How does a school or university determine the energy performance of its window systems? The Efficient Windows Collaborative puts forth four methods for measuring how well a window conserves energy and provides illumination:

  • U-factor.

    The U-factor of a window assembly represents its insulating value. Heat can be lost or gained through the window frame and glazing through conduction, convection and radiation. The U-factor is a measure of the rate of non-solar heat loss or gain through a window assembly. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value. Typical U-factors range between 1.25 for single glazing in aluminum frames and as low as 0.15 for low-E coated triple glazing in insulated frames.

  • Solar Heat Gain Coefficient (SHGC).

    Heat can be gained through windows by direct or indirect solar radiation. The solar heat gain coefficient measures a window's ability to control heat gain. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading.

  • Visible Transmittance (VT).

    Visible transmittance indicates the portion of in­coming visible light transmitted through the window (also referred to as visible light transmittance). It is expressed as a number between 0 and 1; a window with a higher VT transmits more visible light. The collaborative says that VT affects energy by providing daylight, which can reduce electric lighting loads and result in lower cooling loads.

  • Air Leakage (AL).

    Heat loss and gain also occur by air leakage through cracks in a window assembly. Air leakage also may contribute to summer cooling loads by raising the interior humidity level. The air leakage rating is a measure of the rate of air-leakage around a window or skylight in the presence of a specific pressure difference. The lower a window's air-leakage rating, the more airtight it is.

The collaborative states that the window performance measurements should account for the entire window assembly, not just the center of glass.

Read the main story, "Window Systems," to learn more.