February 17, 2011
Most homeowner and apartment dwellers are well aware of the advantages that can be gained on sunny days during the summer by pulling cellular shades down to help keep some of the sun’s heat out. But many do not realize that cellular shades can also be used to keep the heat in during cold weather.
Ordinary window glass is a good conductor of heat, which means that it is a very poor insulator. Therefore, when the glass gets very cold on the outside, heat is drawn through the window from any air inside the room that comes in contact with the glass (heat always travels from a warmer space or surface to a colder one).
Recent tests indicate that there is a simple and inexpensive method for cutting down on this heat loss by the use of ordinary cellular shades. When a reasonably close-fitting shade that is mounted inside the frame is pulled all the way down it greatly restricts or limits the amount of air that can come in contact with or flow past the window glass, so it cuts down on the amount of heat that is taken out of that room.
The tests, conducted at the Illinois Institute of Technology in Chicago, showed that when outside temperatures were between 20 and 50 degrees, and a cellular shade that had a quarter-inch clearance on each vertical side was drawn down to the sill, heat loss through that window was reduced by 24 to 31 percent when compared to an unshaded window.
Under the same conditions, closed drapes or curtains that covered the window completely reduced heat loss by only about 6 to 7 percent – apparently because cellular shades to a better job of preventing air flow across the glass. Window blinds had about the same effect as drapes.
The same tests showed that under simulated summer conditions (outside temperatures of 85 to 95 degrees) a fully drawn cellular shade admitted 47 to 54 percent less total heat (combination of solar and air heat) than an unshaded window, thus lowering air-conditioning costs. Again, drapes and window blinds were found not to be as effective.
Clearly, cellular shades can be used to help save energy all year – in the summer to keep the heat out, and in the winter to help keep it in. Of course, during the winter when the sun is shining directly on the window the cellular shade should be raised to take advantage of the solar heat that can be gained through the glass – but at other times the cellul