Glossary of terms

Learning the glass energy language can assist you with achiving energy-saving results quickly. 

Here's a guide to get you started. 


Glass may be coated “on-line” or “off-line” (independent of the manufacturing process). On-line coatings are called “pyrolytic” and, because of their high durability, can be further processed (cut, toughened, curved etc.). Off-line coated products are often referred to as “sputtered” coatings and some of these coatings need to be protected within a double-glazed unit or a laminate. Once manufactured, off-line coated products are generally not suitable for further processing other than cutting.

Double-Glazed Unit or Insulated Glass Unit (IGU)

A double-glazed unit or IGU is comprised of two panes of glass separated by a cavity containing air (or another gas) and hermetically sealed. An IGU provides thermal insulation and improved acoustic performance. An IGU is described in terms of the thickness of the outer pane in millimetres, followed by the gap width between the panes and finally the thickness of the internal pane (e.g. 4/12/4). The greater the gap width (towards about 20 mm), the better the insulation performance. The inclusion of an inert gas such as argon instead of air and the specification of Low E glass further improves the insulation provided by the IGU.


Laminated glass panes are assembled from two sheets of glass sandwiching an interlayer, which bonds the glass (usually PVB). A heavy impact can break laminated glass, but won’t splinter it. This leads to greater safety and security. Laminated glass eliminates nearly 99% of harmful UV rays, greatly slowing the fading of floors and furniture. A specialized interlayer in laminated glass can further reduce the Solar Heat Gain Coefficient.

Light Transmittance, Visible Transmittance (VT)

VT refers to the proportion of the visible spectrum that is transmitted through the glass.

Low E

Low E is a coating that is deposited on a glass surface to enable it to reflect short wave (direct solar) heat or long wave (re-radiated/reflected) heat.

Reflective Coating

To create a reflective coating, a metallic coating is applied to one side of the glass in order to significantly increase the amount of reflected visible and infra red heat.

Solar Control Glass

Solar control glass is glass that reduces heat gain derived from direct solar radiation. This may be achieved via interlayers, body tints, reflective coatings or Low E coatings.

Solar Heat Gain Coefficient (SHGC)

The SHGC is the ratio of solar heat admitted by the glazing into a building, compared with the energy striking the outside surface of the glazing. It includes directly-transmitted radiation plus indirect heat gain from re-radiation and convection of absorbed heat from the glass into the building. The lower the number, the higher the performance.


Toned/tinted glass is usually green, grey, bronze or blue. It can shade internal areas and reduce the amount of heat entering through the window. This will keep the building cooler and reduce glare and UV rays. 


The U-value indicates the rate of heat flow through a window due to a temperature difference, from inside to outside (in winter) or from outside to inside (in summer). Heat is lost and gained through a window by the combined effects of conduction, convection and radiation. The lower the number, the higher the thermal performance.