Daylight Devices

 

Sunshades modulate light and add color as shown in this project by SmithGroup Architects, Inc. Photo courtesy of EFCO Corporation. Photography by Wes Thompson Photography

 

 

Neither a wall, a window, a skylight or a roof, tubular daylight devices (TDD) have now achieved their own section in the Construction Specification Institute's (CSI) Division 8, which identifies doors and window properties. TDDs use refractive and reflective technologies to direct diffuse daylight into interior spaces from rooftop openings. Technological advances allow for long horizontal runs, as well as providing the ability for designers to dim these natural sunlight fixtures. They can be used in place of electric lights providing a static distribution pattern. The only difference is that the TDD allows for the welcome shifting light levels of natural daylight as it reflects the exterior sunlight condition. TDDs are combined to work with automated lighting to provide a constant source of lighting to any type of interior environment, from displays, conference rooms, offices and even large warehouses. The following three factors are important to specifying a TDD:

1. The optical efficiency of the TDD and the percentage of light that travels from the roof to the interior space
2. A low U-Factor, which measures the resistance to heat loss through the system
3. A low Solar Heat gain coefficient, which is a measure of how well the TDD blocks heat admitted through the product.

TDDs collect daylight not through mirrors, but through angled indices, which as Neall Digert of Solatube International states, "shatter the light to use optics to overlap 100 percent of the sun's rays, which minimizes the potential of glare to diffuse and focus daylight."

Daylight Harvesting

Designers use their knowledge of the sun's angles to design light shelves, as well as light shades to manipulate sunlight. Daylight harvesting can be provided through the use of light shelves installed either to the interior or exterior of a window system.

A window can be divided into two sections. Vision panels are windows within the vision range of an average adult, typically any window below 7 feet. After calculating the solar angle from grade, the designer can determine the length of a light reflective shelf that will interrupt the solar angle of daylight and penetrate the interior of the building. Direct daylight can cause glare on the work plane. A highly reflective light shelf below an upper window deflects light onto other surfaces in the interior. The length of the shelf, the angle and the orientation determine how deeply sunlight will travel. Light is diffracted at a 90 degree angle and additional diffractions within a space can occur by designing ceiling surfaces, as well as additional interior light shelves, which continue to reflect daylight throughout the structure.

Direct sunlight can be deflected away from a building in hot climates or directed to a building in cold climates. Designers often choose to orient buildings so that the north and south faces have the longest solar exposures. This orientation provides greater solar gain for passive solar strategies. In many cases the program of a building requires that the building has longer exposures to the east or west. Shading devices for these exposures must be designed as vertical attachments. The length of these devices can also be calculated through modeling programs based on the latitude and longitude of the project location.

Window blinds are another means to modulate daylight. Window blinds can be used to decrease the air conditioning load caused by solar heat gain. Most blinds can be pivoted to direct and diffuse the sun's rays throughout the day. Blinds can be specified in both vertical and horizontal shade configurations. Dust, airborne allergens and replacement costs from damage are common problems for this convenient solution. Some window systems provide the alternative of blinds sealed within the panes of glass where they are maintenance-free, minimally invasive to interior space and conform to stricter fire ratings of commercial spaces. This solution also provides the opportunity to open the window while still modulating the daylight.

Window Glass Technologies

Building envelopes that protect a building from sun, wind and rain, as well as provide insulation, ventilation and daylight were the dream of Michael Davies. In the 1980s, Davies envisioned these "walls for all seasons" as  polyvalent.⁴ Window glass is required to meet greater-than-ever standards for performance, providing a barrier for unwanted solar radiation while allowing the greatest amount of visible light into the spaces. Windows can comprise between 10 percent to over 25 percent of a heating load of a building and even more of the cooling load. Architects can specify windows with multiple panes of glass, with solar controlled coatings, low emissivity and low-conductance gas fills.

The skin of a building responds to the demands of a climate for heating and cooling. Adding layers to a window increases its insulation values and can minimize heat loss or heat gain in hot or cold climates. The space in between the layers of glass provides insulation by means of an air space. Newer window products include the use of low-conductivity gases such as argon or krypton. Argon is produced by the partial distillation of liquid air, and it is inexpensive to produce since it is already a byproduct of liquid oxygen and liquid nitrogen. Inert argon gas is not flammable, will not explode and is not toxic to the environment, according to the Canadian National Occupational Health & Safety Resource Center. Krypton has even greater insulating properties and it is also nontoxic. These materials increase the energy performance of windows.

Window coatings provide a barrier to the invisible, yet harmful infrared and ultraviolet rays that comprise over 60 percent of the solar spectrum. Low-E coatings are made of metallic oxides bonded in thin layers to glass surfaces and act like a mirror to reflect the rays of the sun. Coatings change the appearance of a window and designers who want to "tune" window performance to orientation of the building façade should be aware of this consequence. New products on the market are coatings that increase the visible light transmittance above 65 percent (a clear single-pane window has a 90 percent visible light transmittance), while having a solar heat gain efficiency rating of .27 (a clear single-pane window has a solar heat gain rating of .87).