An Expert’s Guide to Energy-Efficient Window Coverings

Story at a glance:

• Drapes, curtains, and blinds are some of the most common examples of energy-efficient window coverings.
• Insulated cellular shades and window quilts are two of the most energy-efficient interior window coverings.
• Exterior energy-efficient window coverings like louvers and awnings often require more strategic placement than indoor window coverings in order to be truly effective.

The DOE estimates that, on average, 25 to 30% of the energy used to heat and cool a home is lost through its windows—and while that number is reduced to about 10% in commercial buildings, windows still influence end uses that make up for approximately 40% of a building’s total energy consumption.

All of this to say that windows play a significant role in determining how much energy a building uses, meaning architects are always looking for new ways to improve their overall energy efficiency. One way to do this is by installing energy-efficient window coverings and attachments to help prevent unwanted heat transfer.

In this article we’ll take an in-depth look at the different types of energy-efficient window coverings and see how they compare to one another.

What are Energy-Efficient Window Coverings?

An energy-efficient window covering refers to a type of window design feature primarily intended to provide shading throughout the day, but which may also serve to improve privacy, weatherproofing, and building aesthetics. These window coverings can be attached to a window or window frame either internally or externally.

There are many types of window coverings available, but not all of them are considered energy efficient. When choosing window coverings based on energy efficiency, the DOE recommends looking for products bearing the Attachments Energy Rating Council’s (AERC) Energy Improvement rating.

Products bearing this rating have been certified in accordance with the strict standards set forth by the AERC, an independent non-profit that rates, labels, and certifies the energy performance of window coverings and attachments. Energy Improvement rating labels show how well a product performs in cool climates and warm climates, with higher numbers indicating higher energy savings.

Benefits of Energy-Efficient Window Coverings

Energy-efficient window coverings can also provide a few additional benefits, including:

• Reduced operating costs. When implemented and used properly, energy-efficient window coverings can drastically reduce a building’s heating and cooling loads, which in turn helps reduce overall operating costs.
• Fewer emissions. By reducing the energy required to illuminate, heat, and cool buildings, energy-efficient window coverings also reduce their total GHG emissions.
• Better daylight control. Operable window coverings like blinds, curtains, and roller shades give occupants greater control over interior lighting conditions and can help to reduce distracting glare.
• Improved IEQ. Because energy-efficient window coverings help prevent the unwanted transfer of thermal energy into or out of a building, they help improve indoor environmental quality by making for more comfortable spaces.
• Added privacy. While not necessarily true of all window attachments, many window coverings offer some measure of added privacy when they are in use.
• Aesthetic value. Many window coverings and attachments add aesthetic value to a building, potentially even improving its curbside value.

Types of Energy-Efficient Window Coverings

There are many types of energy-efficient window coverings available for both residential and commercial applications, in two basic categories: operable and static.

Operable Window Coverings

The vast majority of energy-efficient window coverings are operable; they can be opened, closed, moved, or otherwise adjusted in some capacity. Because they can be moved throughout the day in accordance with changing factors and conditions, operable window coverings are considered to be the more effective window covering option.

Shutters

Though they have largely devolved into purely decorative features in the modern age, shutters can, when used properly, be extremely effective at reducing unwanted heat transfer through windows and window frames. Shutters typically consist of a solid frame of vertical stiles and horizontal rails, into which a solid panel or series of horizontal louvers may be set.

When shut, solid panel shutters completely block sunlight and solar energy from entering through a window, whereas louvered blinds in the closed position may be adjusted to allow for ventilation (if the window is open) and filtered sunlight. Shutters are typically made from either wood or vinyl, with solid wooden shutters typically offering the highest insulating R-value.

Shutters may be installed on the exterior or interior of a building; exterior shutters are typically more robust and durable, whereas interior shutters are more convenient to adjust. When closed, the robust nature of exterior shutters can help provide added protection against heavy rains and high winds, making them particularly useful in coastal regions.

Drapes & Curtains

As perhaps the oldest forms of operable window coverings, drapes and curtains are two simple—yet no less effective—means of controlling the amount of daylight and solar energy entering a building. Both drapes and curtains consist of large fabric panels that are hung from a rod and used to cover windows or glass doors, though drapes are typically longer and made from a heavier fabric than curtains.

Most curtains do not completely block out light when drawn and allow some sunlight to filter in depending on the type of fabric they are made from and the amount of lining material they contain. Curtains of the blackout variety, on the other hand, are specifically designed to prevent light from entering or escaping and are functionally identical to drapes.

The Industrialist hotel in Pittsburgh, for example, features curtains instead of blinds in each guest room and drapes in many of its common spaces as a means of providing shade and privacy while still preserving the building’s historic character.

Horizontal & Vertical Blinds

Today blinds are perhaps the most ubiquitous and widespread form of window covering on the market. Extremely simple in their design, blinds are composed of thin slats or vanes arranged in a horizontal or vertical manner, with the former being the more common configuration.

Traditional horizontal blinds (or Venetian blinds) may be raised or lowered via a pull cord and maneuvered with a manual control, allowing the user to incrementally open the slats and let light filter in or close the slats into their overlapping state and block out most of the light. Vertical blinds operate in a similar fashion but are instead installed along a track system, making them comparable to curtains or drapes. When in the closed position, the vanes in a vertical blind may be rotated to change the amount of light entering the room; when the blind is pulled open, all of the vanes traverse the track and stack against one another on one side of the window or door.

Both types of blinds can be made from a variety of materials—which can impact the degree to which they prevent solar heat gain—and mechanized, allowing the user to open or close them via a remote control, app, or building management system.

Interior & Exterior Roller Shades

Roller shades are another great solution when it comes to operable window coverings. These shading devices use a flat material or fabric that can be rolled up or down to control how much daylight comes into a building. Depending on the amount of sunlight the window sees, the shades may be made of a tighter woven material to block direct light or a looser one to offer some visibility.

Once selected and installed these dynamic window coverings can result in significant energy savings. Mermet USA and the Lawrence Berkeley National Laboratory calculate that, compared to a low-e double-glazed window without a shade, a light color E Screen shade in a 3% openness factor diminishes the solar heat gain by 59%—which makes a big difference in how much air conditioning is required to keep a building comfortable.

Screens, blinds, and shades can be of the manual or mechanical variety, with the latter capable of being controlled by sensors or linked to a building automation system for more efficient operation. The Oklahoma City Ballet’s rehearsal space, for example, makes use of Hunter Douglas Architectural’s RB500 automated roller shades with Mermet’s E screen 1% in white fabric filter light—the shades rely on sun sensor technology to open and close with the sun’s movement throughout the day.

And while we often think of solar screens as being indoor shading devices, there are also outdoor versions as well. Mermet offers sun control fabrics suitable for outdoor use in the form of retractable solar screens, roller shades, and zipper systems.

Insulated Cellular Shades & Window Quilts

When it comes to interior window coverings, insulated cellular shades are typically considered to be the most energy efficient option on the market, as they have a higher R-value than curtains, drapes, blinds, and shades. This is thanks to the honeycomb-like structure of insulated cellular shades, as each cell serves as an insulating air pocket. Insulated cellular shades are available with either one or two layers of cells, with the two layer variety offering a higher R-value; the best cellular shades have an R-value ranging between 4 and 5.

Window quilts are another form of energy-efficient window covering made from heavy, quilted fabric. Similar to insulated cellular shades, window quilts serve to both block solar energy and prevent unwanted heat transfer during the day and night—when installed and used correctly, they can stop up to 80% of heat loss in the winter and 86% of heat gain in the summer. In order to prevent drafts, window quilts are designed to fit snugly against the window frame/trim, using either vertical channels or Velcro/snaps to attach the quilt to the trim.

Both insulated cellular shades and window quilts are designed to operate like blinds and can be drawn up to allow unfiltered light in; insulated cellular shades fold up while window quilts roll up.

Electronically Switchable Glass

There are four core subtypes of electronically-switchable smart glass: micro-blinds, polymer-dispersed liquid-crystal devices (PDLCs), suspended-particle devices (SPDs), and electrochromic devices.

• Micro-blinds. Feature microscopic rolls of thin metal blinds on glass; when no voltage is applied, the blinds are rolled and thus let light through; when a voltage is applied, the blinds unroll and prevent light from entering.
• PDLCs. Created by dissolving liquid crystals within a liquid polymer and letting the polymer cure into a solid, at which point the liquid crystals become incompatible with the polymer and form droplets; when no voltage is applied, the crystals are randomly arranged in the droplets and scatter light, creating an opaque surface; when voltage is applied, the crystals align and light is allowed to pass through the droplets undisturbed, resulting in a transparent sheet of glass.
• SPDs. Consist of a thin film laminate of rod-like nano-scale particles suspended in a liquid placed between two panes of glass; when no voltage is applied, the particles are randomly organized and thereby serve to block and absorb light; when a voltage is applied, the molecules then align and let light through.
• Electrochromic. Typically include several thin layers of a ceramic material that are then charged with a large number of lithium ions; when a voltage is applied, the lithium ions and electrons are transferred between layers, resulting in an instantaneous tinted effect; once voltage is shut off, the glass becomes clear again.

HALIO, formerly Kinestral Technologies, is a leader in electrochromic glass technology and developer of the HALIO Smart Glass family of products. Aside from the glass itself, the complete HALIO autonomous system includes tint drivers (which control each window’s tint level), the HALIO Sky Camera (records and reports changes in daylight levels for automatic adjustment), the HALIO Cloud (controls system automation), and HALIO Gateway, which securely manages all communication between HALIO devices and the HALIO Cloud.

SageGlass—a brand under the Saint-Gobain family of companies—is another industry leader in electrochromic smart glass technology. Fentress Architects utilized SageGlass Classic—amongst other energy-efficient features—when designing the Nashville International Airport’s Concourse D and Terminal Wing expansion, helping the project to earn LEED Silver certification.

Responsive Shading Facades

Glass building facades can also be outfitted with responsive shading systems that may be opened, shut, angled, rotated, or otherwise manipulated to carefully control the amount of solar energy entering a building throughout the day.

Facade-mounted dynamic shading devices are typically linked to some manner of building automation system and may use sensors or cameras to automatically adjust to changing sunlight levels. Alternatively, automated shading devices may be controlled via an app, allowing individual users to have greater control over their indoor environmental conditions.

Abu Dhabi’s Al Bahar Towers, for example, makes use of dynamic shaders in the form of screened curtain walls inspired by the mashrabiya—a traditional Islamic lattice shading device—that are composed of triangular actuated panels. Each triangle is coated with fiberglass and has been electronically programmed to respond to the sun’s movement in order to reduce glare and solar heat gain. The unique shape of the panels ensures that they remain useful throughout the year regardless of the sun’s current angle.

Static Window Coverings

While not as common in single-family homes, static window coverings are often an extremely important component in commercial and large-scale multifamily residential development projects. As the name suggests, static window coverings refer to those unmovable window attachments which constantly remain in a fixed position once they are installed.

Because they cannot be moved or adjusted throughout the day, static window coverings often require more strategic placement than operable window coverings in order to fully benefit from their energy-saving abilities.

It should be noted, however, that modern technology has advanced to the point where most fixed solar shading features can be made operable and even automated. The examples below include some of the more common types of traditionally fixed solar shading devices.

Low-E Coatings & Films

From a technical standpoint, low-emissivity or low-E window coatings and films are extremely similar to the micro-blind variety of electronically-switchable glass (sans the switchability) in that they use microscopic metallic particles to reflect long-wave infrared radiation while still allowing some visible light to pass through.

Low-E coatings are applied to glass during the manufacturing process, while low-E films can be bought on their own and adhered to the interior-facing side of the glass, making them extremely retrofit friendly.

On average, low-E coatings and films help conserve 50% of interior heat during the winter and reflect 70 to 80% of solar heat gain during the summer.

Awnings

Architectural awnings are a type of building projection used to provide shade and shelter from the elements, essentially functioning as miniature roofs when installed overtop individual windows. According to the DOE, window awnings can help reduce solar heat gain during the summer by up to 65% on south-facing windows and as much as 77% on west-facing windows.

Awnings are traditionally made from fabrics like canvas or polyester, though they can also be made from various metals like aluminum and copper to improve their overall durability. The design of window awnings is highly flexible, meaning they can be tailored to suit the aesthetics of almost any project.

Brise-Soleil, Louvers & Sunshades

A brise-soleil—or “sun breaker” in French—is a form of static external window or window-wall covering that uses a series of horizontal, vertical, latticed, or patterned blades to block, diffuse, or otherwise limit the amount of sunlight and solar heat entering a building. Also referred to as sunshades or louvers, brise-soleil systems may be oriented vertically or horizontally and support a wide variety of styles, patterns, and material choices, often serving an aesthetic purpose in addition to a practical one.

Like most static window coverings, the specific design characteristics of brise-soleil systems are tailored to the site and project in question, though the general concept remains the same. All brise-soleil are designed to allow the low-level sun to enter in the mornings, evenings, and in winter but eliminate direct high-angled sunlight during midday, afternoons, and throughout the summer.

The BKSK-designed Charlotte of the Upper West Side, for example, is a green building in New York that employs a custom-designed, louver-based solar shading system made from terra-cotta. The louvers were carefully designed by BKSK to achieve an optimal level of solar admittance and were finely crafted over a period of two years by Sannini, a leading Italian manufacturer of terra-cotta products.