Using window coverings to cut heating and cooling bills

Windows make up about 10 percent of a typical home's surface area but can account for 30 to 40 percent of its heat loss in winter. The curtains, shades and blinds you already have can cut some of that loss. When they're open and closed decides how much.

In winter

• Open them to the sun. Open the curtains and shades on south-facing windows during the day to let the sunlight warm the house.
• Close them at night. Closed curtains and shades cut the chill from cold windows after dark.
• Cellular shades. Insulated cellular shades are honeycombed inside, and can cut heating energy needs by almost 25 percent.

In summer

• Close them in the sun. Keep the coverings closed during the day on windows getting direct sunlight, to keep the heat out.
• Outside beats inside. Shading on the outside of the window gives the most protection from solar heat.
• Inside the glass. Anything put up on the inside of a window should be light-colored or mirror-like, and shouldn't make an airtight pocket between the glass and the room.

In a heat wave

Temporary shading keeps a house from getting too hot on sunny days. A reflective emergency blanket, reflective bubble wrap or aluminum foil taped inside the window needs no special tools. These are short-term fixes.

Daylight

Open curtains and shades let in daylight, so the lights can stay off. Light-colored, loose-weave curtains let daylight in while keeping some privacy.

An annotated example

What to do next

Note which windows get direct summer sun and which face south. Close the coverings on the sunny ones on summer days, and open the south-facing ones on sunny winter days. Thermostat settings that cut the bill covers the thermostat side.