Low-emittance (low-E) coatings
are microscopically thin, virtually invisible, metal
or metallic oxide layers deposited on a window or skylight
glazing surface primarily to reduce the U-factor by
suppressing radiative heat flow. The principal mechanism
of heat transfer in multilayer glazing is thermal radiation
from a warm pane of glass to a cooler pane. Coating
a glass surface with a low-emittance material and facing
that coating into the gap between the glass layers blocks
a significant amount of this radiant heat transfer,
thus lowering the total heat flow through the window.
Low-E coatings are transparent to visible light. Different
types of low-E coatings have been designed to allow
for high solar gain, moderate solar gain, or low solar
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