Understanding Glass

The type of glass you choose is perhaps the most important decision you make when purchasing windows. After all, about 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.

Energy Efficiency

Due to rising energy costs, single-pane windows are just about obsolete. Most windows are now built with double or triple-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 improved over time.


Glass Terminology


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.


A measure of heat transfer through a material. Lower numbers indicate less heat transfer (more insulation) with a typical double-pane window achieving U = 0.5. It has a relationship to the R-Value where U-Value = 1/R (approx).

Solar Heat Gain Coefficient (SHGC):

Measures how much heat from the sun is blocked. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the more a product is blocking solar heat gain. Blocking solar heat gain is particularly important during the summer cooling season in hot Southern climates. By contrast, people in Northern climates may want solar heat gain during the cold winter months to lessen the cost of heating the home. Learn more at:

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.


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.


With Aluminum Spacer

With Warm Edge Spacer

Warm Edge Spacer:

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%.

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.

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.

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

Intercept insulating glass units reduce condensation problems. (Cold side temperature = 0 degrees F, room side temperature = 72 degrees F and 25% relative humidity.)


Heat Mirror™ – Voted one of the "Top 100 Inventions of the Millennium"

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 Millennium" 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.


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• 3800 Series from $138 -- a well built budget window you can count on.

• Earthwise 4700e from $198 -- >20% better energy savings than even Energy Star requires.

• Triple Pane 4700t from $268 -- with 2 argon filled airspaces for energy savings and sound control.

• Alpen Fiberglass Windows -- the most energy saving windows on the planet!

Triple-Pane Glass: Is it Right for Your Project?

Triple-pane glass is manufactured much like double-pane, but with an additional airspace. Triple-pane with low E and argon or krypton gas can be extremely effective at reducing a building’s heating and cooling costs. It can also lead to a more comfortable indoor environment as it raises the temperature of a room's coldest surfaces during winter months.

But while triple-pane glass is typically 30% more energy saving than double-pane (our Earthwise 4700t model is 45% more energy saving!), window product with triple-pane can be significantly more expensive than double-pane versions. In order to evaluate whether triple-pane glass is right for your project, you’ll need to know just how much energy you are losing through your windows, and calculate and compare the energy savings you should expect with new double or triple-pane models.

FOR EXAMPLE: Let’s assume your home’s heating and cooling cost is $2,400 each year with its original double-pane windows (U-value = 0.49) and you are considering either a new high efficiency double-pane window (U-value = 0.30) or a triple-pane model (U-value = 0.20). Let’s also assume 25% of your heating and cooling cost is due to your old inefficient windows-- $600/year. So then the new double-pane window you are considering should save you (0.49 - 0.30)/0.49 x $600 = $232.65/year, and the new triple-pane windows should save (0.49 - 0.20)/0.49 x $600 = $355.10/year. The energy saving benefit of the triple-pane windows over double-pane is $355.10 - $232.65 = $122.45/year.

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, krypton or xenon gas injected between the panes to increase their energy efficiency. The spacer bar has 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=0.11 (about 10X better than single-pane).

NOTE: Many people 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. This air movement actually aids in the transfer of heat between the panes, reducing energy efficiency.

But is this worth it if the triple-pane windows cost $100 more for each window? Well, if your house has 9 windows, your payback period over the (new) double-pane model should be ($100 x 9)/$122.45 = 7.35 years. Will you be in the home that long? How much more might you be able to sell your home for with triple-pane vs. double-pane windows? You’ll have to decide. Also please note that this simple calculation does not take into consideration energy savings achieved through the better solar heat gain coefficients (SHGC-- see below) of the new windows, nor future increases in your utility costs.

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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