High performance windows with new glazing technologies not only reduce energy costs but make homes more comfortable as well.
High-performance windows create warmer interior glass surfaces, reducing frost and condensation. High-performance windows with warm edge technology and insulating frames have such a warm interior surface that condensation on any interior surfaces is significantly reduced under all conditions. Condensation occurs when the interior surface temperature of the glass drops below either the dewpoint or frost point. A window's frame and/or glazing system can contribute to the possibility of condensation if they are poor performers for a specific climate. High-performance windows create warmer interior glass surfaces, reducing condensation and frost.
The interior humidity level can be a contributing factor, reducing interior humidity is an important component in controlling condensation. If necessary use vent fans, dehumidifiers, and/or air-to-air heat exchangers to help control a home's humidity levels.
The American Architectural Manufacturers Association (AAMA) has a good reference for understanding indoor condensation. This reference includes a chart for the recommended indoor relative humidity levels for your home based on outdoor air temperatures. It also provides tips to manage indoor humidity levels.
The set of images below show interior surface temperature patterns of a clear double glazed unit (left) and an low-E insulated glazing unit with an improved spacer (right). The cold regions on the clear double glazed unit, displaying the purple and blue colors, result from large amounts of heat flowing through the aluminum spacer. This cold region will also be the site of possible water condensation on the roomside surface of the glass. The low-E window is warmer because of the increase resistance to heat flow caused by the low-E coating on the inner surface of the glazing unit and from the insulating spacer. Under extreme winter conditions (i.e. 0°F outside), the possibility of 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 vertical edges.
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. These values are based on center-of-glass temperatures. Condensation may occur at lower humidity levels on the glass edge.
Under some climate conditions, condensation may occur on the exterior glass surface of a window. This is more likely to occur on higher-performance windows with low-E coatings or films, and low-conductance gas fills that create very low U-factors. By preventing heat from escaping from the interior, the exterior surfaces of the window approach outside air temperatures. These exterior temperatures may be below the exterior dew point causing condensation on the glass surfaces. This is most likely to happen when there is a clear night sky, still air, and high relative humidity, in addition to the right temperature conditions. Like other dew formed at night, exterior window condensation will disappear as surfaces are warmed by the sun. It is the excellent thermal performance of well-insulated glazing that creates the condition where the outer glass surface can be cold enough to cause condensation to form.