What to look for in New Energy Saving Home Windows
In climates with a
significant heating season, windows have represented a major source of unwanted
heat loss, discomfort, and condensation problems. In recent years, windows have
undergone a technological revolution. It is now possible to have lower heat
loss, less air leakage, and warmer window surfaces that improve comfort and
minimize condensation.
Winter Thermal Comfort
An older window with a lower glass temperature feels colder because more heat
is radiated from a person's body to the window. Cold glass can also create
uncomfortable drafts as air next to the window is cooled and drops to the
floor. This sets up an air movement pattern that feels drafty and accelerates
heat loss.
High performance windows
with lower U-factors will result in a higher interior window temperature in
winter and thus greater comfort. Proper installation along with
weather-stripping will also improve comfort by reducing cold air leakage.
Look for the following features when buying new
windows:
Windows
-
Double-pane 2x
more efficient than single
-
Gas-filled
(Argon) centers improves performance 6-9%
-
Emissivity –
“Low-E” glass can cut thermal radiation in half.
-
Details –
thermal breaks, etc.
Doors – similar to windows, details: Thermal breaks, core insulation for metal doors.
Raise your hand if you don't know what a thermal break is.
If the inside frame and the outside frame are both made of aluminum the two need to be kept apart by a material that will slow the flow of heat. Plastic is usually used for this.
Look for Efficient Window Properties on the NFRC
Label
The National Fenestration
Rating Council (NFRC) has developed a window rating system based on whole
window product performance (www.nfrc.org). The NFRC label provides the only
reliable way to determine energy efficient properties and to compare products.
The NFRC label appears on all fenestration products that are part of the ENERGY
STAR program.
The following terms which
indicate the overall performance for well-made, high performance windows:
- U-Factor - The rate of heat loss is indicated in terms of the U-factor
(U-value) of a window assembly. The insulating value is indicated by the
R-value which is the inverse of the U-value. The lower the U-factor, the
greater a window’s resistance to heat flow and the better its insulating
value.
- Solar Heat Gain Coefficient (SHGC) - The SHGC is the
fraction of incident solar radiation admitted through a window. SHGC is
expressed as a number between 0 and 1. The lower a window’s solar heat
gain coefficient, the less solar heat it transmits.
- Visible Transmittance (VT) - The visible transmittance (VT) is an
optical property that indicates the amount of visible light transmitted.
The NFRC’s VT is a whole window rating and includes the impact of the
frame which does not transmit any visible light. While VT theoretically
varies between 0 and 1, most values are between 0.3 and 0.8. The higher
the VT, the more light is transmitted. A high VT is desirable to maximize
daylight.
- Air Leakage (AL) - Heat loss and gain occur by infiltration
through cracks in the window assembly. Air leakage is expressed in cubic
feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the assembly.
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