| The type of glass you choose is perhaps the most important
decision you make when purchasing windows. After all, 80-90% of a window
is its glass unit. Your choice of glass can dramatically affect and
enhance the architectural appeal of the building. It also affects the
building’s energy efficiency. |
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Energy Efficiency
Due to rising energy costs, single-pane windows
are just about obsolete these days. Most windows are now built with
double-pane, insulated glass.
Insulated glass is essentially a “sandwich” of two pieces
of glass separated by a spacer bar and sealed on all four sides to create
a dead airspace between the two panes. This technology was actually
invented in 1935 by the Libbey-Owens-Ford Company (under the trade name
Thermopane™). However, it has been significantly enhanced and
improved over time.
Today, a typical window will have double-pane
glass with an airspace between 1/2" and 1". They may also contain a
special low E coating or argon gas injected between the panes to increase
their energy efficiency. Spacer bars have also been specially engineered
to provide greater energy savings and longevity against seal failure.
Energy efficiencies range from U=0.49 (twice as insulating as single-pane
glass) to U=0.10 (over 10X better than single-pane).
Click
here to see a U-Value Chart.
NOTE: Many consumers are under the false assumption
that more dead airspace means greater energy efficiency. However, studies
have found that while airspaces less than 1/2" are too small to be effective,
airspaces greater than 5/8" allow air currents to develop within the
two panes. Such air movement aids in the transfer of heat between the
panes, actually reducing efficiency. |
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The Downside of Insulated Glass – Seal
Failure
When seal failure occurs, moisture begins
to form between the two window panes. This is one of the top service
issues with today's windows, which requires replacing the glass. It
is not possible to simply re-seal the window, so be sure to choose your
manufacturer wisely!
NOTE: Most WeatherMaster windows come with a
lifetime warranty against seal failure. |
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Glass Terminology
R-Value:
A measure of the resistance to heat flow through a material.
Typically it is a measure only of conduction, not air leakage or radiation
heat transfer. As a result, U-Values are preferred when making comparisons.
Higher numbers mean more insulation. A typical double-pane
window achieves an R-2.0. |
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U-Value:
A measure of heat transfer through a material. Lower numbers
indicate less heat transfer (more insulation) with a typical double-pane
window achieving a U-0.5. It has a relationship to the R-Value where
U-Value = 1/R (approx). Click
here to see a U-Value Chart. |
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Warm-Edge Spacer:
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| With Aluminum
Spacer |
With Intercept
Spacer |
Intercept insulating glass
units reduce condensation problems. (Cold side temperature = 0
degrees F, room side temperature = 72 degrees F and 25% relative
humidity. |
A spacer bar specially designed to minimize
heat transfer by conduction through the spacer bar. Ordinary spacer
bars are often made of aluminum or other alloys that readily conduct
heat or cold from the glass surfaces, while warm-edge spacer bars provide
an insulating barrier. As a result, windows with warm-edge spacers are
less prone to condensate at the perimeter of the glass during cold winter
months. Oftentimes these specially engineered spacer systems incorporate
other advantages that can drastically improve the longevity of the glass
unit, and thus the window's warranty. Some brand names are Intercept,
Swiggle, DuraSeal, and Super Spacer. By using a warm-edge spacer, manufacturers
can increase the energy efficiency of a window by approximately 5%. |
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Argon Gas:
An inert gas that is often injected within
the dead airspace of an insulated glass unit to enhance its efficiency.
Since it is a naturally occurring component of the atmosphere, it is
100% safe. Argon gas-filled windows are approximately 6% more energy
efficient than windows without argon gas. |
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Krypton Gas:
An inert gas that is often mixed with
argon and injected into the dead airspace. It is more prevalent in
triple-pane windows since it is more effective when the glass panes
are close together. |
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Low E Glass:
Low E glass contains a highly engineered
surface designed to reflect heat before it passes through the window.
While early versions were often mirror-like or tinted, today's Low
E glass is designed to allow most visible light to pass through it
and doesn't appear any different than ordinary glass. In addition,
low E coatings often are designed to reflect UV light to protect draperies
and interior furnishings from sun bleaching. By incorporating low E
glass, manufacturers can decrease energy loss through double-pane glass
by up to 50%.
NOTE: Not all low E glass is equal. Be sure
to check both winter and summer U-value ratings, and the Solar Heat
Gain Coefficient, SGHC. For the Mid-Atlantic states lower values are
better. LEARN
MORE |
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Heat Mirror™ – Voted one of the "Top 100
Inventions of the Millenium"
Originally developed by scientists at M.I.T.,
Heat Mirror is a thin film that when suspended between two sheets of
glass, can make a 1" thick window just as insulating as the 6" wall
around it. Not to be confused with low E glass, Heat Mirror creates
two airspaces in an insulated window, and is optimized for maximum energy
savings in nine different climates. In fact, windows built with Heat
Mirror are up to 150% more insulating than windows built with standard
low E glass.
Heat Mirror is so revolutionary that in November
1999 it was voted as one of the "Top 100 Inventions of the Millenium" by
Popular Science magazine. Popular Science called Heat Mirror film "one
of the more dramatic advances" of the millennium. Already it is
credited with saving tens of millions of barrels of oil since its commercial
release in 1980.
Some Additional Benefits
- More sound insulation than low E windows. Windows with Heat Mirror
block up to 85% of all outside noise.
- Improved plant growth. Heat Mirror's outstanding insulation eliminates
the wide temperature swings that can be harmful to plant health. In
addition, Heat Mirror blocks the infrared radiation that can scorch
leaves, while letting in light plants need for healthy growth.
- Heat Mirror "knows" what season it is. It allows radiant heat
from the sun to pass through the glass and warm the home in the winter,
but repels the sun's heat in the summer.
- UV protection. Heat Mirror blocks 99.5% of sun bleaching UV radiation,
but allows visible light to pass straight through.
- Lighter weight. Competitor products offer triple glazing in an
effort to approach Heat Mirror performance. That extra weight is hard
on window hardware, and can make it difficult for older or health-challenged
individuals to operate the windows. In contrast, windows with Heat
Mirror are the same weight as conventional double-pane windows.
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Triple-Pane Glass:
Manufactured like double-pane, but with an
additional airspace.
Triple-pane is viewed by many in the industry
as outdated since similar efficiencies can be achieved with modern day
low E coatings or Heat Mirror at a much reduced cost. |
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Pilkington Activ™ Self-Cleaning
Glass:
Now there's a revolutionary new kind
of glass that actually uses the power of the sun to clean itself – Pilkington
Activ ™ Self-Cleaning Glass. It features a special photo-catalytic
exterior surface that uses the sun's UV light (abundant even on cloudy
days) to gradually break down, loosen and dissolve dirt and soil. Then
a hydrophilic action causes water to sheet on the glass, so a light
rain rinses away the loose dirt, and windows dry with virtually no
spots or streaks. In dry weather, a quick squirt with a hose will have
windows looking like new again.
Click
Here to see a diagram of the Pilkington Self Cleaning process.
Pilkington Activ™ Self-Cleaning Glass is environmentally friendly
in its application, installation, and use, requiring less cleaning and
no chemical cleaners or detergents. Also, since it is an integral part
of the glass itself, it cannot peel, separate or disintegrate over time,
so it never needs re-treatment. For further information, be sure to
see Pilkington's answers to frequently
asked questions. |
