But, soft! What light through yonder window breaks?
It is the east, and Juliet is the sun.
- William Shakespeare, Romeo and Juliet

No one would dispute the undeniable beauty of soft, dappled light shining through a window in the early morning. Unless, of course, you happen to be one of the many Americans whose windows are letting in more than just light. Poorly fitted or sealed windows can be a major frustration during the winter months, letting in cold drafts that blur the lines between indoors and out. Unfortunately, this isn't just an issue of comfort. Windows with air leaks pose a real energy efficiency problem as well. Not only does the cold air that seeps into your home through these leaks force your heating system to work that much harder to maintain a stable and comfortable temperature; poorly fitted and sealed windows also make it that much easier for the warm air inside your home to get out.  In fact, the Energy Information Administration (EIA) estimates that as much as one third of the average home's heat loss occurs through windows and doors.

Long-term fixes for air leaks typically involve the purchase of replacement windows. However, replacement windows aren't an option for everyone. For some, the cost is prohibitive; and for renters, replacement windows might not even be a possibility. Luckily there are a variety of options for consumers who are looking to improve the energy efficiency of their existing windows. Used individually or in combination, the following tips can help to mitigate heat loss, reduce drafts, and generally make your home a more comfortable place to be this winter: 

Caulking and Weatherstripping
Caulking and weatherstripping are two of the easiest and most cost-effective ways to reduce leaks and drafts due to small cracks and gaps around window frames. They are so cost-effective, in fact, that the U.S. Department of Energy estimates that when used correctly, the cost of caulking and weatherstripping will be offset through lower utility costs within one year. Use caulk to seal cracks less than ¼ inch in width on the non-movable parts of your window, mainly around the frame and where the trim meets the wall. Slightly larger or irregularly shaped gaps might better be filled with a small can of spray foam that will expand to fill the space.

Several of our more recent blog posts have mentioned weatherstripping, so I won't repeat the many good tips already provided on this topic. Suffice it to say that weatherstripping should be reserved for use on a window's movable parts, such as where the lower sash meets the sill. If you're worried that caulking or weatherstripping might be too permanent, there are products on the market labeled as temporary caulking. They seal the window for the season, but can be removed once warmer weather rolls around and you're ready to throw open the windows to the spring.

Allison Casey's helpful post on weatherizing the windows and doors in her home noted the usefulness of window coverings as an extra thermal barrier. Keep in mind that no matter how thick your curtains, they won't stop air leaks – that's what strategies like caulking and weatherstripping are for. But appropriately hung draperies can decrease the discomfort associated with drafty windows and reduce heat loss by as much as 25%. And, as Allison pointed out, be sure to open and close your draperies depending on the amount of sunlight an individual window receives during the day.

Insulating Window Panels or Interior Storm Windows
If draperies and caulking just aren't enough, you might consider installing insulating window panels or interior storm windows. Both of these products typically consist of an insulating material encased inside a wooden or metal frame that seals tightly against the interior side of your existing window frame, usually with magnets or Velcro. As they are not permanently affixed, they can be easily removed and are completely reusable from one winter to the next. The main difference between the two types of panels is the degree to which they obstruct your view. Insulating window panels are typically made of rigid insulation; while this provides an R-Value of between 3.8 and 7, it does so by completely obstructing the window. Interior storm windows, on the other hand, are typically composed of a thin but durable sheet of plastic. R-Values for this material are not as good as for insulating window panels, but they do manage to insulate while maintaining your view.

Window Films
There are a variety of plastic window films available to do-it-yourselfers. At the more basic end of the spectrum is heat shrink film. This film looks like a thicker version of plastic wrap and is typically sold in rolls from which sheets can be cut to size to fit the dimensions of a particular window. Once attached to the window frame with double-sided tape, a hair dryer is used to shrink the film, providing a better seal around the window and removing any visible wrinkles. Since the film is clear, light is still able to penetrate into the space. However, because it is attached to the window frame, you're pretty well stuck with a closed window until you're ready to remove the film for good.

At the upper end of the spectrum, Low-Emissivity (Low-E) window films are coated with a thin and nearly invisible metallic coating that helps to reduce infrared heat transfer between the interior and exterior panes of a double-paned window. Because they must be directly in contact with the window pane to function properly, Low-E films are slightly less forgiving that heat shrink films during installation. Low-E window films are also somewhat more expensive than heat shrink films. But both are still a fraction of the cost of replacement windows.

Ready for more? The Office of Energy Efficiency and Renewable Energy has additional tips on how to improve the energy efficiency of existing windows without breaking the bank.