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.