Keep the heat where it belongs, and be stylish too
Energy saving curtains are one of the least expensive ways you can save on home heating and cooling bills. Understanding how curtains work to save energy will help you choose the right kind for your needs.
If your windows are bare it’s much easier for heat to pass through them in several ways. First, let’s consider light transfer. Sunlight entering your house through the windows may get converted into heat inside. This is a welcome source of free energy in winter, but it can add to your cooling bill during hot weather. Curtains with a dark inside surface and a light, reflective liner on the window side will reflect sunlight back outside before it can turn into heat.
Radiative transfer is the process of infrared radiation (also known as ‘heat’) passing through the glazing. This is undesirable any time you’re heating or cooling, because the heat goes from the hot area to the cooler area, which means you lose heat from indoors in winter, and gain heat from outdoors in summer. Putting curtains up adds an effective barrier to radiative heat transfer, because it’s harder for infrared radiation to pass through fabric than through glass (that’s why you feel warm when you stand in a cool room in front of a sunny window). Of course, if you already have energy efficient new windows or your windows are coated with energy efficient window film, the curtains won’t make as much of a difference in this area.
Convection is the process of heat moving through fluids (such as air, a gas, or water, a liquid). Conduction, meanwhile, is the process of heat moving through solids. Windows exhibit a pattern of heat loss to the outside in winter, through convection and conduction, as follows.
First, heat from the room circulates throughout the room through convection. When the hot air from the room circulates past the window, some of that heat is absorbed by the window glazing, warming the window and cooling the air. Since cold air falls, the cooled air at the window flows away from the window, which draws more warm air down behind it, so that you lose more and more heat to the outside.
Second, the heat in the window travels outside through a combination of convection (in the air space between any panes of a double or triple glazed window) and conduction (through the glass, and through the window frame, especially if it’s made of metal).
Of course, you can stop much of this heat loss by installing energy saving windows – for example ones with double glazing and an argon or other low-e gas injected between the panes to stop heat transfer, or windows with reflective coating to reflect infrared light instead of letting it through the glass. But tearing out and replacing a window with a new energy saving window might cost you hundreds of dollars (or a couple thousand for a good quality bay window), while you can put curtains up for a small fraction of that amount. The key is to position your curtains correctly, use the right materials, and of course choose something you like looking at.
Basic facts on energy efficient curtains
ENERGY STAR curtains: There are no ratings from ENERGY STAR for curtains. Some manufacturers will claim that their curtains offer the equivalent of a particular R value (somewhere between R-1 for most standard curtains, to R-6 for curtains with an insulated lining, is typical) but there is no standard rating system for energy saving curtains.
Combine several layers of energy saving curtains to add extra insulation layers. The standard white outer drape and darker inner drape not only give you sunlight and privacy during the day, and darkness and privacy so you can sleep in the next morning – they add two layers of still air between the window and your room, instead of one, improving the insulation factor of the curtains.
Fitted curtains. These energy saving curtains help keep room air separate from the cold window surface in winter (or cool room air away from the hot window surface insummer) thanks to a dead air space that acts as a strong insulator. Regular, unfitted curtains allow more airflow between the curtains and the glass, so the curtains don’t do as good a job of insulating.
Light-colored energy saving curtains reflect the heat from sunlight and prevent it from heating up your home in the summer, while providing partial lighting inside so that you don’t have to turn on lights. They can be opened on colder days to allow the full solar energy to warm your home.
Window orientation affects the efficiency of energy saving curtains, especially in summer. You’ll cut down your cooling costs a lot if you put highly reflective energy saving curtains in a window facing the noon to afternoon sun (south to west in the northern hemisphere). Curtains facing away from the sun obviously don’t keep the sun’s direct heat out (or the visible spectrum light that turns to heat indoors when it strikes a dark object), although they do at least block some infrared heat transfer and offer insulating properties. As for winter, heat losses are greatest in windows facing away from the sun (since there’s no solar energy gain); it’s best to keep those curtains closed as much as possible, while sun-facing windows should have their curtains open when the sun can shine through them and create heat indoors.
Energy saving window quilts are a variation on energy saving curtains. They are sealed on all sides and consist of an insulating layer and a vapor barrier surrounded by regular cloth fabric. They can provide insulation up to an R value of R-5.
Cutting air flow with curtains
Most curtains and drapes only cut heat loss by about 10% because they don’t effectively cut vertical air flows at the top and bottom of the window. Convection carries hot air down along the glass, where the heat from the air transfers to the glass. The cold air then falls below the bottom of the curtain (or out the sides) and more hot air is drawn down from above. (See the lefthand window in the illustration.)
You can readily convert conventional curtains to energy saving curtains by cutting off the air circulation above the window or below it, as the righthand window shows. For instance, you can box in the curtain rods along the top, to keep warm air from going down from within the room to the area at the top of the window, or you can make sure the curtains either reach the floor or are flush with the wall below the window. Any of these changes will reduce the amount of convection moving heat away from your windows, and make a big difference to the efficiency of the curtains.
The best solution, of course, is to close off the top, bottom and both sides, and you can do this by using magnetic tape or velcro fasteners. If you can completely cut off vertical airflow, you can reduce the amount of heat transfer by up to 25%.
When choosing curtain colors, the best energy saving curtains should have a medium colored fabric facing inside, with a white backing on the outside to reflect heat back outside in summer. Dark curtains won’t do anything more to prevent heat from entering, but they’ll make the room so dark you’ll need to use indoor lighting, which will increase your energy use and, especially if you use incandescent or halogen lights, will also increase your cooling load.
The best fabric for curtains is closed-weave fabric. If you can shine a flashlight through the fabric and see direct needles of light coming through (as opposed to just seeing a dull glow across the circle of the flashlight), the fabric isn’t closed weave. Closed weave fabric is more effective than open-weave fabric, both for cutting out direct light (which can turn into heat) and for preventing air flow through the curtain.
For other ideas on cutting heat loss (or air conditioning loss) through your windows, see my main Energy saving window coverings page.