Many people don’t realize they have a variety of options when it comes to installing windows in their homes. Windows are a major source of a house’s heat loss and heat gain- the latter of which is a considerable issue here in Houston. Outside air and solar heat can make way through the centers, edges and frames of windows. A national study conducted in 1990 revealed that the heat lost and gained through the windows of American buildings and homes accounted for one-fourth of all the energy used to heat and cool these structures. That means that 25 percent of the air conditioning and heating in this country was used to simply offset the heat lost and gained through windows.

Windows are a feature of any common building structure. Until recently, they were primarily made out of clear glass, which commonly has poor performance in its ability to resist heat flow. Performance and window efficiency have been improved significantly through the research and development of different types of glazing.

When discussing window efficiency it is important to understand how it is measured. A window’s energy efficiency is measured U-values and R-values. U-values are a measurement of heat conduction and R-values measure resistance to heat flow. R-values for windows typically range from 0.9 to 3.0 while U-values range from 1.1 to 0.3.

Window R-values are commonly affected by the following five factors:

• The type of glazing material
• The number of glass layers
• The size of the air space between the layers
• The thermal resistance or conductance of the frame and spacer materials
• The “tightness” of the installation (Ex: air leaks)

Energy Efficiency

There are some simple methods to help improve window efficiency in your home or workplace. One way is to control air leaks. A reduction in air leaks and heat flow can significantly improve the overall energy efficiency of your home. The most common and least expensive method of controlling air leaks is to use caulking or weather stripping.

Caulks are used to fill cracks and holes by using airtight mixtures. Before being able to apply caulk, old caulk and paint residue should be removed. This can easily be done by using a putty knife, stiff brush, or special solvents. New caulk should be applied at the point where the window frame meets the wall. The best time to apply caulk is during a time of warm temperature and low humidity in order to prevent cracks and ensure it settles properly.

Weather-stripping, on the other hand, seals an area through the use of a narrow piece of metal, felt, rubber, foam, or vinyl. Application should occur between the sash and window frame.

To help control heat loss or gain there are several types of advanced forms of glazing systems available. Advanced glazing types include:

• double- and triple-pane windows
• spectrally selective windows
• tinted or heat-absorbing windows
• reflective windows
• gas-filled windows
• and various combinations

Low-e Glazings

This form of glazing reduces heat transfer by using a special thin coating, in the form of an invisible metal oxide of a semiconductor film, placed on one or more glass surfaces. . Heat reduction between panes of glass is achieved by having the coating facing the air spaces within the window.

Low-e films may be applied as either hard or soft coats. Hard coatings are more long lasting, but receive slightly poorer performance than soft low-e coat films. Soft coat low-e films, on the other hand, may become degraded quite easily if exposed to air and moisture. In addition, they have a low shelf life and are easily damaged. Furthermore, they cost 10 to15 percent more than normal windows but they can reduce the loss of energy by as much as 30 to 50 percent.

Typically, low-e films are applied during a home’s construction; however, retrofitting films are also available. Films can be an inexpensive replacement compared to the cost of total window replacement. With the ability to last up to 15 years, they can help to reduce fabric fading and increase the overall comfort of a home.

Spectrally Selective

These coatings have the ability to filter out 40 to 70 percent of the heat that is transmitted through normal glass, while still allowing for the same amount of light to shine through. Spectrally selective coatings can be used to increase or decrease solar gains, allowing the homeowner to create a completely customized system.

Research has shown that by combining spectrally selective coatings with advanced glazings, new homes in hot climates could reduce their electric space cooling requirements more than 40 percent.

Rebates are now being offered by some utilities to encourage the use of this technology.

Heat-Absorbing Glazings

This form of technology combines the use of heat absorption glazing with tinted coating to increase the absorption of solar hear gain. Although heat is able to pass through the tinted window through conduction, inner layers and spectrally selective coatings can be added to minimize this transfer. Heat absorption glass does not have the same appearance of reflective glass and only allows for the reflection of a small percentage of light.

To reduce light and heat penetration even further, consider using blue- and green-tinted windows. These colors have proven to be better alternatives than using other common colors such as gray and bronze. In hot and desert climates, black tinting should be avoided completely because it will absorb more heat than light.

Reflective Coatings

The greatest advantage of reflective coating is that it can significantly reduce the transmission of daylight simply through clear glass. Reflective coating can be applied to clear or tinted glass, typically blocking more light than heat. These types of coatings are mostly commonly used in hot climates. Unfortunately the reductions in cooling may be offset by the need for additional electric lighting. D

ifferent types of glazings and combinations can be used to achieve various levels of light transmission. Decisions to use selected glazings should be based on the needs of the home, combined with a careful examination of heating and cooling needs based on factors such as shading, ventilation, solar heating, and daylight.

Smart Windows

New technologies called switchable glazing or “smart” windows can alter the light and heat transmittance and transparency of windows either automatically when prompted by environmental factors or manually with the touch of a switch. The Solar Energy Industries Association claims that “smart” windows can cut the energy costs of a building in half.

Electrochromic windows allow you to change the tint of your glass from fully transparent to opaque to completely dark, and everything in between. Electric conductors are sandwiched in between two pieces of traditional glass. When electricity is applied to the conductors, they alter the tint of the window. The voltage needed to operate the windows is quite low- just one to three volts- and the windows only use electricity to change their settings. They do not require continuous energy to maintain tints. An excellent feature- these windows can be programmed to absorb specified parts of the light spectrum. They can be set to block out solar heat or light without needing to obstruct or cover your windows to attain thermal and visual comfort. Imagine being able to keep your windows curtain and blind-free without having to worry about your furniture fading or your house heating up. And though it’s not recommended, on such occasions when you’re watching TV on a beautiful day- you would no longer have to sacrifice your view outdoors just to make out the pictures on the screen made imperceptible by the sun’s glare.

Electrochromic windows are just now being made commercially available and this presents some drawbacks. First of all, they are not widely available. Second, elctrochromic windows are considerably more expensive than other windows- costing as much as ten times more than low-e insulated glass. Third, installation of the windows is more complicated and costly than that of traditional windows because electrochromics require an electrical hookup which will most likely need to be installed by a licensed electrician. The initial costs of these windows are high, but manufacturers of electrochromic windows claim that within three years of installation, the money saved in electricity costs will more than pay for their additional expense.