Protective Exterior Covering
As a rule, most stained glass window installations will benefit from an exterior protective covering. An exterior covering reduces air infiltration, improves the security of the building and reduces the likelihood of vandal or storm damage to the window.
However, it is of the utmost importance that the exterior glazing be properly installed and ventilated or significant damage to the stained glass window could occur.
Unvented protective covering has caused more damage to historic stained glass windows in the U.S.A. than any other factor (see bibliography listing for Inspired Partnerships' study on protective covering). Unvented protective covering causes damage to both the stained glass window and the window's frame in two ways.
First, we have all noticed that in certain weather conditions condensation will collect on the interior surface of a "single layered" glass window. When a protective covering is installed on the exterior of a stained glass window, the condensation will collect on the inside surface of the protective covering. This means that the condensation is occurring in the space between the stained glass window and the protective covering. If this area is not vented to allow the moisture to dry out between condensation cycles, this space stays continuously moist. The dust on the stained glass becomes "hygroscopic" dust, meaning it stays permanently moist, like a wet sponge. Microorganisms grow rapidly in this environment and they secrete acids that attack the lead, the stained glass, and the window frame. This problem does not happen when insulated glass units are used as exterior protective covering.
From our observations while restoring stained glass windows with this type of problem, the lesser 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 inspection will give clear evidence if this 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.
The second problem caused by unvented protective covering is heat build up. Stained glass absorbs much more of the sun's energy than clear window glass. In fact, a dark piece of stained glass can absorb up to 60% of the sun's energy when unvented protective covering is in place. One study, conducted on a sunny January day in Chicago, recorded temperatures in the air space as high as 150° Fahrenheit, when outside temperatures were 20° Fahrenheit. As temperatures change from cold to hot, building materials constantly expand and contract. Severe temperature fluctuations, as indicated in this study, produces unnecessary expansion and contraction cycles and is a major contributor to premature metal fatigue in the lead came that could result in buckling of leaded glass windows.
A properly vented protective covering should be designed and installed by a qualified stained glass professional. The Stained Glass Association of America (SGAA) in Kansas City, Missouri, has published standards for protective glazing installation.