Energy Efficient Window Coverings

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, from the bottom, or both.

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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 10% heating energy savings. In cooling seasons, cellular shades can reduce unwanted solar heat through windows by up to 60%, reducing the total solar gain to 20% 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, however they are often more complicated to operate and provide less visible light when closed.

Learn more about cellular shades and window quilts.

Roller and 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.

Louvered Blinds

Window blinds&#;vertical (Venetian blinds) or horizontal slat-type (louvered-type)&#;are effective at reducing summer heat gain and reducing glare, while providing good daylight indoors. 

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 glare, while allowing you to take additional advantage of natural daylighting.

Curtains and Drapes

Curtains are fabric interior coverings 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% and increase the thermal comfort of the home. 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 loss, 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 will further reduce heat loss. 

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 (applied to the glazing surface) 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.

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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 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:

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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).

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. More recently, there are solar control films that have a more neutral appearance and are effective at blocking near IR solar radiation in the summer.  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.

There are also window films that have low-e coatings, which can also be effective in reducing winter heat loss and increasing thermal comfort of occupants.  They also typically are effective at reducing near IR solar radiation. 

Window films can be professionally applied by a skilled installer or are available for do-it-yourself projects at home improvement stores.

Window films require extra care for cleaning.  If they are primarily absorbing solar radiation (tinted films), they can damage insulated glazing unit (IGU) seals.  Most window manufacturers will void their warranties if the film is installed on an IGU.

Read more about window films at the Efficient Window Coverings Collaborative.

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