The adjacent images show interior surface temperature patterns of a clear double glazed unit (left) and an energy-efficient Low-E insulated glazing unit with an improved spacer (right).

Under typical winter conditions, (i.e. 20°F outside), condensation on the glass under typical humidity levels is shown by purple and blue. With a conventional clear double glazing (left), condensation occurs in a band a couple inches wide along the edge of the sightline, with more condensation along the bottom than at the top. With the energy-efficient Low-E insulated glass unit (right), condensation will be greatly reduced (a small strip less then 1" high along the bottom).

Under extreme winter conditions (i.e. 0°F outside), condensation is shown by purple, blue and green. With clear double glazing, there is condensation over the entire unit. With energy-efficient Low-E glazing, there is only condensation on a band along the bottom and up along the edges.

Source: Lawrence Berkeley National Laboratory.

The graph below shows condensation potential on the center of glass area (the area at least 2.5" from the frame/glass edge) at various outdoor temperature and indoor relative humidity conditions. Condensation can occur at any points that fall on or above the curves. As the U-factor of windows improve, there is a much smaller range of conditions where condensation will occur.

Source: Lawrence Berkeley National Laboratory. These values are based on center-of-glass temperatures. Condensation may occur at lower humidity levels on the glass edge.