Buying Energy-Efficient Windows

With energy costs skyrocketing the new home building and renovation industries are finally on track to build and remodel homes for power savings including high-efficiency windows. This is in contrast to the old-style "McMansion" style of house that needed huge amounts of power to both heat the place in the winter and then air-condition it in the summer.

One of the past strategies for insulating against the cold is to provide a thermal rating of at least R-20 in the walls and R-40 in the attic. During the past few years the basic requirements for building new homes has pushed the thermal requirements much higher, depending on where in the country the home is being built. However, despite huge amounts of insulation in the attic and walls the R-values will not improve the overall energy-efficiency of a structure if the windows are barely an R-2.

Windows Suck Energy

For almost a century the popular belief was that two panes of glass were sufficient to insulate the home from the elements. In the colder areas of the country there was an autumn tradition of putting up storm windows over the single pane of the glass which was the sole barrier to the elements. Of course installing storm windows was better than having just one sheet of glass against temperatures that could go down as low as minus forty-degree temperature. But back then few homes even had insulation the walls and so only a few rooms were ever heated.

There are three reasons that two panes are not great stabilizers of interior temperature:

1. Glass has no thermal-break qualities: Glass panes conduct heat and cold almost as efficiently as metal which makes them poor insulators. Because one of the laws of thermodynamics dictates that heat is drawn to cold the interior heat is drawn to the cold glass surface and then conducted to the colder, outer pane and, finally, outside.

2. The dead airspace between the glass panes is a poor insulator: The interior area between the glass panes is actually its own weather system. In northern areas the cold outer pane and warmer inner pane causes the air in the space between them to circulate. This movement creates a convection current that speeds up the transference of warm air from the inside of the home to the exterior and, in warmer climates, brings heat inside the home.

3. Most spacers between the panes are metal: To hold the panes of glass at an equal distance apart the window manufacturers use spacers and most of these companies make these spacers from aluminum. So even if the sashes are made from foam-filled vinyl the heat can still be lost through the metal spacers.

To overcome these deficiencies windows manufacturers have made improvements in the design and fabrication of the windows.

1. Low Emissive Coatings: One of the best ways to make glass a better blocker of energy loss is to prevent the heat from penetrating the glass. Windows in sunny areas are coated with a protective, metallic coating to reflect the heat away from the home before it can get to the inner glass pane. New innovations in these coatings have allowed a transparent, reflective coating directly on the glass. In cold regions of the country this coating reflects the heat back into the home.

2. Gas-Filling: To prevent the connection currents from forming between the glass panes window manufacturers have begun to fill the air spaces with heavy gases such as argon, krypton and xenon. Originally, this space was filled with just air or dry nitrogen gas. However, in a sealed window unit, air currents between the two panes transfer heat to the top of the unit where it is cooled. This creates a continuous cycle as the air is heated and cooled. By using less conductive, heavy gas the convection currents within the space are minimized which means that the transfer of heat between the inside and outside is greatly reduced.

3. Plastic/Foam Glass Spacers: The old metal spacers failed on three accounts:

  • First, they allowed heat and cold transfer through conduction
  • Second, metal and glass do not expand and contract at the same rate. Therefore, after a few years of this unequal movement there were many instances of the seal failing and moisture getting between the panes.
  • Third, the pressure from wind activity would not be transferred equally and that can also cause seal failures.

4. Vinyl Window Frames

As much as wood and metal still have a place in high-end homes vinyl-framed windows are the most cost-effective way to purchase energy-efficient windows. This is because vinyl:

  • Provides a great thermal break
  • Holds the panes securely.
  • Can be custom-manufactured in hundreds of shapes and sizes
  • Is inexpensive

For more information on replacing your present windows consult our Contractor Directory or simply post your project online.

Posted by: kim
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