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Energy Savings - Questions & Answers

Control your environment—control your costs

When it comes to choosing high performance insulated windows, thermal resistance in the spacer system plays an extremely important role in energy savings. But to understand how windows affect heating and cooling costs, you need to know a little about how energy flows through them. The primary ways heat is transferred through windows are:

  • Sunlight (solar radiation):
    An important source of heat that consists of visible light and a part of the solar spectrum that is heat, but not visible light (or infrared heat radiation). Solar heat gain coefficient (SHGC) is the measure of the amount of energy that passes through the window. The higher the SHGC, the greater percentage of solar energy that is transmitted to the inside.
  • Radiant Heat:
    Radiant heat is a type of heat loss given off by warmer objects to cooler objects. Things warmed by sunlight become stronger sources of radiant heat, and radiant heat is blocked by most window glazings. Sunlight passes through glazings, warming objects indoors, but the heat from those objects does not quickly escape back through the glazings. A good example of the radiant heat dynamic is a greenhouse.
  • Conduction:
    Conduction is the transfer of the heat through physical contact. Heat conducts from the warmer to the cooler side of the window. Conduction occurs not only through solid materials (spacers, glass, and even window frames), but also through the air space between the layers of glass. The amount of heat transmitted through a material due to a temperature difference is given by its U-value. The smaller the U-value the less heat that is transmitted. Special gases, such as Argon, injected between the layers of glass conduct heat less readily and therefore, result in lowering U-values.
  • Convection:
    Refers to the transfer of heat as molecules of air are physically moved from one place to another. A warm glass surface heats the air next to it, causing the air to rise. A cold glass surface is warmed by the air next to it, and that air mass will fall as it gives up heat. These convection currents occur on the inside of a window, on the outside, and between layers of glass.
  • Infiltration:
    The process that carries heat through cracks and gaps around the window frames. Infiltration can carry cold air into a house and carry warm air out. It is driven by wind and other differences in air pressure, such as warm air rising inside a house.