|
WINDOW
PROPERTIES
Heat
is lost and gained through windows by direct conduction through
the glass and frame, by air leakage through and around the window
assembly, and by the radiation of heat into the house (typically
from the sun) and out of the house from roomtemperature objects
such as people, furniture, and interior walls. The National Fenestration
Rating Council operates a voluntary certification program for
windows that documents three measured criteria corresponding to
each of these heat loss and gain methods: U-factor, air leakage,
and solar heat gain coefficient (SHGC). In addition, NFRC labels
the visible transmittance (VT) of the window.
•
U-factor — U-factor is a measure of the rate at which a
window conducts non-solar heat flow, usually expressed in units
of Btu/hr-ft2-°F. The U-factor may refer to just the glass
alone but, under the NFRC ratings, it represents the performance
of the entire window, including the frame and spacer materials.
A window with a lower U-factor is more energy-efficient than one
with a higher U-factor. Window U-factors generally range from
0.2 to 1.2.
•
SHGC — The SHGC is the fraction of solar radiation admitted
through a window, either transmitted directly and/or absorbed
by a window assembly and subsequently released as heat inside
the home. The lower a window’s SHGC, the less solar heat
it transmits and the greater its shading ability. A window with
a high SHGC is more effective at collecting solar heat gain during
the winter. A window with a low SHGC is more effective at reducing
cooling loads during the summer by blocking heat gained from the
sun. SHGCs range between 0 and 1.
•
Air Leakage — Air leakage is a measure of the rate of air
infiltration around a window in the presence of a specific pressure
difference across the window. The air leakage is expressed in
units of cubic feet per minute per square foot of frame area (cfm/ft2).
A window with a low air leakage rating is tighter than a window
with a high air leakage rating. Air leakages are usually less
than 1.
•
VT — VT is the fraction of the visible spectrum of sunlight
(380 to 720 nanometers) weighted by the sensitivity of the human
eye that is transmitted through the window’s glazing. A
window with a higher VT transmits more visible light. VT is expressed
as a number between 0 and 1. Window selection is primarily based
on U-factor and SHGC, although VT and air leakage can be important.
Thus, the NFRC label helps consumers make educated purchasing
decisions.
Next:
WINDOW DESIGN CONSIDERATIONS
Energy
Efficiency Articles
Hardwood
vs Laminate
The difference is more than you think, or maybe not!
Good
Carpet is best!
There is nothing like walking barefoot on great carpet
Caring
for Hardwood is easier than you think.
Just vacuum and you are done.
More
Articles...
|
