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
• Air Leakage

Advanced

• Glass
• Dynamic Windows
• BIPV
• Automated Shading

Glass Panes (Layers)

One of the shortcomings of glass is its relatively poor insulating qualities. Multiple panes of glass with air spaces in between improve the insulating value considerably (see figure to the right). Relative to all other glazing options, clear single glazing allows the highest transfer of solar energy while permitting the highest daylight transmission.

Double glazing reduces heat loss (as reflected by the U-factor) by more than 50% in comparison to single glazing. Although U-factor is reduced significantly, the VT and SHGC for a double-glazed unit with clear glass remain relatively high. Adding a low-E coating to a surface of the double-pane unit will increase the energy performance. Depending on the type of low-E coating, the SHGC and VT will also be affected. Adding a gas fill between the layers of glass will also improve energy performance.

As each additional pane of glass adds to the insulating value of the assembly, it also reduces the visible light transmission and the solar heat gain coefficient. Adding a low-E coating to a surface, or multiple surfaces, of the triple-pane unit will increase the energy performance. Depending on the type of low-E coating, the SHGC and VT will also be affected. Adding gas fills between the layers of glass will also improve energy performance.

Additional panes of glass increase the weight and thickness of the unit, which makes mounting and handling more difficult and transportation more expensive. There are physical and economic limits to the number of glass panes that can be added to a window assembly. However, multiple-pane units are not limited to glass assemblies.

Suspended Films
The middle layers(s) of glass can be substituted with an inner plastic suspended film (see figures below). The light weight of plastic film is advantageous, and because it is very thin, does not increase the unit thickness. Windows using plastic films decrease the U-factor of the unit assembly by dividing the inner air space into multiple chambers. When protected by glass panes from scratching, wear, weathering, and visual distortions caused by wind, the limited strength and durability of the plastic film is overcome. The plastic films are specially treated to resist UV degradation and are heat shrunk so they remain taut and flat. Like glass, a low-E coating can be bonded to the plastic film to lower the assembly U-factor. The plastic film can also be treated with spectrally selective coatings to reduce solar gain in hot climates without significant loss of visible transmittance. The low-E coatings can be applied to the glass or plastic. The combination of multiple glass panes and plastic films with low-E coatings and gas fills achieves very low center-of-glass U-factors—as low as 0.08.

Center-of-glass values of triple and quadruple window units using suspended film.