A drapery's ability to reduce heat loss and gain depends on several factors, including fabric type (closed or open weave) and color. With such a wide variety of draperies available, it's difficult to generalize about their energy performance.
In the summer during the day, you should close draperies on windows receiving direct sunlight to prevent heat gain. According to the University of Florida, medium-colored draperies with white-plastic backings have been found to reduce heat gains by 33%. Draperies also stay cooler in the summer than some other window treatments because their pleats and folds lose heat through convection.
When drawn during cold weather, most conventional draperies can reduce heat loss from a warm room up to 10%. Therefore, in winter, you should close all draperies at night, as well as draperies that don't receive sunlight during the day.
To reduce heat exchange or convection, draperies should be hung as close to windows as possible. Also let them fall onto a windowsill or floor. For maximum effectiveness, you should install a cornice at the top of a drapery or place the drapery against the ceiling. Then seal the drapery at both sides and overlap it in the center. You can use Velcro or magnetic tape to attach drapes to the wall at the sides and bottom. If you do these things, you may reduce heat loss up to 25%.
Two draperies hung together will create a tighter air space than just one drapery. One advantage is that the room-side drapery will maintain around the same temperature as the interior space, adding to a room's comfort.
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