Church Window Protection

Our Patented System for Stained-Glass protection

stained-glass Protection

This rose window aluminum protective glazing system was
designed to match the frame's tracery in an outset system at
First United Methodist Church, Iowa City, IA.

Proper ventilation is one of the essential factors to consider when installing glazing systems to protect stained-glass windows.

Stained-glass windows artistically illuminate and beautify the interior of a building and form an integral component of the architectural texture of the building's exterior. Traceries of a compound window frame, as well as the texture of the stained-glass windows' leaded panes, can be obscured by protective glazing, reducing the exterior reveal or even eliminating the aperture. This can drastically reduce or even eliminate the visibility of significant and costly architectural features on a building's facade.

The professed savings in heating from protective glazing on buildings such as churches that are intermittently heated has been shown to be exaggerated.
Stained-glass Protection

An aluminumframe ventilation system was
installed in this stained-glass window
in a private chapel in Wichita, KS.

Research conducted on 160 churches in the Chicago area and published in 1996 by Inspired Partnerships Inc. clearly shows that the return on a bank certificate of deposit or even a passbook savings account exceeds the return on investment of protective glazing on stained-glass windows for energy savings alone in intermittently heated buildings.

There are really only a few reasons to add protective covering to stained-glass windows in churches that are heated intermittently, such as protection from storm damage or vandalism.

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 adopt a system without the glazing. The reasoning here is that the occupants are no longer used to drafts from air infiltration through a stained-glass window and to the water that enters their building from condensation inherent in any single-glazed system 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 stained 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.

Traditionally, the condensation and leakage was handled by condensationcollection pans at the bottom of the stained-glass window. The stone frames of a medieval Gothic cathedral usually have the water-collection trough built in. Some large frames have the water-collection trough sloped from both sides to the middle with a weep hole through the frame for the collected water to run through to the exterior of the building, in some cases through a gargoyle's mouth.

In America, many older churches that originally had condensationcollection pans, often made of copper, have had them removed during remodeling some time after the addition of protective glazing as they no longer had a function.

Dangers of Glazing

Studies on the effects of improperly designed protective glazing systems in the U.S. indicate that since protective covering has been installed, more damage has been caused to stained glass in church windows and their frames from improperly designed protective glazing systems than from storm damage, 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 if it is continuously damp, the dust is conducive to the growth of microorganisms that secrete organic acids that attack the stained glass, oxidize the lead and metal frames and rot wooden frames.

Stained glass collects significant solar gain. An analogy I use to compare the 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 solar gain is trapped between the stained-glass window and protective glazing. This exaggerates expansion and contraction cycles. Solar gain is more of a problem in the winter than in the summer because the sun is lower in the sky and shines more directly on the stained-glass windows.

The 1996 Inspired Partnerships' research found that air reaches temperatures of up to 165-deg. F when trapped in the space between the unvented protective glazing and the stained-glass window.

Expansion and contraction cycles deteriorate most building materials, including stained-glass windows, causing reinforcing systems to fail, bulging and cracking of the stained glass and premature metal fatigue and deterioration of the lead in a stained-glass window. 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 contribute to the deflection of the stained-glass window. Proper venting is critically important for the preservation of your stained-glass heritage. One of ventilation at the top and bottom of the stained-glass window is the minimum ventilation recommended for 16 sq.ft. of stained glass in a protective glazing system.

From our observations made while restoring stained-glass windows with these types of problems, the less the space between the window and the unvented protective covering, the more severe the damage becomes. However, a minimum of a 1-in. air space between the window and protective glazing is needed for the effective conservation of the stained-glass window. The greater the space, the less severe the damage. A quick 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 single-glazed protective covering. If you detect a white “lead oxide” powder (the equivalent of rust on steel); this 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 the glass pulling out of the flanges of the lead came in these areas.

Venting Systems

Venting Systems

This craftsman at Bovard Studios is fabricating a ventilation
system in a mahogany wood frame for the St. Cecilia Chapel,
Dominican Sisters Motherhouse in Nashville, TN.

For the past decade, venting in protective covering for stained-glass windows has largely 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 though. I felt it was not aesthetic and in glass it adds a structurally weak point to the glazing. Over time, working with our engineers and our experienced field staff, we came up with solid designs with a venting system built into the frames that prevents water and insect infiltration. We refined our designs and arrived at our Precision Flow ventilation system designed for the conservation of stained-glass windows. We received our final Patent approval in November 2009 for our system. We install our Precision Flow ventilation system into all of our frames, protective covering, 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 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 the heat buildup and condensation to escape at the top of the unit.

Holes randomly placed in the framing system without a gravity air-flow system will not be as effective for the conservation of stained-glass windows. On exterior venting, we incorporated a water-guard system into our frame design to prevent wind-driven rainwater from entering the system. Perforatedaluminum screens are incorporated into the vents of the frame to keep insects out and are placed flush with the exterior surface area so no indentations or hooded areas exist for bugs to nest in, which would block the air flow and cause the ventilation system to fail.

Working with our clients' local architects and our engineers, we fabricate framing and glazing systems for stainedglass 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 manmade structures cannot resist all of nature's forces. Nothing made by man can stand against the strongest forces of nature.


There are several types of glazing materials available for protective glazing systems for stained-glass windows: modern float glass, laminated glass, tempered glass, tempered laminated glass, polycarbonate (Lexan is a brand name of polycarbonate), acrylic (Plexiglass is a brand name of acrylic) and extended-life polycarbonate (polycarbonate with a coating of acrylic).

The advantage of plain float glass is that it stays clear and is less expensive than the 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 is no stronger but holds together when broken. This is an important safety feature in a storm or in earthquake zones. Tempered glass maintains all of the attributes of float glass plus is up to 10 times more resistant to breakage from impact 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 that can cause fatalities when blowing through the air at high velocity. Laminated glass, like that used in a car windshield, will hold together when broken and will continue to protect the window from most hurled projectiles.

Tempered laminated 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 its high cost.

Plastic polycarbonate and acrylic protective glazing are much stronger. Polycarbonate is virtually shatterproof and acrylic is shatter resistant. Polycarbonate and acrylic haze from 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 of the concave or convex surfaces. I recommend a minimum of ¼ 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 the polycarbonate with acrylic, manufacturers have developed a product with a significantly increased life expectancy. Acrylic is harder than the much stronger lycarbonate. 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.

Venting System

The working of a sputter coat typically used on surfaces 2
or 3 of an insulated glass unit (surface 2 is the interior
side of the exterior glass pane; surface 3 is the exterior
side of the interior glass pane) is graphically displayed here.

Low-E glass used as exterior glazing for stained-glass windows can cause significant problems. There are various type of low-E coatings for glass on the market today.

The way the various coatings function when stained-glass windows are installed inside of the low-E glazing systems vary with the type of low-E coating on the glazing system.

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 window glass (see diagram), If stained glass is placed on the interior of the low-E glass in winter, the solar energy passes through the low-E window glass and is blocked by the stained-glass window from entering the building. The heat is trapped between the stained-glass window and the low-E glass, which is designed to block heat transfer back through the window glass in the amount normal window glass would allow. This can exaggerate heat buildup.

Typically, the most immediate failure is in the low-E glass itself.This was explained to me by a technical person at Atofina Chemicals, Inc., a lead ing manufacturer of the low-E chemicals supplied to major glass manufacturers for the production of low-E glass.

Venting System

This 1/2in plate-glass protective glazing system was
installed in a new mahogony wood frame at St. Paul's
Episcol Church in Kansas City, MO.

The low-E glass, exposed to the solar-energy heat gain, gets hot. The portion of the low-E glass that is set into the sash is not exposed to the solar-energy heat gain and stays cooler. When exaggerate 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.

During the past year, we have discussed this issue with two large glass companies' laboratories as well as with commercial window companies. The concurrence 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.

Some low-E coatings reflect significant amounts of solar energy at all times allowing less solar energy to enter the building. These types of low-E coatings would reduce the amount of heat from solar energy entering through the window glass to be trapped between the stained-glass window and exterior glazing, but the heat entering the space has resistance escaping back out through the low-E window glass,and would be trapped in a non-vented system, untill a scientific study is conducted,the precise effects of stained-glass windows installed behind low-E glass are anecdotal and observable only in specific cases. The installation of stained-glass windows behind low-E glass voids the warranties for low-E glass.

All of the experts with whom I have discussed this subject strongly state that adequate ventilation is required between low-E glass and stained-glass windows as with stained-glass windows and any exterior glazing systems.

We do not know the proper amount of venting required for stained-glass windows set on the interior side of low-E glass installations as we do for exterior glazing systems that do not include low-E glass. The minimum per the top and bottom of the unit,required for standard protective glazing systems, has been shown to work.

Our conclusion is, use caution when installing stained-glass windows inside of low-E glass exterior glazing systems and absolutely use adequate ventilation.

Architectural Impact

If you add a protective glazing systems to stained-glass windows, you need to be aware of the protective glazing's impact on the architectural features of the building and mitigate its impact as much as possible. A properly designed system requires a minimum of 1 in. of air space between the stained-glass window and protective glazing. Therefore, at least 1 in. of the frame's reveal will be lost. We want to leave as much of the reveal as possible exposed as a significant architectural feature. The divisions for the 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.

Protective glazing systems need to have as little aesthetic impact as possible on the stained-glass window as well aas on the architectural integrity of the building. It is essential that prot glazing systems are properly designed to preserve our nation's precious stained-glass heritage for future generations.

Our Precision Flow® (Patented) venting system designed to comply with the engineering studies showing the requirements as determined by the National Preservation Center’s 1996 Protective Glazing Study for stained glass windows. For a summary, click here for Ron Bovard's Traditional Building magazine article, "Stained Glass Protection".