Energy Efficient Window Attachments

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Photo of a window shade.
Operable window shades help control daylighting. 
Chris Gunn / NREL

Window coverings can reduce energy loss through the windows, lower heating and cooling bills, and improve home comfort.

About 30% of a home's heating energy is lost through windows. In cooling seasons, about 76% of sunlight that falls on standard double-pane windows enters to become heat.

Most types of window treatments will result in energy savings, but the exact savings will depend on the type of attachment, the season, the climate, and how the attachment is used.

In addition to the window treatments discussed below, storm windows with low-e coatings are effective at improving thermal performance of windows and reducing solar heat gain.

 

Operable Window Coverings

Operable window coverings give you the flexibility to choose whether to keep your window coverings open or closed for privacy, and to maximize natural light, take advantage of heat from the sun in the winter, and reduce heat gain in the summer. Options include shades, blinds, draperies or curtains, and some shutters.

Operation of Window Coverings

Not all window coverings are operable, but of those that are, one study found that 75% of residential window coverings remain in the same position every day. If this describes your habits, be strategic about which coverings you open in the morning.

If it’s winter and likely to be sunny, open the blinds or curtains in the morning to allow the sun to heat your home through the day—especially those that receive direct sunlight.

In the summer, you may want to keep certain window coverings closed to reduce heat gain. For natural light, open those coverings that don’t get direct sunlight.

You may also want to try switching the ones that are opened and closed through the day to maximize light and heat from the sun when you want it.

Some window coverings offer automated options. Learn more about the benefits, drawbacks, and installation of automated window coverings.

 

Interior Window Treatments

Insulated Cellular Shades

Insulated cellular shades are made of pleated materials that are designed to fold up, accordion-like, usually at the top of the window, but sometimes at either the top or the bottom. Insulated shades contain one or more air layers in a honeycomb cross-section. Some can be adjusted from the top or the bottom.

Insulated cellular shades are typically considered to have the highest R-values of all window coverings. The air pockets in the honeycomb cross-sections act as insulators, increasing the R-value and reducing the conduction of heat through the window.

Insulated cellular shades can be a good choice if you are looking for significant energy savings from their window coverings, as well as comfort, privacy, and increased home resale value.

In heating seasons, tightly installed cellular shades can reduce heat loss through windows by 40% or more, which equates to about 20% heating energy savings. In cooling seasons, cellular shades can reduce unwanted solar heat through windows by up to 80%, reducing the total solar gain to 15% or less when installed with a tight fit.

Cellular shades that operate on side tracks are most effective at increasing the R-value of windows, and those that open from both the top and bottom allow users to most effectively control daylight entering the home.

Some cellular shades include the option of automation, allowing the blinds to open and close on a set schedule. The schedule can be seasonally optimized to reduce heating and cooling loads while maximizing natural light and home comfort.

 

Window Quilts

Window quilts have a sheet of quilted material that can be opened by rolling and closed by unrolling. They typically fit snug against the trim, either on tracks or with an attachment such as Velcro or snaps.

Because of their snug fit, window quilts offer R-value increases similar to cellular shades, and they typically cost less. 

Learn more about cellular shades and window quilts.

Roller or Roman Shades

Roller shades are usually inexpensive shades that are raised or lowered from a roller bar fitted at the top of the window. Roman shades are fabric window shades that are drawn up into a series of evenly stacked folds when raised or lowered.

These shades typically fit inside of the window casing, or just outside, and they come in a variety of fabrics, colors, and weaves. Heavier fabrics will typically offer slightly better thermal performance, but roller and roman shades offer only a small amount of insulation and are most effective for privacy, room darkening, and blocking sunlight.

Blinds

Window blinds—vertical or horizontal slat-type—are more effective at reducing summer heat gain than winter heat loss.

Because of the numerous openings between the slats of blinds, it's difficult to control heat loss through interior window blinds, but the slats offer flexibility in the summer. Unlike shades, you can adjust the slats to control glare, light, and solar heat gain.

When completely closed and lowered on a sunny window, highly reflective blinds can reduce heat gain. Horizontal slat-type blinds can also be adjusted to block and reflect direct sunlight onto a light-colored ceiling. A light-colored ceiling will diffuse the light without much heat or glare, while allowing you to take additional advantage of natural daylighting.

Curtains and Drapes

Curtains are fabric interior attachments that are sized to fit the window, while drapes reach all the way to the floor.

A drapery's ability to reduce heat loss and gain depends on several factors, including fabric type (closed or open weave) and color. With such a wide variety of draperies available, it's difficult to generalize about their energy performance.

During summer days, you should close draperies on windows receiving direct sunlight to prevent heat gain. Studies demonstrate that medium-colored draperies with white-plastic backings can reduce heat gains by 33%. 

When drawn during cold weather, most conventional draperies can reduce heat loss from a warm room up to 10%. Therefore, in winter, you should close all draperies at night, as well as draperies that don't receive sunlight during the day.

To reduce heat exchange or convection, draperies should be hung as close to windows as possible and fall onto a windowsill or floor. For maximum effectiveness, install a cornice at the top of a drapery or place the drapery against the ceiling. Then seal the drapery at both sides and overlap it in the center. You can use Velcro or magnetic tape to attach drapes to the wall at the sides and bottom. Taking these steps may reduce heat loss up to 25%.

Two draperies hung together will create a tighter air space than just one drapery. One advantage is that the room-side drapery will maintain around the same temperature as the interior space, adding to a room's comfort.

Window Films

Window films help block against solar heat gain and protect against glare and ultraviolet exposure. They are best used in climates with long cooling seasons, because they also block the sun's heat in the winter.

They can be useful for homeowners who don’t want to block views with other window treatments, but who have issues with glare and solar heat gain. They can also be a good choice on windows that are difficult to fit with other window treatments, or in places where artwork, furniture, or carpeting could be faded by UV exposure.

Films typically have three layers: an adhesive layer that sits against the glass, a polyester film layer, and a scratch-resistant coating. You may also choose options such as tints, UV blockers, or thicker films that offer security. Low-e films are also emerging as an energy-saving option.

Window films are the first window attachment to be rated by the National Fenestration Rating Council (NFRC), which also created window labeling and ratings for consumers. The NFRC’s Window Film Energy Performance Label looks like this:

Window film energy performance label.
A) Rated by NFRC. B) Manufacturer and product name. C) Details on NFRC rating procedures. D) Reference Product column shows what type of glazing works best with the film. E) Solar Heat Gain Coefficient (SHGC). F) Visible Transmittance (VT).
National Fenestration Rating Council (NFRC)

It includes the solar heat gain coefficient (SHGC) and visible transmittance (VT) of the window film, both numbers between 0 and 1. The lower the SHGC, the better the film is at blocking heat gain. The higher the VT, the more potential for daylighting. Read more about energy performance ratings.

The effectiveness of these reflective films depends on:

  • Size of window glazing area
  • Window orientation
  • Climate
  • Building orientation
  • Whether the window has interior insulation.

Silver, mirror-like films typically are more effective than the colored, more transparent ones. East- and west-facing windows, because of their greater potential for heat gain, can benefit more from these films. North-facing windows won't benefit from them, and south-facing windows may benefit somewhat, but the benefit could be offset by the reduction of heat from the winter sun.

Some window manufacturers also make reflective glazing or glass.

Window films have some overall disadvantages:

However, these disadvantages are dependent on the specific product you choose and its features. Read more about window films at the Efficient Window Coverings Collaborative

Exterior Window Treatments

Exterior Shutters and Shades

Exterior shutters and shades are usually made of a variety of materials, including fabric, wood, steel, aluminum, or vinyl. They are most effective at reducing solar heat gain.

Shades are typically fabric or vinyl and the material may have openings that allow some visibility through the window. The larger the openings, the less protection from solar gain. They are usually manually operated, though some can be opened or closed with a crank inside the home.

Roller shutters are usually mounted above the window and side channels guide them as they're lowered and raised. When you lower these blinds completely, their slats meet and provide shade, privacy, security, and protection from storms. If partially raised, the blinds allow some air and daylight to enter through windows.

Most exterior shutter systems include a mechanical crank, rod, or motor to allow operation from indoors. This can help encourage daily use of the shutters, and may be required by local fire codes.

Learn more about exterior shutters and shades from the Efficient Window Coverings Collaborative.

Awnings

An awning is a roof-like shelter installed on a home’s exterior that shades windows from the sun’s heat and glare. Awnings can also shade outdoor living spaces. Awnings can be fixed or retractable.

Window awnings can reduce solar heat gain in the summer by up to 65% on south-facing windows and 77% on west-facing windows. You can use an awning to shade one window or have an awning custom-made to shade the entire side of your house.

In the past, most awnings were made of metal or canvas, which need to be re-covered every five to seven years. Today, awnings are made from synthetic fabrics such as acrylic and polyvinyl laminates that are water-repellent and treated to resist mildew and fading. Whatever the fabric, you should choose one that is opaque and tightly woven. A light-colored awning will reflect more sunlight.

Awnings require ventilation to keep hot air from becoming trapped around the window. Grommets (eyelets) or other openings along the tops and sides of an awning can provide ventilation. The awning may also open to the sides or top to vent hot air.

While awnings can save energy during the cooling seasons, they can increase energy used for heating, so keep this in mind when deciding whether awnings are right for you. You can also adjust your use depending on the season: keep awnings installed or closed in the summer and remove or open awnings in the winter. Fixed awnings can sometimes be installed to allow the lower-angle winter sunlight to reach windows.

You can roll up adjustable or retractable awnings in the winter to let the sun warm the house. New hardware, such as lateral arms, makes the rolling up process quite easy. Some awnings can also be motorized for easy operation.

Exterior Solar Window Screens

Solar screens can reduce solar heat gain, UV damage, and glare. They can be installed on the interior or exterior as roller shades or fixed panels, and they typically allow for a view out the window and light transmission. They look similar to regular insect screens but provide more efficiency benefits.

The openness factor on solar screens varies and affects the efficiency benefits; greater openness reduces the protection against glare and solar heat gain but increases visibility and light transmission.

Read more about solar screens from the Efficient Window Coverings Collaborative. 

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