Environmental Protective Glazing - Safeguarding Historic Windows

An Environmental Protective Glazing (EPG) system is a form of secondary glazing designed to safeguard fragile historic windows—particularly stained glass—from environmental damage such as moisture, condensation, temperature  fluctuations, and air pollution.

The system functions by creating a controlled air space between the original  historic glazing and the protective outer layer, which is typically made of mouthblown glass. This air pocket may be sealed or ventilated, depending on the design and environmental conditions.

The primary purpose of an EPG system is to keep the original glass dry and  stable, thereby reducing deterioration and limiting the need for future  invasive conservation treatments. By mitigating environmental stressors, EPG systems help preserve both the material integrity and aesthetic value of historic glazing for the long term.

• Creates a protective barrier:
  The Environmental Protective Glazing (EPG) system is installed behind the original window, separated by a carefully measured air gap. This design provides a physical shield that reduces the impact of external  environmental factors.
• Manages moisture:
  The EPG system protects stained glass from external weathering—such as rain,  wind, and pollution—as well as from internal condensation. By controlling moisture exposure, it helps prevent corrosion, staining, and  structural weakening of the historic glass.
• Provides ventilation:
  The air cavity between the original glazing and the protective layer is  often ventilated, either to the interior or the exterior of the building. Ventilation helps stabilize temperature and humidity levels, reducing the risk of condensation and ensuring both sides of the glass remain dry. Internal ventilation is generally preferred for its  effectiveness in minimizing internal condensation.
• Preserves original materials:
  The EPG system enables the historic glass to remain in its original  architectural setting while being protected from environmental stress. This approach allows us as conservators to employ less invasive treatments, such as cold painting or localized stabilization, rather than full re-leading or structural replacement.

• Preserves historic glass:
  The Environmental Protective Glazing (EPG) system helps slow the  deterioration of fragile stained glass by creating a controlled and  stable micro climate. This protective environment reduces exposure to damaging fluctuations in temperature, humidity, and pollution.
• Minimizes intervention:
  By preventing further decay, EPG lessens the need for invasive or  irreversible conservation treatments on the original glass, allowing it to remain largely untouched and preserving its historical authenticity.
• Aesthetic considerations:
  EPG systems are designed to be as discreet as possible, ensuring that the visual impact on both the interior and exterior of the building is minimal. When properly installed, they maintain the appearance and integrity of the original window, allowing natural light and color to be viewed as intended by the original artist or craftsman.

The Epg system is a great tool to help stabilize deterioration, yet it is not the overall solution. Other areas of the building must be considered for maintenance to ensure the EPG is as affective as can be. For example leaking roofs, or overflowing blocked external drainage that may be above the window surround.