Facility executives considering window replacements
should look to the National Fenestration Rating Council
(NFRC) window energy rating system. The NFRC label
provides a reliable way to determine window energy
properties. It appears on all products certified to the
NFRC standards on window, door and skylight products, and offers ratings for U-factor, SHGC, visible light
transmittance (VT), and (optionally) air leakage (AL)
and condensation resistance (CR) ratings.
Solar heat gain coefficient was a significant factor in
the decision to upgrade the glazing of a New York City
landmark: the United Nations building. Soon after its
completion, the building showed large heat gain and
glare problems on its east- and west-facing facades,
which had tinted, single-pane glass.
the window frame and glazing by the combined
effects of conduction, convection, and radiation.
This is indicated in terms of the U-factor (the
rate of heat loss) of a window assembly.
■ Heat gain from solar radiation. Regardless of
outside temperature, heat can be gained through
windows by direct or indirect solar radiation. The
ability to control this heat gain is measured in
terms of the solar heat gain coefficient (SHGC).
■ Infiltration. Heat is also lost and gained by air
leakage through cracks in the window assembly.
This effect is measured in terms of the amount
of air (cubic feet) that passes through a unit area
of window under given pressure conditions. In
reality, infiltration varies slightly with wind- and
temperature-driven pressure changes.
EACH WINDOW COMPONENT
interacts with the others in
complex ways that determine
its heat transfer characteristics
and overall energy performance.
SOURCE: W WW. ENERGY.GOV