Reviewed and Revised on 11/13/2013
Window treatments can help improve the energy efficiency of windows in your home. However, other improvements such as:
can have a higher impact on reducing your home energy use and should be done before considering window treatments. Window treatments are generally especially helpful, If there are problems with solar heat gain (sun's heat travelling through a window and heating the inside of a home), feeling cold from losing body heat (feeling chilly in front of windows) and with drafts through the window.
Some window treatments will reduce heat losses through the glass only, while other block drafts as well.
In cold weather:
window treatments need to have insulating properties to reduce the U-factor (increase the R-value) of the windows. The U-factor is the rate of heat loss from the window, so in this case, you want to keep heat from being lost from the inside of the home to the outdoors.
In hot weather:
window treatments are needed to reduce the solar heat gain. For exterior applications these could include sunscreens and awnings; and interior treatments - window films, white/light colored shades, blinds, and shutters. Additional options could be draperies, insulative panels, high-reflectivity films, mesh window screens and storm panels.
In order for a window treatment to achieve the intended insulative value (i.e. R-value which measures the ability of that insulation to resist heat travelling through it) a tight seal all around the window treatment is needed to trap air.
Multiple layers of shades and/or drapes, and shutters can help to achieve this effect. A tight fit close to the window is needed as well. Cornices or shelves at the top of the window treatment help trap air and reduce convection air currents. Convection is the naturally occurring process in which warm air rises and cool air sinks.
Draft stoppers or cloth tubes filled with sand help block drafts at the bottom of the window treatment. Some treatments can be fit into metal or plastic tracks for a tight fit.
Select window treatment materials that trap air. Using firmly woven heavy fabric reduces air passing through the fabric. Adding a white or light colored lining either as a separate lining or with the window treatment helps reduce heat loss in the winter and heat gain in the summer.
White or light colored window shades hung inside the window casement or frame rather than on the window trim or frame are more effective in reducing heat gain (in summer) and heat loss (in winter).
Insulated Roman shades filled with fiber batting and layers of fabric mounted inside the window frame are also energy efficient.
Open weave fabrics, reed, or bamboo shades or treatments filter direct sunlight, but have little insulation value. Window blinds with multiple slats allow air movement, thus providing little insulative value. However, blinds are effective in reducing solar heat gain.
Things to be careful about when buying/installing window treatments:
1. R-value: Pay attention to the R-value of the window treatment. Ask how this was determined, under what types of testing conditions, and if it is for the total treatment or at the center of the treatment. Ask if it is an estimate, or tested in a laboratory. Installation and management of the treatment will impact its energy insulating effectiveness.
Condensation can sometimes become a problem with window treatments. A well-insulated and tight window treatment can successfully keep heated room air away from the window. In winters, this window glass will become however cold due to low outside temperatures, resulting in moisture condensation on the glass. Condensation can cause mildew formation and deterioration of window frame. It is thus important to consider the occurrence of potential situation when creating a tighter insulative window treatment. Solutions include using a well-insulated, tight window treatment which includes a vapor barrier.
Another solution is to monitor condensation at the windows, wiping away condensation whenever it occurs.
Other alternatives to tight interior treatments include adding shades on the window exterior by using trees, awnings, or other shading devices for summer; weatherizing the window using caulk and sealers; or upgrading the window to a more energy efficient window, or adding a storm window.
3. Interior storm windows
Storm windows increase the thermal efficiency of a window. Interior storm windows are an option, but can damage casings and sills, and may cause condensation on the interior face of the window sash if they are not tight. When using interior storm windows, they get mounted on the inside of the home, leaving the exterior face of the window sash unprotected from the natural elements.
Important considerations when putting a storm window include being able to exit through the window in an emergency, and the fumes emitted when window plastic burns or melts in case of fire at the home. Thus, using rigid, clear plastic as a permanent interior storm window can be a problem. All rooms in any home should have two or more escape exits – one or more doors and a window.
Glass, polycarbonate, or clear acrylic are sometimes used for the interior glazing. Plastic can scratch when being cleaned. Flexible plastics may last only a few years before needing to be replaced.
Interior storm windows may be attached using a channel,hook, loop tape or any other technique. Some of these techniques are clearly visible, while others can blend in with the window structure itself. One needs to make sure that the attachment technique is such that the window is secure and fastened tightly so as to be able to reduce the air infiltration.
4. Heavy duty, clear, plastic, flexible sheeting on a frame or clear plastic film can be attached to the inside of your window frames during the cold winter months to serve as window treatments. Depending upon their method of attachment (tape) the window frame may be damaged when removed.
5. Any window treatment added to a home must be safe and should not block the ability to escape from the room in case of fire. Some window treatment materials give off fumes when burned and hence should not be used.
For more information on interior and exterior window treatments for energy efficiency, read about Window Treatments and Coverings.