I've done nearly 400 blog postings in the last 2 years, demonstrating how elementary school math can be used in real life. We've examined several kinds of labels and what they communicate. For example, Fuel Economy and Food labels. Today we will look at the kind of numbers used on labels you might find on windows and skylights.
Here's a typical Energy-Star label. Energy-Star is a government program encouraging efficient windows in residential low-rise dwellings. The window ratings are completely different in commercial and high-rise buildings.
When we say window, we're talking about an object placed into an opening in a wall (window) or ceiling (skylight).
An object made of glass (or other similar transparent substance) framed with plastic, wood or metal and often capable of opening or closing to let light and air in/out of the room.
There are a number of unique units of measure used with windows:
U-Factor: A lower number is better
U-factor measures how well a window prevents heat from escaping from inside the house. The rate of heat loss is indicated by the U-factor (or U-value) of a window assembly. U-Factor ratings generally fall between 0.20 and 1.20. The lower the number, the greater a window's resistance to heat flow and the better its insulation. In cold climates, a low U-Factor is better. To get a tax credit, windows must rate .30 or lower.
Solar Heat Gain Coefficient: A lower or higher number might be better
Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat from direct sunlight. The SHGC is the percentage of solar radiation entering via the window (both transmitted and absorbed). SHGC is a number between 0 and 1 - a lower number transmits less heat into the house. In warm climates, less heat is better. In cold climates, more heat is better. To get a tax credit, windows must rate .30 or lower.
Visible Transmittance: A higher number is better
Visible Transmittance (VT) indicates how much light comes through a window. VT is a number between 0 and 1 - the higher the VT, the more light is transmitted through the glass. In most cases, windows are installed to provide light, so a higher number is better.
Air Leakage: A lower number is better
Air Leakage (AL) means the volume of air, in cubic feet, that can leak through a square foot of window area (cfm/sq ft). Heat loss and gain occur by these leaks around the window assembly. The lower the AL, the less air will pass through the window assembly when it is closed. An ideal window is rated .30 or lower. This rating is optional.
Condensation Resistance: A higher number is better
Condensation Resistance (CR) measures the ability of a window to resist the formation of condensation on the interior surface of the glass. The higher the CR number, the better the window is at resisting condensation formation. This rating is a number between 0 and 100, and provides a way to compare the likelihood of moisture condensing on the glass. This rating is optional.
Noise Reduction / Sound Attenuation
Reducing noise entering a home through windows can be as important as reducing the heat that comes in or goes out. Sound Transmission Class (STC) is one way to represent sound transmission. This frequently-used rating is based on reducing sound frequencies found indoors, such as voices or appliances. Outdoor-Indoor Transmission Class (OITC) is another scale that measures stronger, lower frequency sounds (traffic, trains, aircraft). OITC ratings are usually lower than STC ratings because these sounds are harder to block. The higher the number, the more noise is blocked. This rating does not appear on energy labels.
For more information on how your location may require different types of windows, check here. We didn't need to worry about energy ratings on our windows, as they were selected to block airplane noise, and not primarily for climate reasons - we're already in the perfect climate!