There’s more than meets the eye when it comes to protective glazing. The goal of protective glazing is twofold: 1. Protect the Asset (Stained Glass Window); 2. Don’t draw attention aesthetically.

The window in Fig. A by most standards fails the aesthetic criteria.

But what about its protective function? How can we tell if protective covering is truly being protective?

Once protective glazing has been added to the exterior of a stained glass window, it becomes a significant part of the window environment. It is not uncommon to discover that when there is a significant issue with the stained glass, there are issues with the protective covering also.

Such systems are typically installed to prevent exterior forces such as weather and projectiles from damaging the stained glass window. If any component of the system fails or is missing, the window may be compromised.

Fig A. Covered but Not Vented
Double Trouble for a Large Window

Lack of proper venting and sealant failure are two major culprits. Venting allows the space between the stained glass and outer covering to “breathe” as changes in air temperature occur. Without venting, significant expansion and contraction of this airspace can cause bulging of the stained glass over time, weakening the entire window. Venting can be accomplished to the interior or exterior depending on how the covering system is configured.

When sealant fails, excess moisture and the elements can enter the interspace, negatively affecting the exterior lead surface and any wood surfaces, or worse yet, bypassing the window to the interior. Inferior sealants, or those that do not allow for expansion and contraction, are the main problem here.

Hardened Glazing Compound Falling Out

Nature can be cruel to the matrix that is a stained glass window, so spending money to help preserve a valuable asset can be a wise investment, as long as it is done properly and built to last.