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Building Materials and Insulation | Windows | Roofing Materials

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Windows

Windows bring light into buildings and give a feeling of openness to rooms. However, they can also be major sources of heat loss in the winter and heat gain in the summer.

In 1997, the energy used to offset heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion. However, when properly selected and installed, windows can help minimize a home's heating, cooling and lighting costs.

There are several types of energy efficient windows that are available for reducing a home's heating and cooling energy requirements.

The energy efficiency of windows is expressed in terms of R-value or U-value. The greater the R-value, the greater the efficiency of the windows. Conversely the greater the U-value the less the efficiency of the windows. The U-value is the reciprocal (opposite) of the R-value.

There are three main factors that effect the R-value of a window:

1. The type of glazing material. E.g. glass, low-e glass, etc.
2. The number of layers of glass.
3. The type of gas between the layers of glass.

Types of glazing materials:

1. Low emissivity (low e) glass has a special surface coating to reduce heat movement back through the window. These coatings reflect from 40% to 70% of the heat that is normally transmitted through clear glass, while allowing the full amount of light to pass through.
2. Heat absorbing glass contains special tints that allow it to absorb as much as 45% of the incoming solar energy.
3. Reflective glass has been coated with a reflective film and is useful in controlling heat gain in the summer.

Layers of glass and air spaces:

1. Single pane glass has very little insulating value.
2. Double and Triple pane windows have air or gas filled spaces between each pane. Each layer of glass helps resist heat flow. Advanced multipane windows are now manufactured with inert gases such as argon and krypton. These gases transfer less heat than air does. Although multipane windows tend to be more expensive than single pane windows, the annual energy savings outweigh the initial cost.