How To

Understanding Energy Ratings

The following is from U.S. Department of Energy ENERGY EFFICIENCY AND RENEWABLE ENERGY website (link available here)

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. For more information about SHGC and windows, see passive solar window design.
  • 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.

Window Treatments and Coverings

You can choose window treatments or coverings not only for decoration but also for saving energy. Some carefully selected window treatments can reduce heat loss in the winter and heat gain in the summer.

Window treatments, however, aren’t effective at reducing air leakage or infiltration. You need to caulk and weatherstrip around windows to reduce air leakage.

Window Blinds

Window blinds—vertical or horizontal slat-type—are more effective at reducing summer heat gain than winter heat loss.

Interior Blinds

Because of the numerous openings between the slats, it’s difficult to control heat loss through interior window blinds, but the slats offer flexibility in the summer. Unlike shades, you can adjust the slats to control light and ventilation. For example, when completely closed and lowered on a sunny window, highly reflective blinds can reduce heat gain by around 45%. They can also be adjusted to block and reflect direct sunlight onto a light-colored ceiling. A light-colored ceiling will diffuse the light without much heat or glare.

Exterior Blinds

Exterior roller blinds are usually made of wood, steel, aluminum, or vinyl. They’re mounted above the window. Side channels guide them as they’re lowered and raised. When you lower these blinds completely, their slats meet and provide shade. If partially raised, the blinds allow some air and daylight to enter through windows.

Window Shades

When properly installed, window shades can be one of the simplest and most effective window treatments for saving energy.

Shades should be mounted as close to the glass as possible with the sides of the shade held close to the wall to establish a sealed air space. You should lower shades on sunlit windows in the summer. Shades on the south side of a house should be raised in the winter during the day, then lowered during the night.

For greater efficiency, use dual shades—highly reflective (white) on one side and heat absorbing (dark) on the other side—that can be reversed with the seasons. The reflective side should always face the warmest side: outward during the cooling season and inward during the heating season. They need to be drawn all day to be effective.

Quilted roller shades, and some types of Roman shades, feature several layers of fiber batting and sealed edges. These shades act as both insulation and air barrier. They control air infiltration more effectively than other soft window treatments.

Pleated or Cellular Shades

Several manufacturers have designed two- or three-cell pleated or cellular shades with dead air spaces, which increase their insulating value. [ see R-values of Hunter Douglas Duette honeycomb shades here — link supplied by Hirshfield’s ]

Window Draperies

A drapery’s ability to reduce heat loss and gain depends on several factors, including fabric type (closed or open weave) and color. With such a wide variety of draperies available, it’s difficult to generalize about their energy performance.

In the summer during the day, you should close draperies on windows receiving direct sunlight to prevent heat gain. According to the University of Florida, medium-colored draperies with white-plastic backings have been found to reduce heat gains by 33%. Draperies also stay cooler in the summer than some other window treatments because their pleats and folds lose heat through convection.

When drawn during cold weather, most conventional draperies can reduce heat loss from a warm room up to 10%. Therefore, in winter, you should close all draperies at night, as well as draperies that don’t receive sunlight during the day.

To reduce heat exchange or convection, draperies should be hung as close to windows as possible. Also let them fall onto a windowsill or floor. For maximum effectiveness, you should install a cornice at the top of a drapery or place the drapery against the ceiling. Then seal the drapery at both sides and overlap it in the center. You can use Velcro or magnetic tape to attach drapes to the wall at the sides and bottom. If you do these things, you may reduce heat loss up to 25%.

Two draperies hung together will create a tighter air space than just one drapery. One advantage is that the room-side drapery will maintain around the same temperature as the interior space, adding to a room’s comfort.

Window Shutters

Window shutters—both interior and exterior—can help reduce heat gain and loss in your home.

Interior shutters need a clear space to the side of the window when they’re opened. They also require hardware that is fastened to the window jams or trim. Properly designed exterior shutters may provide the best possible window insulation system. They offer several advantages:

  • Weather protection
  • Added security
  • No use of interior space
  • No thermal shock to windows if left closed.

Exterior shutters must be integrated into your home’s architecture. Their mounting, drainage, and hinging will require special consideration; it’s easier to address these design issues in new construction.

Most exterior shutter systems include a mechanical crank, rod, or motor to allow operation from indoors. This can help encourage daily use of the shutters, and may be required by local fire codes.

Roll-down metal exterior shutters are often used as protection against storms and/or vandalism. While metal shutters provide protection against these hazards, they don’t provide much of a barrier against air infiltration and heat.

Like window blinds, louvered shutters work best for summer shading. Movable or fixed louvers allow ventilation and natural daylight to enter a room while blocking some direct radiation. However, they won’t provide much insulation against heat loss in the winter.

Solid shutters will decrease both heat loss and summer heat gain. These insulating shutters consist of wood panels, a vapor barrier, and sometimes a decorative covering. If you fit them tightly against a window frame, they’ll provide an insulating air space between the shutter and the window.

You can combine shutters with other window treatments, such as draperies, for greater insulating ability.