The quick and easy guide to the
glass energy language.
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
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
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 refers to the proportion of the visible spectrum
that is transmitted through the glass.
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.
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.