Windows For The Soul
Protective Exterior Covering

Protective Covering

As 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 website: http://www.ncptt.nps.gov/

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 window.

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 wooden frames.

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 system).

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.
 
 

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