Rating Window Efficiency Systems
When looking at different models of windows you want to be aware of the rating that the manufacturer applies. That rating will either be a "U Value" which gives an indication of that windows tendency to transfer heat either in or out, or by an "R Value" which describes the windows resistance to transferring heat in or out.
U-Values are a measure of heat conductance
R-Values are a measure of heat flow resistance
If a window's R-value is high, it will lose less heat than one with a lower R-value.
If a window's U-value is low, it will lose less heat than one with a higher U-value. NOTE: Most window manufacturers use R-values in rating their windows.
Usually, window R-values range from 0.9 to 3.0
U-values range from 1.1 to 0.3).
When comparing different windows, you should ensure that all U- or R-values listed by manufacturers:
1. Are based on current standards.
2. Are calculated for the entire window, including the frame, and not just for the center of the glass.
3. Represent the same size and style of window.
5 factors affecting the R-value of windows
- The type of glazing material (e.g., glass, plastic, treated glass)
- The number of layers of glass
- The size of the air space between the layers of glass
- The thermal resistance of conductance of the frame and spacer materials
- The "tightness" of the installation.
ENERGY STAR qualified windows, doors, and skylights save you energy and money, increase the comfort of your home, and protect your valuable possessions from sun damage. They are also better for the environment because lowering your energy use means less air pollution from power plants.
NFRC is a non-profit organization that administers the only uniform, independent rating and labeling system for the energy performance of windows, doors, skylights, and attachment products.