There are many
technologies used today to manufacture windows.
Typical
materials are aluminum, wood or vinyl frames. Manufacturers
often combine few of those materials; for example to achieve a
good thermal insulation they use wood as a primary frame
material and then clad it in some more weather resistant
material such as vinyl or aluminum to provide durability and low
maintenance.
In terms of
glass, you can have double
or even triple glazed windows. Combination of the window frame
and glass is what in the end makes the window either a good or bad
choice for achieving energy efficiency that you desire in a
green home.
Window Frames
Few of the technologies
seem to be excelling in achieving high thermal performance, such as
wood clad, insulated vinyl or insulated fiberglass frames.
Wood is
traditionally considered a good insulator and wood clad windows
combine a good performance with durability. They are however one
of the most expensive choices.
Vinyl
windows are least expensive and if combined with a triple glass
panes, some insulation within the extruded cavities and
insulating spacers between panes, they can offer good
performance for a moderate price.
What seems to be
coming through lately as a superior technology for highly energy
efficient windows are Pultruded Fiberglass frames. The frames
are made by pulling the resin saturated strands of fiberglass
through the heated dies to form the window. Window then becomes
a dimensionally very stable system. Fiberglass frames have high
durability and thermal insulation properties, which is achieved
by filling the cavities of the profile with foam insulation.
Glass
Glass plays a
major role in the overall performance. Glass can be
either installed with double or triple panes and to really have
the characteristics of a thermally intelligent window, it should come
with low emissivity (Low-E) coating. But we will talk
about this a bit later.
Simply switching
to a multiple pane windows provides a benefit of a drastic heat
loss reduction. Images above provided by FLIR Systems made by
infrared camera, show clearly a difference between the single
and double pane window. Orange areas on the window wall indicate
a high heat loss. You can see the double pane window colored in
purple, showing a lower temperature, hence lower heat loss.
Low Emissivity Glass
Low Emissivity
glass is a window pane that has a
metallic-oxide layer (Low-E coating) applied to the inner side of one of the
glass panes. It's main purpose is to limit the heat transfer from a
warmer to a cooler glass pane. Using an expert explanation, it does that
by allowing the solar energy (light) in the form of
short-waves to come into the home. Short waves from visible
light get absorbed by the objects in the home, get heated by
these rays and then radiate the heat back into the room as a long wave energy.
Now the purpose of the
Low-E coating is to block the heat energy from within the home
(long-waves) to escape to the outside. In a nutshell,
Low-E glass allows the sun light to enter while also providing
thermal insulation.
An example of
how different low-emissivity glass panes contribute to making a
great quality energy efficient windows is illustrated here in
the Zo-e-shield windows made by
Weather Shield Windows and
Doors.
Multi-pane
configurations by Zo-e-shield offer various degrees of thermal
efficiency. Configurations include Zo-E-SHILED 5, a base model with 2 panes of glass, multiple layers of Low-E coating separated by a warm edge spacer
and inert gas-filled insulating airspace. Next model offers 3 panes with one
laminated double pane, same warm edge spacer and inert
gas-filled insulating airspace, where their highest performer
offers 3 separate
panes, 2 warm edge spacers, multiple layers of Low-E coating and
2 inert gas-filled insulating airspaces.
Short Movie
about Zo-e-shield windows series
explains benefits of multi-pane Low-E coated glass in energy
efficient windows.
The placement of the Low-E coating
is important because it determines for which climate that
particular window configurations is best suited for.
For example, in hot climates, and
situations where sun is directly hitting the window (east-west
orientation, un-shaded south-facing), we need to limit the solar
heat gain. The Low-E coating is then applied to (inner side of)
the outside pane.
For cold climates, where we want to
keep most of the heat inside the home, Low-E coating is applied
to the inside pane (outer side). See images below for
illustration.
Tech Talk!
What
is U-Value?
Measure of
material's heat-conducting properties: Heat
conduction rate per unit surface area per unit
temperature difference between its two sides.
How
do I convert U-Value from SI units to
English units?
Divide the SI number by 5.678. For example, If U=1.4
W/m²K
in SI units, then 1.4/5.678 = 0.246 Btu/hr-sf-°F in
English units.
Conversely to convert from English to SI, multiply
by 5.678: 0.246*5.678 = 1.4 W/m²K.
The SHGC (Solar
Heat Gain Coefficient) is dimensionless,
so it has the same value in both systems.
What is R-Value?
R
= 1/U-Value
A measure of the
capacity of a material, such as insulation, to
impede heat flow, with increasing values indicating
a greater capacity
I
found that an excellent resource for explaining
different types of glass and their thermal
properties is
Efficient Windows Collaborative. Their
informative web site is a knowledge base that
provides wealth of data on window technologies,
energy efficiency and selection criteria.
To achieve energy efficiency, window manufacturers
can offer different kinds of Low-E coatings
to provide best thermal properties of
your windows for your particular climate conditions.
Let's explore some of them.
High Solar
Gain, Low-E Glass
According to the
Efficient
Windows Collaborative explanation, this type of
glass is designed to admit solar gain but to reduce heat loss.
One can imagine that this glass is best applied in colder
climates where heating season is longer then the cooling season
and you want to allow solar energy to penetrate the windows and
supplement the heating of your home. At the same time though, you do not want the
energy that you spent to heat your home easily escape through the
windows. For this kind of glass, you can expect to find high
U-vales (resistance to heat loss) on the order of 0.29 or so.
SGHC
(Solar
Heat Gain Coefficient) could be also rather high,
~0.71, which means about 71% of solar heat gain is transmitted
through the pane. There is another parameter that describes this
glass and it is a measure of visible light transmittance (VT).
For this glass VT=.75 or 75% of visible light is transmitted.
What I learned
from the information provided at Efficient Windows
Collaborative's site, this glass is most suitable for climates
that require a balanced amount of heating and cooling through
out the year - moderate climates.
The glass has a Low-E coating that reduces heat loss but allows
a reasonable amount of solar gain to penetrate through the pane.
In terms of performance parameters, typical values that one can
find on the National Fenestration Rating Council sticker (NFRC)
are U=0.27, SHGC=0.58, and VT=0.78.
There are other
factors that can influence the ultimate performance of your
windows, such as placement and orientation, potential shading
from trees or other structures, etc. More information about
window placement can be found in Passive
Solar section.
If you live in hot climate where cooling is
predominant, then this type of glass is ideal for
your location.
Windows with this type of glass allow low solar heat
gain in the summer while at the same time reducing
the heat loss in the winter. U-values for this glass
are about 0.25.
Since reducing the solar heat gain is the primary
goal of this glass, it has a low SGHC ,on the order
of 0.39 (only 39% of solar heat gain transmitted).
In terms of visible
light transmission, it allows relatively higher amount of light
to penetrate through the pane while reducing the
solar heat gain. VT for this glass then could be ~
0.70 that compared to the other 2 types of glass
indicates a clear reduction of visible light
transmittance and glare.
Complete window
system
performance is what matters in the end.
An important
aspect of how the whole window will perform is the space between
the panes and the connection between the panes and the frame.
The space
between the panes is usually filled with argon or krypton gas
(or some blend of them) to serve as an insulator. Spacers that
separate the glass panes need to be with very low conductance to
complete the energy efficient window system.
Energy
Performance Guidance
When purchasing
your next energy efficient windows,
the best way to
compare the energy performance ratings is to refer to the NFRC
label that is affixed to every window. This information about
the energy performance ratings refers to the complete window
system.
National Fenestration
Rating Council - NFRC (www.nfrc.org)
is a nonpartisan coalition of professionals whose purpose is to
provide fair accurate and credible energy performance ratings
for fenestration products. The information you should look for
on the label are the U-factor and Solar Heat Gain Coefficient (SHGC).
Lower the U-factor value, lower the amount of heat loss through
the product. Equally, lower the SHGC value, lower the heat gain
that enters through the product.
