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window must be energy efficient and durable. Below are a few of
the many terms used by builders and manufacturers to evaluate these
qualities:
NFRC: The National Fenestration Rating
Council has developed a new window efficiency rating system (appearing
in 1998) using numbers between 0 and 10 to represent a window’s
efficiency for each of the heating and cooling seasons.
ER: The North American Energy Rating
assesses the overall performance of the glass, the frame, resistance
to air infiltration, and beneficial solar gain.
R-Value: How the window resists heat
transfer. The higher the R-value, the better the insulating effect
and the lower the rate of heat flow.
U-Value: How well the window conducts
heat. The lower the U- value, the better the insulating effect.
Low-E: A transparent coating fused into
the glass surface. Standing for “low emissive,” it allows
the sun’s shortwave energy in from the outside while resisting
heat transfer from inside to outside (good during cold weather).
Long wave energy from the outside, e.g. reflected heat from sidewalks,
is reflected off the glass (good during hot weather, also reduces
harmful ultraviolet rays).
Insulating Glass System: Double or triple
glazing (i.e. 2 or 3 panes of glass) with a scaled space in between.
The space may be filled with a gas (often Argon) to provide greater
insulation. They insulating ability of the spacers separating the
panes also greatly affects the window’s performance.
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