Why

Windows are the primary weak spot for heat loss in any building. In the summer they can let in unwanted heat, and in the winter they can account for as much as 25% of heat leakage.

Dramatic improvements in window technology during the past 30 years are largely responsible for our ability to create buildings with superior energy performance. These technologies are widely available and relatively inexpensive.

Recommendation

All windows should be high performance windows. These score 0.4 or less in both of the main ratings of energy efficiency: thermal transference (called the U-factor) and Solar Heat Gain Coefficient (SHGC). This recommendation applies to any project with windows.

Exception: Passive solar buildings are designed to make use of the solar heat that enters the space at certain times of day or season, so south-facing windows should not have low-e glazing and may require a higher SHGC.

Look for This Label

This label assures you that the window has been independently rated using a procedure established by the National Fenestration Rating Council (NFRC). Although a window may also have other labels, the NFRC label is the best source of energy performance data. The label is useful for comparing energy loss between window types, frame materials, glass coatings, brands and styles.

Low U-Factor/Low Solar Heat
Gain: How They Do It

For years the standard in Title 24 has been dual-pane windows, which have two pieces of glass with an air space in between them for insulation. While dual-pane windows insulate almost twice as well as single pane, this is just the beginning in terms of potential energy efficiency.

High performance windows include features such as low-emissivity ("low-e") coatings, tinting where applicable and nonmetal (or low conductivity) frames.

Low-e coatings: Low-e coatings, which are virtually invisible, are installed inside the air space of a double-paned window and block the sun's heat energy from entering a building. These types of coatings are also called "spectrally selective" because they block specific wavelengths of light (heat) while allowing others (visible light) to pass through. They can also reduce the loss of heated indoor air.

Tinted glass: Tinted glass, which is common in commercial buildings, blocks heat like low-e coatings, but does not admit as much visible light.

Frame materials: The metal that frames aluminum-framed windows acts as a thermal bridge between the inside and outside air, allowing valuable heat to escape. Wood, vinyl, composite and fiberglass all perform better than aluminum. If aluminum frames are selected, then a product with thermal breaks between the interior and exterior panes of glass is strongly recommended.

Note: Vinyl windows present some concerns for those looking to truly build "green." Lifecycle costs, durability, and the nature of the component materials should be researched by those pursuing a green building.

Window Placement

Window placement greatly affects energy consumption and comfort. South-facing windows admit desired light and heat in the winter, but transmit excessive heat in the summer unless they are properly shaded. Windows that face south can be effectively shaded by overhangs. The ideal overhang is one that provides shade during the hot months but allows direct solar gains during cold months. Deciduous trees (those which lose their leaves in winter) can provide these qualities naturally.

Shading is much more difficult on the east and west sides of the house. When the sun strikes these façades it is fairly low in the sky, making overhangs ineffective. The best approach is to minimize windows that face east and west.

Cost and Cost Effectiveness

Windows are sold in a wide variety of styles, materials, features and costs. While high performance windows do cost more, their added energy features represent only a small part of their total cost. Enhanced energy features are available in lower-cost windows found in home improvement centers as well as in costlier windows used in custom homes.

Depending on their specific energy features, high performance windows typically cost $1-$3 per square foot more than conventional dual-pane windows. This extra cost can be recouped two ways:

Energy use: Efficient windows reduce the amount of energy needed each month for heating and cooling. Depending on building design, local climate, amount of building glass area, etc., these savings can often repay the extra cost of high-performance windows within five years.

System costs: Since better windows can significantly reduce both heating and cooling loads, it may be possible to install smaller and less costly furnaces and air conditioners. The reduction in summer heat gain can eliminate the need for air conditioning.

Code Considerations

Title 24 assumes a "worst case" U-factor of 0.79 and SHGC of 0.65 for dual-pane windows, although certain compliance approaches require better performance numbers. Additionally, the code specifies a limit to the amount of glass (20% of the total floor area) unless other performance options are incorporated. Windows with low U-factors and SHGC can often allow for the use of more glass area.