Window Basics

Do words like Low-E, argon, R-values and shading coefficient confuse you? Do they sound like a foreign language you will never understand? We can help.

Do words like Low-E, argon, R-values and shading coefficient confuse you?Do they sound like a foreign language you will never understand? Then prepare yourself, you have just entered the fascinating but confounding world of windows! 

But before you decide that the subject is too difficult and you don't really need to replace your drafty old windows after all, we have simplified the entire matter for you.

The first item you need to consider when looking at windows is where you live.

If You Live In the South... 

Heating issues probably are not your primary concern--air conditioning loads are. The term you need to learn is shading coefficient, or the measure of heat gained, which is what drives air conditioning loads. In this case, the lower the number, the better it is.

Most homeowners want a lot of light in their home, which is the reason behind the daylighting trend. A clear, insulating glass unit transmits 80 percent visible light. An insulating glass unit with a Low-E pane of glass will admit at least 73 percent visible light. The difference between these two figures is not discernible to the human eye. Through new window technology, window manufacturers have been able to merge homeowners' desires to have less heat conduction and more light.

There are three versions of the sun's energy that are factors in window technology. They are:

  • UV light, which fades fabrics and discolors pigments in the skin
  • Visible light, which is what the eye sees
  • Infrared, short wave warmth

Visible light is desired (although it can cause some levels of fading). The infrared energy can be fought with shading coefficient. The problem is the UV light.

The best way to combat the harmful effects of UV light is to reduce the UV light transmitted. The only way to do this in the past was through tinted glass, which diminished the visible light, leaving the room dark or tinted with color. However, recent window technology has developed a new version of Low-E glass which combines improved R-value, lower shading coefficient and low UV light transmittance. These Low-E glasses transmit at least 73 percent of the visible light, nearly the same amount as a clear, insulating glass unit.

You must realize that not one type of glass or window is going to meet all your needs. Work with a professional to choose the correct window for your home, your budget and your region of the country.

There is one additional type of glass that is frequently required by building codes for specific applications in the home--safety glass. Most building codes require this type of glass in and around exterior doors and walkways. Generally there are two types of safety glass available today: tempered glass and laminated glass.

Tempered glass is a piece of glass that has been thermally strengthened so that it is nearly four times stronger than a regular piece of glass. This is often the safety glass of choice because of its cost-effectiveness. The second type of safety glass offered is laminated glass.

Laminated glass is the same type of glass that is found in your car's windshield. A piece of laminate (PVB) is placed between two panes of glass to increase the safety factor. The laminate effectively glues the two pieces of glass together, while remaining invisible to the eye. Should the glass break, the laminate prevents it from falling out of the opening. The benefits of laminated glass include sound control and safety. It also transmits a minimum of UV light. However, this type of glass is expensive and offers minimal energy efficiency.

Your best bet is to consult your professional remodeler for further information about windows, window technology and energy efficiency. If you have questions which your remodeler cannot answer, call the manufacturer for more details.

If you live in the North...

You need to be concerned about heat loss and will want to find the best insulation you can in a window so you can retain the most heat.

Look for a high R-value. In the home, windows have long been called the "bad actor" of energy conservation because they are not as efficient as a wall's insulation. However, through technological advances, there are options available to raise the energy efficiency of windows.

  • The primary way is through a Low-E glass coating. The technical name of Low-E glass is actually Low-Emissivity, High-Transmittance, which means that Low-E reduces the amount of heat loss while still maintaining a high level of visible light coming through the glass. Low-E glass is one of the factors that determines the R-value, or insulation quality, of the glass.
  • A standard window with a single pane of clear glass will have an R-value of approximately one, while an insulating window with Low-E coated glass can range from R4 to R8 or more, depending on the window construction. In the winter, Low-E glass will reduce heat loss, while in the summer it will reflect the re-radiated long wave energy from asphalt or other similar objects outside, thus keeping the house cooler.
  • Another method of increasing the R-value is through the use of inert gasses. Argon and other inert gasses also will reduce the loss of heat through an insulating glass unit. Since inert gasses are heavier than air, their molecules do not move as easily. This makes the space between the panes of glass in the window less conductive. Inert gasses between the two panes of glass make it more difficult for the warmer inside air to pass through the glass to the colder outside. With lower heat loss, the R-value is increased.
  • The latest method of increasing the R-value is through warm edge technology. It deals with the edge condition of a window. The spacer material to which the panes of glass are glued creates the edge condition. A conventional aluminum spacer is extremely conductive. In other words, it transfers heat through the aluminum spacers much the same way heat from a fire would travel through a steel rod that has one end placed in the fire. If you held onto that rod with your bare hand, you eventually would burn yourself. Similarly, if heat is moving between double panes of glass in a window that is edged with aluminum, the heat eventually will pass through the aluminum to the outside edge and disappear into the environment--i.e., heat loss.

    It is a natural tendency for heat to move toward cold and for heat molecules to rise. The warm air inside a home will naturally make its way to the outside cooler temperatures. Warm edge spacer materials decrease the window's tendency to transfer that heat from the inside to the outside because it lessens the conduction at the window's edge. The less heat lost to the exterior of the home, the warmer the home is inside--i.e., the lower the energy bills.

Source: National Association for the Remodeling Industry (NARI)

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