IGU
made of glass is called insulated glass, which refers to heat insulation,
not sound or electricity. A less accurate term is "insulating glass",
since the glass itself has no insulative properties. It is the air space
between the glass layers (lites) that provides the insulation.
It is important that the air remains as immobile as possible to prevent
convection currents transferring heat across the insulating (resisting) gap. This limits
the thickness of the air gap used and is the reason for triple glazing.
The space between the lites may be filled with air or an inert gas like
argon or krypton which would provide better insulating performance. Argon
has a thermal conductivity 67% that of air. 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. Similar techniques are also
used in insulation products called vacuum insulated panels.
Often the insulating quality is used in reference to heat flow where the
gap is the insulating medium. The
gap is usually 12mm to 20mm thick.
Within this range, the thickness does impact the insulating properties
substantially, but smaller gaps have greater heat conduction through the air
or other gas, and larger gaps allow more convection within the space leading
to higher convective heat loss. A 16mm air gap is often considered the
optimum thickness for air although this depends on many factors such as the
size of the window, the temperature difference between the two panes and
whether it is vertical.
In
general, the more effective a fill gas is at its optimum thickness, the
thinner the optimum thickness is. For example, the optimum thickness for
krypton is lower than for argon, and lower for argon than for air. However,
since it is difficult to determine whether the gas in an IGU has become
mixed with air at manufacture time (or becomes mixed with air once
installed), many designers prefer to use thicker gaps than would be optimum
for the fill gas if it were pure. 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.
As of 2007, argon is commonly used in insulated glazing as it is
affordable. Krypton, which is considerably more expensive, is not generally
used except to produce very thin double glazing units or relatively thin, or
extremely high performance triple glazed units.
- In principle, xenon would be even more effective than krypton.
- Insulated glass assemblies cannot 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.
- The effectiveness of insulated glass can be expressed as an R-value.
The higher the R-value, the greater is its resistance to heat transfer.
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