Protective Glazing and Screens

     The use of protective glazing (also known as secondary or storm glazing) 
is highly controversial. Potential benefits of protective glazing are that it 
can shield windows from wind pressure; increase energy savings; protect 
against environmental pollutants and UV light; provide vandalism and security 
protection, and reduce window maintenance. Potential drawbacks are that it can 
promote condensation; cause heat to build up in the air space and thereby 
increase the window's expansion/contraction; eliminate natural ventilation; 
reduce access for maintenance; offer only minimal energy payback for 
intermittently heated buildings (such as churches and temples), and mar the 
appearance. Protective glazing can also be presented as a cheaper alternative 
to full-scale restoration. And all too often protective glazing is installed 
as a routine matter when there is little threat of damage from vandalism or 
other causes. Protective glazing, especially when improperly installed, may 
hasten deterioration of stained glass windows.

     Various types of metal grills or screens are also used. They add 
security and vandalism protection but often impair the appearance of the 
window (inside and out) by creating new shadows or diffusing transmitted light.

     As a general rule, protective layers should not be added. In most cases 
the potential drawbacks outweigh the potential benefits.

     Under some circumstances, however, protective glazing or screens may be 
necessary.  A real vandalism or security threat warrants protective glazing, 
such as when the windows can be reached easily or are in an isolated location. 
Protective glazing is also warranted when employed historically on a 
particular window as original plating (Tiffany Studios, for example, often 
used plate glass to keep dirt and moisture out of their multi-plated windows). 
Unusual circumstances (such as when the windows are painted on the outside) 
may also dictate the use of protective glazing. Finally, protective glazing 
is warranted when a UV filter is needed to prevent epoxy glass repairs from 
breaking down.

     A variety of protective glazing materials are available. They include 
polycarbonates, acrylics, laminated glass, plate glass, and tempered glass. 
The plastic products are very strong, lightweight, and relatively easy to 
install, but tend to scratch, haze, and yellow over time, despite UV 
inhibitors. They also have a high coefficient of expansion and contraction, so 
the frames must be designed to accommodate change induced by temperature 
fluctuations. Poor installations in restrictive frames cause distorted 
reflections from bowing panels. Protective panels of glass are heavier and 
more difficult to install, making them more expensive than plastic. However, 
glass will not bow, scratch, or haze and is usually the best option in 
aesthetic terms; laminated glass provides additional impact resistance.

     A common error in installing protective glazing is to create a new window 
configuration. Insensitive installations that disregard the original tracery 
destroy the window's aesthetics-and the building's. When protective glazing is 
added, it should be ventilated. If a window is not ventilated, heat and 
condensation may build up in the air space between the ornamental glass and 
the protective glazing. The surface temperature of unvented glass has been 
measured up to twice the outdoor ambient temperature. This differential affects
the expansion and contraction of the support system, particularly lead cames, 
thereby accelerating metal fatigue. Protective glazing may also cause 
condensation on the historic window, depending on the window's orientation, 
indoor/outdoor humidity, and whether or not the building is air conditioned.

     When absolutely necessary, protective glazing should be installed in an 
independent frame between _" (16mm) and 1" (25mm) from the leaded glass. This 
allows the protective panel to be removed for periodic maintenance of both the 
historic window and the new feature. The conditions of the air space between 
the two elements should be monitored on a regular basis; the glass should not 
feel hot, and condensation should never collect on the window.

     No ideal formulas have been developed for venting the air space between 
the ornamental glass and the protective glazing, but it is typically vented to 
the outside (unless the building is air conditioned most of the year). 
Generally, a gap of several inches is left at the top and bottom when glass 
is used, or holes are drilled in the protective glazing at the top and bottom 
when polycarbonates and acrylics are used. Small screens or vents should be 
added to keep out birds and insects. Finally, it is important to realize that 
some original plating of glass softened or tinted the transmitted light 
intentionally, as designed by the original window maker; in this case any new 
or replacement plating should simulate this effect to respect the artisan's 
intentions.