Things to Consider

Evaluation of Advantages and Disadvantages of External Glazing of Windows

April 09, 2007

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A number of factors must be considered when determining whether external glazing is a wise decision for leaded and painted windows.  External glazing, properly installed, is not necessarily heat or energy saving.  In order to preserve the leaded window, the external glazing must have an air flow between the stained glass and the external glazing.  There must be venting at the top and bottom of the window to permit the passage of air and moisture.  This is called "the chimney effect."  If the external glazing is not vented, it will allow condensation, which will destroy the lead cames and possibly attack the surface of the glass in the space of a very few years.  It is possible to destroy stained glass by installing poorly designed "protective glazing" more quickly than by leaving it alone.

External glazing must be:

1. Historically sympathetic with the building;
2. Unobtrusive against the original design of the stained glass when seen from the interior;
3.  Structurally and materially sound;
4.  Installed and vented properly.

A protective barrier of some sort may be advised to protect against projectiles, e.g., the BB guns or rocks of ill-advised individuals, hail, or flying objects experienced during times of heavy wind.  Glass would seem to be better than plastics.  This is the opinion of most European professionals in the historical monuments commissions.  It is a tried and true element of windows.  Plastic is only recommended when the issues of an impenetrable barrier against projectiles is of the highest priority.  Even then, a thin wire mesh can be associated with glass for such purposes.  Glass can be slumped or treated so that it has considerable transparency and the original tracery and leading systems can be seen.  Because of the discoloration experienced with plastics such as LEXAN, they are not recommended for historic or artistically significant structures.  Glass products that can be textured to reduce glare are the most recommended.

Contours of tracery are an important element of the building.  The question of the ability of the external glazing to follow the contour of the tracery is an aesthetic, as well as structural, issue.  Given the simple trefoil tracery and double Gothic arched lancet heads, matching the pattern in the external glazing should not be difficult.

Venting is a CRUCIAL issue.  Often a steward of religious property is faced with the previous generation's installation of highly inadequate plastic external sheeting often called "protective glazing."  Unvented sheets inserted into the frame (and usually sealed with putty) or screwed on top of the window sash do not protect glass.  European scientific studies have now shown that unvented external glazing is worse for the windows than no protection at all.  In hot and moist climate, e.g., the southern U.S. and Atlantic coast, it is advisable to vent to the interior of the building.  Condensation will form on the interior side of the external glazing, leaving both inner and outer surfaces of the historic stained glass as moisture free as possible.  Since the painted details in windows are major elements, protecting the integrity of the painted surface is paramount in these recommendations.  Venting to the exterior is far more likely to cause condensation on the interior painted surfaced.  Over time, any pollutants in the air will attack the glass through this liquid medium.

In many cases, this issue is most appropriately handled by firms specializing in glazing, not in conservation and restoration of stained glass.  The supervising architect/engineer should coordinate the installation of the external glazing and the original stained glass windows.  Stained glass restoration and installation of external glazing are two entirely different crafts.  Owner or caretakers of a building, reasonably, should deal with the most experienced professionals for any building need.  Certainly this would mean as a first step, demanding that a "restorer" define clearly the costs of restoration between the stained glass itself and the design and installation of external glazing.

For new buildings, including those that will reuse historic windows from no-longer functioning churches (e.g., the Archdiocese of Boston has removed many windows from closed historic churches), there are new options.  Since congregations demand climate control, for which leaded windows were never designed, consider installing high quality modern glazing, with the stained glass mounted as a screen on the interior.  This removes the weight bearing function of the window and allows it to function purely decoratively.  Such construction will necessitate far less repair for the leaded window in the future, and thus, be highly cost-saving in the long run.

Dr. Virginia Raguin is Professor Art History at The College of the Holy Cross, Worcester, MA.  She is author of Stained Glass: From its Origins to the Present (New York: Harry N. Abrams, 2003) and The History of Stained Glass: The Art of Light, Medieval to Contemporary (London: Thames & Hudson, 2003).  She is also the editor of Catholic Collective, Catholic Reflection 1538 - 1850: Objects As a Measure of Reflection on a Catholic Past and the Construction of a Recusant Identity in England and America (Washington, DC: The Catholic University Press, 2006). 


Stained Glass: Old Windows and New Views
Stained Glass Restoration

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