ENERGY GLOSSARY

To help you better understand the research and data presented in this section, we've assembled a few terms that you'll need to be familiar with. These terms are listed in alphabetical order:

• Air Leakage Rating
  a measure of the rate of infiltration around a window or skylight in the presence of a strong wind. It is expressed in units of cubic feet per minute per square foot (cfm/ft²) of window area or cubic feet per minute per foot (cfm/ft) of window perimeter length. The lower a window's air leakage rating, the better is its airtightness.
   

• Conduction
  The flow of heat through a solid material, such as glass or wood, and from one material to another in an assembly, such as a window, through direct contact.
   

• Convection
  The flow of heat through a circulating gas or liquid, such as the air in a room or the air or gas between windowpanes.
   

• Fenestration
  A window or skylight and its associated interior or exterior elements, such as shades or blinds… check with ASHRAE Handbook for definition.
   

• Gas Fill
  A gas other than air placed between window or skylight glazing panes to reduce the U-factor by suppressing conduction and convection.
   

• Glazing
  The glass or plastic panes in a window or skylight.
   

• Infiltration
  The inadvertent flow of air into a building through breaks in the exterior surfaces of the building. It can occur through joints and cracks around window and skylight frames, sash, and glazings.
   

• Low-Emittance (Low-E) Coating
  Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow through the window or skylight.
   

• Radiation
  The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation, and a person's body can lose heat to a cold window or skylight surface in a similar way.
  

• R-Value
  A measure of the resistance of a material or assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-ft2-°F/Btu. A high window R-value, has a greater resistance to heat flow and a higher insulating value.
   

• Shading Coefficient (SC)
  A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch clear, double-strength, single glass. It is equal to the Solar Heat Gain Coefficient multiplied by 1.15 and is expressed as a number without units between 0 and 1. The lower a window's Shading Coefficient, the less solar heat it transmits, and the greater is its shading ability.
   

• Solar Heat Gain Coefficient (SHGC)
  The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The Solar Heat Gain Coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number without units between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater is its shading ability.
   

• Spectrally Selective Glazing
  A specially engineered low-e coated or tinted glazing that blocks out much of the sun's heat while transmitting substantial daylight.
   

• U-Factor (or U-Value):
  A measure of the rate of heat flow through a material or assembly. It is expressed in units of Btu/hr-ft2-°F or W/m2-°C. Window manufacturers and engineers commonly use the U-factor to describe the rate of non-solar heat loss or gain through a window or skylight. Lower window U-factors have greater resistance to heat flow and better insulating value.
   

• Visible Transmittance
  The percentage or fraction of visible light transmitted by a window or skylight.

Note: This section is information published by The Department of Energy - it does not imply an endorsement to The Window Place.