By Ron Bovard, Bovard Studio, Inc.
Glazing For Protection, Not Energy Savings
Damage Caused By Protective Glazing
The Solution: Proper Venting
Efficient Venting System Design
Materials Options For Protective Glazing
Problems With Low-E Protective Glazing
Reducing Negative Architectural Impact
Stained-glass windows beautify the interior of a building and are an important part of the architectural texture of the exterior. Adding protective glazing can obscure the tracery of the window frame, as well as the texture of the stained-glass windows' leaded panes, drastically reducing the visibility of these significant architectural features on a building's facade.
GLAZING FOR PROTECTION, NOT ENERGY SAVINGS
The professed savings on heating bills from protective glazing on buildings such as churches that are intermittently heated has been shown to be exaggerated. Research conducted on 160 churches in the Chicago area and published in 1996 by Inspired Partnerships, Inc. shows that the return on a bank C.D. or even a passbook savings account exceeds the return on investment of protective glazing on stained-glass windows for energy savings in intermittently heated buildings. Protection
from storm damage or vandalism is one of only a few reasons to add protective covering to stained-glass windows in religious buildings that are heated intermittently.
Churches that have never had protective glazing should not add it for reasons other than protection from vandalism and storm damage. Once a building, especially in cold climates, has had protective glazing, however, I do not recommend they revert to art glass windows without protective glazing. The reason is that the congregation is no longer used to drafts from air infiltration through stained-glass and to the water that enters their building from condensation and from leaks in
wind-driven rain. Stained-glass windows, due to expansion and contraction cycles loosening the glazing cement packed between the flanges of the lead came and the glass, tend to leak after a few years. My experience is that clients who have become used to protective glazing systems become dissatisfied once it is removed and not replaced with a new system.
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| This rose
window aluminum-framed protective glazing system was designed to match the
window's original tracery in an outset system at First United Methodist
Church, Iowa City, IA. This careful attention to detail preserves an
important architectural feature of the building's facade. |
DAMAGE CAUSED BY PROTECTIVE GLAZING
Studies on the effects of protective glazing systems in the U.S. show that improperly designed protective glazing has caused more damage to stained glass in church windows and their frames than all damage from storms, fires, and vandalism combined. On a single-glazed stained-glass window, condensation forms on the interior of the window. In an unvented, sealed protective glazing system, the moisture from condensation is trapped between the stained glass and protective glazing.
The dust in this airspace can stay moist, and continuously damp dust nourishes growth of microorganisms that secrete organic acids, which attack the stained glass, oxidize the lead and metal frames, and rot wooden frames.
Stained glass also collects significant solar heat. An analogy I use to compare solar gain of clear glass versus stained glass is the difference between standing barefoot on white concrete on a sunny day and stepping onto black asphalt. In unvented protective glazing systems, the heat is trapped between the stained-glass window and protective glazing. This exaggerates expansion and contraction cycles. Solar gain is more of a problem in winter than summer because winter sun is
lower in the sky and shines more directly on stained-glass windows. The 1996 Inspired Partnerships' research found that air reaches temperatures of up to 165 F. when trapped in the space between the unvented protective glazing and the stained-glass window.
Expansion and contraction cycles cause major stained-glass damage: Reinforcing systems fail, leading to bulging and cracking of the stained glass, and premature deterioration of the lead cames. In a tightly sealed glazing system in a new properly cemented stained-glass window, the increased pressure from the heated air space between the window and protective glazing can cause deflection of the stained-glass panel.
| An aluminum-frame protective glazing system was installed
over this stained-glass window in a private chapel in Wichita, KS. The vents
in the frame were sized according to the author's formula: A minimum of 1
sq.in. vent space at the top and bottom of the frame per 16 sq.ft. of
stained glass. Proper venting prevents potentially damaging solar-gain heat
buildup in the space between the protective glazing and the stained glass. |
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THE SOLUTION: PROPER VENTING
When protective glazing is installed over stained glass, proper venting is critically important. One sq.in. of ventilation at the top and bottom of the stained-glass window is the minimum recommended for every 16 sq.ft. of stained glass in a protective glazing system.
Our field observations have shown that the narrower the space is between the window and the unvented protective covering, the more severe the damage becomes. A minimum of a one-inch air space between the window and protective glazing is needed for satisfactory conservation of the stained-glass. The greater the space, the less severe the damage.
To check for moisture problems, from the outside of the building, look at the surface of the lead behind the protective glazing. If you detect a white lead oxide powder (the equivalent of rust on steel) the window has a problem. If your 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 glass pulling out of the flanges of
the lead came in these areas.
EFFICIENT VENTING SYSTEM DESIGN
For the past decade, venting the protective glazing space has usually been achieved by making holes in the single-glazed protective covering and adding louvered and/or screened vent plugs with a rain-guard feature. This approach always bothered me; I felt it didn't look good and, in glass, it structurally weaken the glazing. Over time, working with our engineers and our experienced field staff, we came up with designs for a venting system built into the frames that prevents water
and insect infiltration. The result was our Precision Flow ventilation system for conservation of stained-glass windows. We received our Patent Pending status in November 2003 for our system and are hoping to receive our final patent approvals at any time. We install our Precision Flow ventilation system into all of our frames, protective coverings, double-glazed and thermal barrier aluminum frames, as well as into our wood frames.
Precision Flow venting in our frames is designed to hold both stained glass and exterior glazing, whether in a single-glazed protective covering system or insulated-glass exterior glazing system. It is designed to the guidelines of 1 sq.in. of ventilation to 16 sq.ft. of stained glass in the bottom and top of the unit, with a gravity flow system: Hot air rises, causing heat and condensation to escape at the top of the unit.
Holes randomly placed in a framing system without a gravity air flow will not be as effective in conserving stained-glass. On exterior venting, we incorporate a water-guard into our frame design to prevent wind-driven rainwater from entering. Perforated-aluminum screens are incorporated into the frame vents to keep insects out and are placed flush with the exterior surface so no indentations or hooded areas exist for bugs to nest in, which would block air flow and cause the
ventilation system to fail.
Working with our clients' architects and our engineers, we can fabricate framing and glazing systems for stained-glass windows that meet Florida's hurricane codes and California's earthquake codes. Our new framing systems survived Florida's 2004 quadruple hurricane onslaughts without failure or leakage. Of course, the forces of nature are unpredictable and man-made structures cannot resist all of nature's onslaughts.
MATERIALS OPTIONS FOR PROTECTIVE GLAZING
Several materials are available for protective glazing systems, each with their own advantages and limitations.
FLOAT GLASS: Plain float glass stays clear and is less expensive than other materials. Its disadvantage is its lack of strength and, when broken, the shards are a safety hazard, especially in storms and earthquakes.
LAMINATED GLASS: Laminated float glass is no stronger that plain float glass, but holds together when broken. This is an important safety feature in storm or earthquake zones; laminated glass, like that used in a car windshield, will and will continue to protect the window from most projectiles -- even when broken.
TEMPERED GLASS: Tempered glass has all the advantages of float glass plus is up to 10 times more impact-resistant than annealed float glass. Non-laminated tempered glass cannot be used in areas with hurricane codes because once it breaks, it shatters into countless small razor-sharp shards. Tempered laminated glass combines the clarity and beauty of float glass with the strength of tempered glass and the safety of laminated glass. The only drawback is its high cost.
PLASTICS: Polycarbonate (eg. Lexan) and acrylic (e.g. Plexiglass) protective glazing are more impact-resistant than glass. Polycarbonate is virtually shatterproof and acrylic is shatter resistant. However, both polycarbonate and acrylic get hazy over time from etching by wind-blown dust. Both have relatively large coefficients of expansion that must be handled in a framing system designed for their expansion and contraction cycles. These plastics flex during these cycles causing
a glare that is unattractive as light reflects off the concave or convex surfaces. I recommend a minimum of 1/4 in. thickness to give the plastic materials enough rigidity to minimize this unattractive effect.
Polycarbonate yellows in a few years when exposed to ultraviolet light. By coating polycarbonate with acrylic, manufacturers have developed a product with significantly longer life. Acrylic is harder than the much stronger polycarbonate. The acrylic coating provides more resistance to scratching, and blocks the UV light that causes the yellowing in polycarbonate, thus extending the useful life of polycarbonate.
LOW-E GLASS: Low-E glass used as exterior protective glazing can cause significant problems. There are various types of low-E coatings, and the way various coatings function when stained-glass windows are installed inside of low-E glazing vary with the type of low-E coating used.
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The stained-glass windows in the West Los Angeles Cathedral in Los Angeles, CA,
are protected by 1/2-in. laminated tempered glass, a premium-quality protective
glazing system that is approved for earthquake and hurricane zones. |
PROBLEMS WITH LOW-E PROTECTIVE GLAZING
A low-E sputter coat is designed to pass solar energy through the low-E glass and prevent heat from exiting out of the low-E glass (see above graph). If stained glass is placed on the inside of the low-E glass, the solar energy passes through the low-E window glass and is blocked by the stained-glass window from entering the building. Heat is trapped between the stained-glass window and the low-E glass, which can cause rapid heat buildup.
Typically, the most immediate failure is in the low-E glass itself. The low-E glass, exposed to solar heat gain in the space next to the stained glass, gets hot. The portion of the low-E glass that is set into the sash is not exposed to the heat gain and stays cooler. When exacerbated by heat buildup from stained glass, this difference in temperature can create enough stress in the low-E glass pane to crack it.
We have seen specific examples of damage when a combination of low-E glass and stained-glass windows is installed in non-vented protective glazing systems. The low-E glass cracks and the new stained-glass windows severely buckle within a short period of time.
Over the last year, I have discussed this issue with two large glass companies' laboratories as well as with commercial window companies. Our representative, Mark Davidson, has discussed this issue with five low-E glass manufacturers. The consensus is that different coatings will produce different results and precise conclusions cannot be arrived at until scientific studies are done on the various low-E coatings in installations with stained-glass windows. (NOTE: The installation
of stained-glass windows behind low-E glass voids the warranties for low-E glass.)
All experts seem to agree that adequate ventilation is required between low-E glass and stained-glass windows -- as with any protective glazing system. However, we do not yet know with precision the proper amount of venting required for stained-glass windows set on the interior side of low-E glass installations. The minimum 1 sq.in. venting per 16 sq.ft. of stained glass at the top and bottom of the unit, required for standard protective glazing systems, has been shown to work in
many cases.
Bottom Line: Use caution when installing stained-glass windows inside of low-E glass exterior glazing systems and absolutely use adequate ventilation.
REDUCING NEGATIVE ARCHITECTURAL IMPACT
If you add protective glazing to a stained-glass window, be aware of the building's architectural features and mitigate negative impact as much as possible. A properly designed system requires a minimum of one inch of air space between the stained-glass window and protective glazing. Therefore, at least one inch of the window frame's reveal will be lost. Because the window reveal is a significant architectural feature, try to leave as much of the reveal exposed as possible.
Divisions for protective glazing should follow the design of the original frame and stained-glass window as much as possible. Stone frames after the 1920s often have the channel for the protective glazing built into them.
ABOUT THE AUTHOR: After several years as an independent artist, Ron Bovard started Bovard Studio in Fairfield, IA, in 1986, where he has assembled a team of leading stained-glass artists and craftspeople. In addition to designing a vented framing system for stained-glass windows, he is he author of Windows For the Soul, and has been the subject of three PBS shows. For more information, go to
www.bovardstudio.com
FOR MORE INFORMATION about stewardship and preservation of historic religious properties, see the website of Partners For Sacred Places
