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Windows can account for a great deal of energy loss from your home. The U.S. Department of Energy estimates that heat loss through windows may account for 10 to 25 percent of your heating bill.¹

Not all windows are created equal

The least efficient windows are single-paned with no storm windows. The most efficient windows are certified by the National Fenestration Rating Council and carry an NFRC label. Windows that carry the ENERGY STAR are certified by the NFRC to meet U.S. EPA and U.S. Department of Energy criteria for your climate zone.

Most of us are familiar with R-value, which measures insulating value—resistance to heat transfer. Windows, however, are rated by U-value, which is the inverse of R-value. U-value measures the rate of heat transfer through a substance—its conductivity. If you were buying insulation, you would want a product with a high R-value. If you were buying windows, you would want a product with a low U-value, indicating that you will lose less heat through your windows. In Maine, you would want windows with a U-value of 0.35 or less.²

Reducing heat loss from your current windows

If buying new windows is not in your immediate future and you still want to decrease your energy costs, there are some quick and easy ways to make the windows you have lose less energy.

First, clean, caulk, seal, and lock your windows. Locking the window will keep the sashes in the frame where they belong, and help keep either sash from working its way open even slightly. If you have shades or heavy curtains, secure them to the window frame at night with painters tape, draft stoppers, draft snakes, magnetic tape, or hook and loop tape. Add a cornice, valance, or lightweight draft stopper to the top of the curtain rod or shade roller. This will keep warm ceiling air from dropping down across the glass and losing its heat to the cold window.

Instructions for removable indoor shutters

While sunny windows, especially if they are south-facing, can help warm a room during the day, they can be a source of heat loss at night or on cloudy days. Here is a thrifty practice worth reviving: you can make your own removable indoor shutters to reduce nighttime heat loss. All you need are some simple tools and easily available materials.

	First, measure your windows accurately by taking two horizontal and two vertical measurements. The shutter should fit onto the windowsill and cover the entire window. Gather some clean cardboard from used boxes, in chunks large enough to cover your window. Appliance stores may have large boxes that they would give you. Cut out one piece of cardboard about 1/8-inch smaller than your window measurements. Check the fit of this piece in your window. If it fits, cut three more pieces the same size and layer one on top of the other with some dots of glue to hold them together. Weight the layers with some heavy books or bricks while the glue dries. If you do not have enough cardboard for four layers, you can cut two layers and fit bubble wrap or padded envelopes in between the cardboard.
	When the glue dries, cover both sides of the shutter with heavy-duty aluminum foil, glued on with craft glue thinned with a small amount of water. Spread the thinned glue all over the cardboard surface with a paintbrush or foam brush and apply the foil.
	Seal the edges of your shutter with duct tape.
	Add foam weather stripping to the edge to seal the shutter snugly in your window. Add some handles or a duct tape tab to make it easy to remove the shutter when you want to see out or let the sun in. You can also decorate your shutter with fabric or wallpaper to match your room.

This simple shutter is easy and inexpensive to make. It can help you reduce your heating bill and be more comfortable in your home this winter. You can also use your shutters in the summer to reflect sunlight from your windows and keep your house cooler.

^{1, 2} U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy, “Windows,” Energy Savers (2010). http://www1.eere.energy.gov/consumer/tips/windows.html.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

© 2008, 2012

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