INSULATED GLASS UNIT (I.G.U.)
 IGU
made of glass is called insulated glass (which refers to
heat insulation, not sound). A more technically correct
term, though, is insulating glass, since the glass
itself has no insulated properties. It is actually the
air space between the glass layers (lites) that provides
the insulated qualities.
The air space
between the lites may be filled with air or an inert gas
like argon or krypton which would provide better
insulating performance. Argon (Ar) has an atomic mass of
39.9, which is much more than nitrogen (N2) and oxygen
(O2) molecules, which have a molecular mass of 28.0 and
32.0 respectively. As a result, argon atoms move
significantly slower than nitrogen and oxygen molecules
at the same temperature. This reduces convection and
decreases the energy transfer between one side of the
glass and the other. Typically the spacer is filled with
desiccant to prevent condensation and improve insulating
performance. Less commonly, most of the air is removed,
leaving a partial vacuum, which drastically reduces heat
transfer through convection and conduction. This is
called evacuated glazing.
Often the
insulating quality is used in reference to heat flow
where the gap between glazed sheets is optimum at about
one centimeter. A larger gap allows for convection
currents and negates the dead air space. However, in
some situations the insulation is in reference to noise
mitigation. In these circumstances a large gap improves
the noise insulation quality or Sound Transmission
class.
Insulated
glass may not be cut to size in the field like plate
glass but must be manufactured to the proper size in a
shop equipped with special equipment.
GLASS COATING
The heat and
sound insulation of glazing may also be improved through
the use of a film or coating applied to its surface.
This film is typically made of polyester or metal, and
may give a reflective appearance and one-way mirror
effect to the window, and may improve both heat and
sound insulation. This may be used on single-glazed
windows as an alternative to insulated glazing, or on
the outside layer of insulated glazing to further
improve its effectiveness. Such coatings may reduce
fading of fabric and improve safety in the case of
breakage.
"Secondary
glazing" is sometimes used as a cheaper alternative.
This consists of a layer of glazing placed retrofitted
inside the window, to provide sound and heat insulation.
Plastic sheet may be used for heat insulation, but may
only last for one season.
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