 Windows
in Your Home
When replacing windows in an older house, your decisions on
what type of windows to buy will be among the most important
decisions you will make in terms of energy use. Windows can
account for as much as 75% of heat gain during the summer
months, adding to air conditioning costs. Also, it is not
uncommon for windows to account for a third of the total heat
loss from a house.
Because of the impact windows have on both heat loss and heat
gain, proper selection of products can be confusing. To add to
the complexity, window glazing technology has changed a great
deal in recent years. The best window glazings today insulate
almost four times as well as the best commonly available
glazings of just ten years ago. Because of this rapid change,
designers and window installation companies are often not fully
aware of the potential these new glazings offer for
energy-efficiency.
 How
Do Your Windows Work?
To understand how windows affect heating and cooling costs,
we need to know a little about how energy flows through them.
This illustration shows the primary mechanism of heat transfer
through windows.
 Sunlight
(solar radiation)
An important source of heat, and is transmitted directly through
most windows. Solar radiation consists of visible light and a
part of the solar spectrum that is heat but not visible light
(infrared heat radiation.) A window's solar heat gain
coefficient (SHGC) is the measure of the amount of solar energy
that passes through the window; typical values range from 0.4 to
0.9, and the higher the SHGC, the greater the percentage of
solar energy that is transmitted to the inside.
 Radiant
heat
Given off by warmer objects to colder objects. Things warmed
by sunlight become STRONGER sources of radiant heat, and radiant
heat is blocked by most window glazings.
 Conduction
The mechanism of heat transfer through physical contact.
Heat conducts from the warmer to the cooler side of a window as
each molecule excites its neighbor, passing the energy along.
Conduction occurs not only through solid materials (window glass
and frames), but also through the air space between the layers
of glass. The amount of heat transmission through a material due
to a temperature difference is given by its "U-value" or
"U-factor." The smaller the U-value, the less heat is
transmitted.
 Convection
The movement of heat in a fluid, such as air. The heat is
transferred as the molecules of air are physically moved from
one place to another. A warm glass surface heats the air next to
it, causing the air to rise. A cold glass surface is warmed by
the air next to it, and that air mass will fall as it gives up
its heat. These convection currents occur on the inside of a
window, on the outside, and between layers of glass.
 Infiltration
The process that carries heat through cracks and gaps around
window frames. Infiltration through leaky windows can carry cold
air into a house and carry warm air out. Infiltration is driven
by wind and other differences in air pressure, such as warm air
rising inside a house.
Window technology has improved dramatically in recent years,
with the net result of lowering your energy bills. Some of the
most important energy features of windows are explained below.
Energy-saving Features
Multiple layers of glazing
Until the 1980's the primary way manufacturers improved the
energy performance of windows was to add additional layers of
glazing. Double glazing insulates almost twice as well as single
glazing. Adding a third or fourth layer of glazing results in
further improvement.
Thickness of air space
With double-glazed windows, the air space between the panes
of glass has a big effect on energy performance. A very thin air
space does not insulate as well as a thicker air space because
of the conductivity through that small space.
During the 1970's a lot of window manufacturers increased the
thickness of the air space in double-glazed windows from 1/4" to
1/2" or more. If the air space is too wide, however, convection
loops between the layers of glazing occur. Beyond about 1", you
do not get any further gain in energy performance with thicker
air spaces.
Low-conductivity gas fill
By substituting the air in a sealed insulated-glass window
for a denser, lower conductivity gas such as argon, heat loss
can be reduced significantly.
Most major manufacturers offer argon gas-fill as an option.
Other gases that have been or are being used in windows include
carbon dioxide (CO2), sulfur hexafluoride (SF6), krypton (Kr),
and argon-krypton mixtures.
 Low
E Coatings
More than any other single improvement, the invention and
commercial development of low-emissivity (low-e) coatings in the
1980s revolutionized window technology.
Thin, transparent coatings of silver or tin oxide permit
visible light to pass through, but they effectively reflect
infrared heat radiation back into the room in the winter This
reduces heat loss through the windows in the winter, and cooling
loss in the summer. These coatings are inexpensive compared to
total window replacement, save energy, reduce fabric fading, and
increase comfort.
 Edge
Spacers
As window glazings have improved in performance, what
happens at the edges of the windows becomes increasingly
significant. The edge spacer is what holds the panes of glass
apart and provides the airtight seal in an insulated glass
window.
Traditionally, these have been hollow aluminum channels, usually
filled with desiccant beads (which absorb small amounts of
moisture that might get into the glazing unit). Aluminum has
extremely high conductivity. That didn't matter when the glazing
did not insulate very well, but as better performing glazings
were developed, proportionately more heat was lost through the
edges.
Beginning around 1990, a number of improved edge spacers have
come onto the market. Some are made of thin-walled steel and
have a thermal break. Others are made of silicone foam or butyl
rubber. The net effect of improved edge spacers can be a 10%-20%
improvement in window energy performance, depending on the other
performance characteristics of the window. With new edge
spacers, however, pay particular attention to warranties against
seal failure, which results in fogging and loss of any low
conductivity gas-fill. Choose windows with long warranties.
 Window
Frame and Sash Construction
Window frame and sash construction has a big impact on
energy performance. Wood is still the most common material in
use, and it insulates reasonably well.
Aluminum has been used extensively, particularly in the western
part of the U.S., but unless a thermal break is incorporated
into the design, aluminum frames conduct heat very rapidly and
are therefore inefficient.
Vinyl (PVC) windows are gaining in popularity, especially in
the replacement market, and vinyl frames insulated with
fiberglass insulate better than wood.
Another important property is the
air-tightness of windows. Windows vary dramatically in how
effectively they block infiltration. Air-tightness is usually
measured in cubic feet of air per linear foot of crack (cfm/ft)
at specified testing conditions. The tightest windows have air
leakage rates as low as 0.01 cfm/ft, and the industry standard
is 0.37 cfm/ft. Most of the better windows have leakage rates in
the range of 0.01 to 0.06 cfm/ft.
In general, casement and awning
windows are tighter than double-hung
and other sliding windows. This is because when a casement or
awning window is closed it is pulled in against a
compression-type gasket. Sliding windows
have to use seals that permit the sash to slide, so they are
rarely as airtight. You will find, though, that double-hung
windows from a few manufacturers are tighter than casement
windows from others, so it makes a lot of sense to examine air
leakage specifications carefully when selecting windows.
Window Dimensions
With high-performance windows, the dimensions of the windows
have a big impact on total energy performance. This is because
the glazing (glass, low-e coatings, gas fill, etc.) generally
insulates a lot better than the edge of the window (edge spacer,
sash, and frame.)
This aspect of window performance has led to a great deal of
confusion. If you consider only the energy performance of the
glazing itself (center-of-glass performance), you get a very
high insulating value. But if you factor in the energy
performance of the window edges to arrive at a unit or average
performance value, the insulating value is significantly lower.
The smaller the window in question, the more significant the
effect of the window edges, because a small window has
proportionately more edge area than glazing area. Windows with
true divided lights have a great deal more edge area.
NFRC TO THE RESCUE
If you're shopping for new windows, look for the NFRC
(National Fenestration Rating Council) label as your guide to
their energy performance. California is now requiring all
windows installed to be certified by the NFRC. The NFRC is a
nonprofit collaboration of window manufacturers, government
agencies, and building trade associations, founded to establish
a fair, accurate, and credible energy rating system for
fenestration products (windows, doors, and skylights). Remember
that a window's ability to insulate is given by its U-value, and
the lower the U-value, the more efficient the window.
 NFRC
rates all products in two standard sizes so that
consumers and others can be sure they are comparing
products of the same size. On the label, these two sizes
are listed as "Res" and Non-Res."
ENERGY STAR Qualification is
based on NFRC product ratings. UniFrame windows have
some of the best energy performance specifications on
the market... saving you money and energy!
CLICK HERE TO
SEE OUR NFRC RATINGS! |
|
|
|
U-Factor
U-factor measures how well a product prevents heat from
escaping. The rate of heat loss is indicated in terms of
the U-factor (U-value) of a window assembly. U-Factor
ratings generally fall between 0.20 and 1.20. The
insulating value is indicated by the R-value which is
the inverse of the U-value. The lower the U-value, the
greater a window's resistance to heat flow and the
better its insulating value. |
Solar Heat Gain Coefficient
Solar Heat Gain Coefficient (SHGC) measures how well a
product blocks heat caused by sunlight. The SHGC is the
fraction of incident solar radiation admitted through a
window, both admitted through a window, both directly
transmitted, and absorbed and subsequently released
inward. 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
Visible Transmittance (VT) measures how much light comes
through a product. The visible transmittance is an
optical property that indicates the amount of visible
light transmitted. VT is expressed as a number between 0
and 1. The higher the VT, the more light is transmitted. |
Air Leakage*
Air Leakage (AL) is indicated by an air leakage rating
expressed as the equivalent cubic feet of air passing
through a square foot of window area (cfm/sq ft). Heat
loss and gain occur by infiltration through cracks in
the window assembly. The lower the AL, the less air will
pass through cracks in the window assembly. |
|
* This rating is optional and
manufacturers can choose not to include it. |
|
When selecting new windows, look for windows with these
energy-saving features!
- Low E coatings, and Low-Conductivity Gas-Fill between
panes.
- Select windows with low air leakage ratings-between 0.01
and 0.06 cfm/ft.
- Choose windows with good warranties
against the loss of the air seal. If the glazing seal is lost,
not only will fogging occur, but also any low-conductivity gas
between the layers of glass will immediately be lost.
- To ensure that your new windows perform as well as
they should, hire skilled contractors
with references to install them.
|
