Energy-efficient windows, doors, and skylights—also known as fenestration—can help lower a home's heating, cooling, and lighting costs.

• Windows- Windows provide our homes with light, warmth, and ventilation, but they can also negatively impact a home's energy efficiency. You can reduce energy costs by installing energy-efficient windows in your home. Some energy efficiency improvements to existing windows can also help.
• Exterior Doors- An exterior door can contribute significantly to air leakage in a home—as well as some heat transfer—if it's old, not properly installed, and/or not properly air sealed. This can result in energy losses.
• Skylights- A skylight can provide your home with daylighting and warmth. When properly selected and installed, an energy-efficient skylight can help minimize your heating, cooling, and lighting costs.

Improving the Energy Efficiency of Existing Windows

You can improve the energy efficiency of existing windows by doing the following:

• Adding storm windows- Reduce air leakage and some heat transfer.
• Caulking and weather stripping- Reduce air leakage around windows
• Using window treatments or coverings- Reduce heat loss and/or gain

However, if your home has very old and/or inefficient windows, it might be more cost effective to replace them than to make these improvements. New, energy-efficient windows eventually pay for themselves through lower heating and cooling costs, and sometimes even lighting costs.

News Highlights

President Signs Green Building Tax Deduction Into Law

On October 3, 2008, President Bush signed into law H.R. 1424 and extended the Energy Efficient Commercial Building Tax Deduction as part of the Emergency Economic Stabilization Act of 2008.

Ensuring Success in Green Building

Steps to Ensure Success

• Establish a vision that embraces sustainable principles and an integrated design approach.
• Develop a clear statement of the project’s vision, goals, design criteria, and priorities.
• Develop a project budget that covers green building measures. Allocate contingencies for additional research and analysis of specific options.
• Seek sponsorship or grant opportunities.
• Seek advice of a design professional with green building experience.
• Select a design and construction team that is committed to the project vision. Modify the RFQ/RFP selection process to ensure the contractors have appropriate qualifications to identify, select, and implement an integrated system of green building measures.
• Develop a project schedule that allows for systems testing and commissioning.
• Develop contract plans and specifications to ensure that the building design is at a suitable level of building performance.
• Create effective incentives and oversight

Energy Performance Ratings for Windows, Doors, and Skylights

You can use the energy performance ratings of windows, doors, and skylights to tell you their potential for gaining and losing heat, as well as transmitting sunlight into your home.

Heat Gain and Loss

Windows, doors, skylights can gain and lose heat in the following ways:

• Direct conduction through the glass or glazing, frame, and/or door
• The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
• Air leakage through and around them

These properties can be measured and rated according to the following energy performance characteristics:

• U-factor- The rate at which a window, door, or skylight conducts non-solar heat flow. It's usually expressed in units of Btu/hr-ft2-ºF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. But National Fenestration Rating Council U-factor ratings represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.
• Solar heat gain coefficient (SHGC)- A fraction of solar radiation admitted through a window, door, or skylight—either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat gain during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gained from the sun. Therefore, what SHGC you need for a window, door, or skylight should be determined by such factors as your climate, orientation, and external shading.
• Air leakage- The rate of air infiltration around a window, door, or skylight in the presence of a specific pressure difference across it. It's expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.

Sunlight Transmittance

A window's, door's, or skylight's ability to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:

• Visible transmittance (VT)- A fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through a window's, door's, or skylight's glazing. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home's daylighting requirements and/or whether you need to reduce interior glare in a space.
• Light-to-solar gain (LSG)- The ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn't always provided.

Energy Performance Testing, Certification and Labeling

The National Fenestration Rating Council (NFRC) operates a voluntary program that tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings. The NFRC label provides a reliable way to determine a window's energy properties and to compare products.

The NFRC label can be found on all ENERGY STAR® qualified window, door, and skylight products, but ENERGY STAR bases its qualification only on U-factor and SHGC ratings.

Passive Solar Window Design

Windows are an important element in passive solar home designs, which can reduce heating, cooling, and lighting needs in a house.

Passive solar design strategies vary by building location and regional climate. The basic techniques involving windows remain the same—select, orient, and size glass to control solar heat gain along with different glazings usually selected for different sides of the house (exposures or orientations). For most U.S. climates, you want to maximize solar heat gain in winter and minimize it in summer.

Heating-Dominated Climates

In heating-dominated climates, major glazing areas should generally face south to collect solar heat during the winter when the sun is low in the sky. In the summer, when the sun is high overhead, overhangs or other shading devices (e.g., awnings) prevent excessive heat gain.

To be effective, south-facing windows usually must have a solar heat gain coefficient (SHGC) of greater than 0.6 to maximize solar heat gain during the winter, a U-factor of 0.35 or less to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer.

Windows on east-, west-, and north-facing walls are reduced in heating climates, while still allowing for adequate daylight. East- and west-facing windows are limited because it is difficult to effectively control the heat and penetrating rays of the sun when it is low in the sky. These windows should have a low SHGC and/or be shaded. North-facing windows collect little solar heat, so they are used just to provide useful lighting.

Low-emissivity window glazing can help control solar heat gain and loss in heating climates.

Cooling-Dominated Climates

In cooling climates, particularly effective strategies include preferential use of north-facing windows and generously shaded south-facing windows. Windows with low SHGCs are more effective at reducing cooling loads. The following types of glazing help reduce solar heat gain, lowering a window's SHGC:

• Low-E
• Tinted
• Reflective
• Spectrally Selective.

Most of these glazing types, except for spectrally selective, also help lower a window's VT.