Properties Primer

• Introduction
• Transmittance
• Reflectance
• Absorptance
• Emittance

Glass

• VT
• SHGC
• U-factor
• Low-E Coatings
• Reflective Coatings
• Tints
• Laminates
• Surface Treatments
• Applied Films

Assembly

• Multiple Panes
• Gas Fills
• Spacers
• Frames Types
• Operability
• Air Leakage

Advanced

• Glass
• Dynamic Windows
• BIPV
• Automated Shading

Emittance

When solar energy is absorbed by glass, it is either convected away by moving air or reradiated by the glass surface. This ability of a material to radiate energy is called its emissivity. Window glass, along with all other objects, typically emit, or radiate, heat in the form of long-wave far-infrared energy. The wavelength of the long-wave far-infrared energy varies with the temperature of the surface. This emission of radiant heat is one of the important heat transfer pathways for a window. Thus, reducing the window's emission of heat can greatly improve its insulating properties.

Standard clear glass has an emittance of 0.84 over the long-wave infrared portion of the spectrum, meaning that it emits 84% of the energy possible for an object at room temperature. It also means that for long-wave radiation striking the surface of the glass, 84% is absorbed and only 16% is reflected. By comparison, low-E glass coatings have an emittance as low as 0.04. This glazing would emit only 4% of the energy possible at its temperature, and thus reflect 96% of the incident long-wave infrared radiation.