installations. Lowemittance
(a.k.a. Low-E) glass is coated with microscopic layers of antireflective
metal or metallic oxide designed to suppress the radiative
transfer of heat. Various types of Low-E coatings have been developed
that allow for high solar gain, moderate solar gain, or low solar gain.
One variation of this glass, used in colder climates, has the dual effect of
admitting heat (high solar gain) through the glass while at the same time
reducing heat loss from inside the building. Another type, (low solar
gain) is used in warmer climates and reacts in the reverse manner, to
deflect heat away from the window glass on the exterior.
Low-E coated glass has promising possibilities to enhance heating and
cooling efficiencies, however if the wrong Low-E type glass is used as an
exterior glazing for stained glass windows it can cause significant
problems. The Low-E coating functions to either block the solar energy
transfer or allow it to pass through, depending on the specific
characteristics of the coating type.
A 'high solar gain' Low-E glazing system allows the solar energy to
pass through the Low-E window glass while suppressing the heat from
passing back out through the glass. If a stained glass window is installed
on the interior side, the heat is blocked from entering the building by
the stained glass window. This heat is then trapped between the stained
glass window and the Low-E glass (that prevents transfer back out)
thereby amplifying the heat build up.
The 'low solar gain' type of Low-E coating reflects a significant
quantity of solar energy. This effectively reduces the amount of heat that
enters through the exterior glass that could be trapped between the
stained glass window and protective glazing. However any heat that
does enter the space, either from the inside or outside, has resistance
escaping back out through the Low-E window glass and would be
trapped in a non-vented system.
There are several other variations in coating types. One type, called
'sputter coating' allows a greater amount of heat transfer as the sun
becomes lower on the horizon, as typically occurs during the winter
months. This is seen as an advantage when the consideration is for
heating the interior of the building, but as you might imagine this
special function would increase the heat build up if a stained glass
window were installed on the inside, thereby blocking the heat transfer.
This acrylic coated polycarbonate exterior glazing was retrofitted to
this older window frame. It was vented by drilling holes at both the
top and bottom. These holes were then fitted with vent plugs that are
covered with screen and have a rain guard feature.
Typically, the most immediate failure
is in the Low-E glass itself. This glass
can get very hot due to its exposure to
the solar energy, however the portion
of the Low-E glass that is set into the
sash is not exposed to this heat gain
and stays cooler. When this difference
in temperature is exaggerated by the
heat buildup in an unventilated space it
can create enough stress to crack the
Low-E glass pane. We have seen
specific examples of damage when a
combination of Low-E glass and stained
glass windows are installed in nonvented
protective glazing systems. The
Low-E glass cracked and the new
stained glass windows severely buckled
within a short period of time.
We have had meaningful discussions
with several Low-E glass manufacturers
concerning the efficacy of using this
treated glass in combination with
stained glass. All of the experts agree
that adequate ventilation is required
between Low-E glass and stained glass
windows (as is true with stained glass
windows and any exterior glazing
We have installed stained glass
windows on the inside of Low-E glass
exterior glazing systems using the
calculation of 1 square inch per 16
square feet at the top and bottom of the
unit, the same ventilation allowance
that we use for standard protective
glazing systems. Every job that we have
installed with this allowance has been
successful (to date). The consequence
of installing stained glass windows
behind Low-E glass is anecdotal at best.
Clear-cut recommendations cannot be
determined until systematic studies
have been conducted on the various
Low-E coatings in installations with
stained glass windows.
Above: An engineered protective glazing installation for First
United Methodist Church in Iowa City, Iowa. The elaborate
glazing process uses aluminum framing material that is
custom bent to match the rose window's mullion tracery. The
center circle and each petal on the rose design has a new
aluminum frame secured to the top of the original wooden
frame. Then each section is fitted with laminated glass (double
glass with a plastic core). The venting for this installation was
accomplished by creating hidden portals strategically located in
the bent metal frame.
|PROTECTIVE GLAZING CONCLUSIONS
|Protective glazing systems should be designed to minimize the
aesthetic impact on the stained glass window and on the
architectural features of the building. In addition it is essential to
build in adequate ventilation to ensure the preservation of our
nation’s precious stained glass heritage for future generations.