Window condensation is usually a wintertime situation that occurs when outside temperatures are low. But condensation on windows can also happen during the summer. The condensation usually happens when replacement windows have recently been installed, or on a house with energy-efficient windows. The worry is that something must be wrong with the windows. In reality, the condensation is an indication that the windows are working as intended.
Winter condensation occurs on the inside of windows, while summer condensation occurs on the outside of windows. (Condensation forming between the panes of a double- or triple-pane window is a different situation.) Summer condensation problems are mostly visual, whereas winter condensation problems can be destructive. Since outside window and building surfaces often get wet from rain, a little extra liquid water will not be detrimental. The condensation disappears as outside air temperatures rise.
Energy-efficient windows and other window treatments reduce the amount of heat moving through a window system. Double- and triple-pane glass, low-E coatings, and inert gas fill help to reduce the flow of heat. This reduced heat flow results in cooler surfaces on the cold side of the window and warmer surfaces on the warm side of the window.
As the summer sun warms a window, a low-E coating or other energy-efficient treatment reduces the amount of heat that moves inward. (This is a good thing, and why people purchase energy-efficient windows). However, at night during the summer, heat is radiated from the outside glass to the cold outside air. The low-E coating reduces the heat transfer from inside, so the outside glass surface can cool significantly below outside air temperatures. Under the right combination of inside temperature, outside temperature, and outside humidity, windows with low-E coatings or other energy-efficient treatments can develop summer condensation, while inefficient windows do not.
The two ways to prevent condensation are to warm the surface and to dry the air in contact with the surface. Since the outside temperature and relative humidity cannot be controlled, the way to preventing summer window condensation problems is to warm the window. Raising the thermostat setting a couple degrees is an effective way to warm the inside surface of the window which, in turn, warms the outside surface. In some cases, installing a deflector on a floor register or moving furniture can help keep air-conditioned air from blowing directly on a window. Since the condensation usually occurs on windows that have a large unobstructed view of the outdoors, exterior shutters, shades, window screens, or even trees can help reduce summer condensation problems, as well.
Condensation on the outside of a window in the summer does not mean that the window is defective. Instead, the condensation indicates that the window is doing its job well: keeping the inside and outside separate.