UNITS / TERMS:
U-factor (U-value) measures the rate of heat loss, or how well a product prevents heat from escaping. U-factor ratings generally fall between 0.20 and 1.20. The lower the U-factor, the greater a product's resistance to heat flow and the better its insulating value.
R-value is the insulating value, which is the inverse of the U-factor. Window R-values usually range from 0.9 to 3.0, with the exception of some highly energy-efficient windows.
Low-E (low-emittance) coating can be applied to glazing surfaces to reduce heat loss, improving both heating and cooling performance.
Solar heat gain coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is expressed as a number between 0 and 1. The lower the SHGC, the less solar heat it transmits.
Spectrally selective Low-E coating can be applied to glazing to reduce SHGC. This type of Low-E coating can reduce heat loss in the winter as well as solar gain in the summer. Technologies such as these widen the range of attractive options for constructive use of solar energy for heating and cooling applications.
U.S. Department of Energy's Energy Efficiency and Renewable Energy Clearinghouse: Energy-Efficient Windows
Visible transmittance (VT) measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
Air leakage (AL) is the heat loss and gain by infiltration through cracks in the window assembly. Air leakage is expressed as the equivalent cubic foot of air passing through a square foot of the product's area. The lower the AL, the less air infiltration.
Condensation resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product resists condensation formation. CR is expressed as a number between 0 and 100.
U.S. Department of Energy's Building Technologies Program: Windows, Doors, and Skylights
National Fenestration Rating Council (NFRC): Window Energy Performance Label
National Fenestration Rating Council (NFRC): About
BACKGROUND FACTS:
Windows can be a major factor in heat loss during the winter and heat gain during the summer. Energy used to offset heat losses and gains through windows in homes and buildings across the U.S. accounts for approximately one-fourth of all energy used for space heating and cooling.
Window frames come in a variety of materials including aluminum, vinyl, fiberglass, and wood. Each of these materials has advantages and disadvantages. For example, while aluminum frames are strong and ideal for customization, they are also prone to heat loss and condensation. Most energy-efficient windows have wood or vinyl frames that do not transmit heat as readily as other materials.
If a window has a high R-value (high resistance to heat-flow), it will lose less heat than a window with a lower R-value. There are a variety of factors that impact the R-value of a window. These factors include glazing material (e.g., glass, plastic, treated glass), the number of layers of glass, the size of the air space between layers of glass, the thermal resistance or conductance of the frame and spacer materials, and the "tightness" of the installation (i.e. prevention of air leaks).
Energy Star performance requirements for windows are tailored to fit the varied energy needs of the northern, southern, and central states' climate regions in the United States. The Energy Star label identifies that a window qualifies for Energy Star status and indicates in which region(s) the window qualifies.
Energy Star: Windows, Doors, and Skylights Key Product Criteria
Efficient Windows Collaborative: Energy & Cost Savings
When purchasing a new home, homeowners want their home to be correct in every detail while costing as little as possible. However, savings to a homeowner can also occur even after the home is purchased, by using energy-efficient components as the home is being constructed.
One example involves a homeowner's choice of energy-efficient windows which have a tremendous impact on energy performance (and costs!) of heating and cooling a home, as well as the seasonal comfort level experienced by the home's occupants. While energy-efficient windows will cost more initially, they often return their additional cost investment through substantial savings to the homeowner on monthly energy bills.
Installation of energy-efficient windows is just one area where homeowners can decrease their energy usage, save money, and help the environment all at the same time. Other areas range from having the correct (cost-effective) level of insulation in a home's walls, ceilings, and floors to simple, periodic maintenance of the furnace and air-conditioning system.
Investing in energy-efficient options provides a continued payback not only in dollars and cents, but also in a more enjoyable and comfortable living environment for your family for many years.
Windows provide less resistance to heat flow than walls, ceilings, and floors of your home. Even when windows comprise a small area of a home, they are the area of greatest heat loss and gain, and air leakage. Windows can account for as much as 25-30% of the heat loss in a home. This increases energy use and costs, and decreases your comfort.
The performance of windows, walls, ceilings, and other building components determine the monthly energy cost as well as the required size of your heating and cooling equipment. The installation of energy- efficient windows (as well as other aspects of a home's construction) reduces not only your monthly energy use (and costs) but also means that a smaller, less expensive furnace and air-conditioning system will be required.
Therefore, while energy-efficient windows will cost more initially, the monthly savings on your energy bills coupled with a reduction in the purchase price of the heating and cooling system, can more than offset the higher initial cost.
