How to Choose Energy Efficient Windows
What are the other concerns I should have about windows?
In addition to NFRC ratings of Solar Heat Gain and
U-factor, Energy Star buyers should consider a number
of other factors when choosing windows.
These include:
Air infiltration, water infiltration, structural performance,
acoustical performance, Security performance, Product
cost, and Warranty.
Product cost and warranties are issues
when making any major purchase, and this information is
available from the window distributor or manufacturer.
Air infiltration measures the amount of air that leaks into
your house from outside. The lower the air leakage rate,
the less air is exchanged between outside and inside the
window.
Water infiltration measures the amount of water and
pressure that a window can resist to keep the water from
leaking through it. The higher the water infiltration rating,
the better the window is at resisting water leakage.
Structural performance ratings measure the amount of air
pressure (wind load) a window can resist before failing.
The amount of structural pressure ratings required for
windows in your area is often determined by local code
requirements. The higher the structural performance
ratings, the more wind load a window can resist.
Acoustical performance ratings measure the amount of
sound transmission through a window. The higher the
sound transmission rating, the better the product is at
blocking noise from coming through the window.
Security performance ratings measure the ability of a
window to resist different types of forces. For example,
there are burglar-resistant windows, fire-resistant
windows, bullet-resistant windows, wind-borne debrisresistant
windows, and many others.
Many of these products have special uses for different building types and
may be covered by local building code requirements.
NFRC has additional information for selecting energy
efficient windows on its Web site www.nfrc.org. Of
special interest, see the NFRC Certified Products Directory,
which lists hundreds of window manufacturers and
thousands of windows, doors, and skylights that have
been authorized for certification by NFRC. If you need
further information, please contact us at 301-589-1776.
June 2010
U-Factor measures how well a product prevents heat from
escaping a home or building. U-Factor ratings generally fall
between 0.20 and 1.20. The lower the U-Factor, the better
a product is at keeping heat in. U-Factor is particularly
important during the winter heating season. This label
displays U-Factor in U.S. units. Labels on products sold in
markets outside the United States may display U-Factor in
metric units.
Solar Heat Gain Coefficient (SHGC) measures how well a
product blocks heat from the sun. SHGC is expressed as a
number between 0 and 1. The lower the SHGC, the better
a product is at blocking unwanted heat gain. Blocking
solar heat gain is particularly important during the summer
cooling season.
Visible Transmittance (VT) measures how much light
comes through a product. VT is expressed as a number
between 0 and 1. The higher the VT, the higher the
potential for daylighting.
Air Leakage (AL) measures how much outside air comes
into a home or building through a product. AL rates
typically fall in a range between 0.1 and 0.3. The lower
the AL, the better a product is at keeping air out. AL is
an optional rating, and manufacturers can choose not to
include it on their labels. This label displays AL in U.S. units.
Labels on products sold in markets outside the United
States may display AL in metric units.
1. Look For The NFRC Label
NFRC is a nonprofit organization whose goal is to provide uniform,accurate information about the energy performance of windows, doors,and skylights. In addition to publishing consensus standards (for consistent ratings), NFRC administers a third party certification and labeling program to provide the window buyer with verified product information.
So look for the NFRC label on windows to compare products on a fair and equal basis.
2. Compare Product Performance
The NFRC label provides information on how a
window performs. The two most important energy
ratings are U-factor and Solar Heat Gain. By
reviewing the label information, consumers make
an informed choice about the product that is better
for their individual situation.
What is U-factor?
U-factor is also known as thermal transmission.
It is a measure of the rate of heat loss through a
product. Therefore, the lower the U-factor, the
lower the amount of heat loss. In cold climates
where heating bills are a major concern, choosing
windows with lower U-factors will reduce
the amount of heat that escapes from inside
your house.
What is Solar Heat Gain?
The Solar Heat Gain Coefficient, also know as
SHGC, measures the rate of heat gain through a
product. Therefore, the lower the SHGC, the lower
the amount of solar heat gain. In hot climates
where air-conditioning bills are a major concern,
choosing windows with lower SHGC will reduce
the amount of heat that comes in from the outside.
3. Look For The ENERGY STAR® Label
The U.S. Department ofEnergy and the Environmental
Protection Agency have
developed an ENERGY STAR
Designation for products
meeting certain energy
performance criteria. Since
the energy efficiency performance of windows,
doors, and skylights can vary by climate, product
recommendations are given for four climate zones:
a mostly heating zone (Northern), two heating and
cooling zones (North/Central and South/Central);
and a mostly cooling zone (Southern). For more
information about ENERGY STAR windows see
www.energystar.gov. Please note that windows
must be certified and labeled by NFRC in order to
be designated as an ENERGY STAR