Energy Efficient Windows

According to ENERGYSTAR, windows typically comprise 10–25% of a home’s exterior wall area, and account for 25–50% of the heating and cooling needs, depending on the climate. So it is critical to consider high-performance, energy efficient windows when constructing a new home. Many improvements have been made in recent years advancing the insulating quality of windows, such as:

• Improved Window Materials: Advances in window technology such as double glazing and low-e coatings substantially reduce heat loss in cooler climates and heat gains in warmer climates. Look for ENERGY STAR or National Fenestration Rating Council (NFRC) labels to be sure you are getting high-efficiency windows.
• Improved Framing Materials: Low conductance materials, such as wood, vinyl, and fiberglass perform better than aluminum. Look for “thermal breaks” where aluminum frames are used in heating-dominated climates to avoid condensation. Insulated frames, including insulating spacers between glazings, also perform better than uninsulated frames.
• Air Tightness: High-performance or advanced windows need to be sealed around framing and other gaps that may exist. Caulks, foams, and weather-stripping work well to keep drafts out.

High-performance windows contribute to:

• Quieter home interior — multiple panes and insulated frames block outside noise.
• Reduced fading of curtains, furniture, and flooring — low-emissivity (solar window) coatings can block up to 98% of UV rays.
• Reduced utility bills — houses lose less heat in winter and absorb less heat in summer.
• Improved quality windows are made from better-quality materials easier to operate and carry extended warranties.

Low-Emissivity & Solar Heat Gain Co-Efficients (SHGC)

Not all low-e coatings are the same when it comes to associated solar heat gain coefficients (SHGC). Pyrolitic or hard-coat low-e coatings typically have much higher SHGC, so it’s worth shopping around for higher SHGC for cold climates. Low emissivity refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect and emit radiant energy.

Solar heat gain can provide free heat in the winter but can also lead to overheating in the summer. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. In Oregon and Washington, look for a high SHGC (0.30-0.60) so that winter solar heat gains can offset a portion of the heating energy need. If cooling is a significant concern and no shading is available, select windows with a SHGC less than 0.40. Select skylights with a SHGC of 0.40 or less.

Performance Standards

The National Green Building Standard provides recognition for sustainable and energy-saving building practices, including the use of energy-efficient windows, in all types of residential construction. This standard has been developed by a consensus committee assembled by the National Association of Home Builders (NAHB) and the International Energy Code Council (IECC).

Quality Window Installation

Quite simply, windows are only as good as their installation. Proper installation is necessary for optimal window performance, to ensure an airtight fit and avoid water leakage. Proper installation will:

• Protect from water damage. Windows should form a continuous water barrier where they meet the wall. With improper installation, water may penetrate and cause damage—often unseen— in the wall.
• Limit air leakage. Windows must complement the wall’s air barrier. Even tiny cracks around the window frame can lead to substantial heat loss unless properly sealed.
• Prevent condensation. Windows must complement the wall’s vapor barrier to prevent water vapor from passing around a window frame. If vapor condenses on cold surfaces between the frame and the wall, rot or other damage may follow.