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 ThermopaneTM). 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).

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.

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.

Window 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.

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).

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.

Heat MirrorTM:
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.

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.

Pilkington ActivTM Self-Cleaning Glass:
Now there’s a revolutionary new kind of glass that actually uses the power of the sun to clean itself… Pilkington ActivTM 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.

Pilkington ActivTM 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.