Volume 10, Issue 9 - November/December 2009

AAMA Analysis

Daylighting Rediscovered
Reduce Energy Demand Through This Natural Method
by Ken Brenden

With the continued growth of green trends, there has been a proliferation of green-related standards and rating systems, as consumers in all sectors are looking to decrease energy usage, with the added benefit of saving on energy bills.

The primary touchstone for window greenness in such standards is maximizing resistance to conductive and radiant heat transfer. But greenness doesn’t end with achieving energy efficiency. A prime example is daylighting. Not only is natural daylighting being rediscovered as a means to reduce the energy demand for electric lighting, it also reduces the heating load that electric lighting places on air conditioning systems. Daylighting also can extend the life of lamps and ballasts by as much as a factor of two*, as they are not used as often during the day. This means that embodied energy, use of mercury and disposal costs are also roughly halved. Furthermore, post-occupancy studies comparing buildings with optimized daylighting systems to those without come to the same conclusions: people simply function better in a daylit environment.

The reemergence of natural daylighting has spurred a distinct reversal of the thinking prevalent in the 1970s when big windows were viewed as energy drains. In a quest for energy efficiency, architects adopted a “bunker” approach in which artificial light replaced natural light as a necessary consequence.

Today, architects are using the latest science and technology to take full advantage of sunlight’s benefits while minimizing its drawbacks, and daylighting has come into its own as a vital component of green design.

Effective Daylighting
For some, the term “daylighting” can imply vast expanses of glass with intense direct sunlight pouring in, creating glare and oppressive heat gain. But that is a misconception. Good daylighting practice achieves a more diffuse, evenly spread light. The goal is to determine how to bring natural light to all rooms—whether or not they have exterior views. Reaching this goal means making a number of decisions that go beyond window selection to orientation and floor plans.

A good daylighting design, in addition to minimizing U-factor, also must forge a balance between solar heat gain coefficient (SHGC) and visual transmittance (VT).

The key to resolving the conflict lies in the choice of glazing. Spectrally-selective low-E glass, for example, transmits visible light waves and reflects infrared heat waves, keeping SHGC low yet preserving a high VT to ensure bright interiors.

In addition to windows, other devices can bring diffuse natural light to interior spaces. Judicious skylighting, for example, introduces daylight throughout the interior rather than just around the perimeter. The newer tubular skylights can channel daylight from the roof down through a ceiling-mounted diffuser that looks much like a conventional ceiling-mounted light fixture. These are increasingly popular for interior areas such as bathrooms, hallways and kitchens that receive limited daylight. The NAHB Green Home Building Guidelines specifically recommend the installation of tubular skylights in rooms without windows.

In addition to adding pathways for light to enter, design strategies can enhance the availability of natural light. For instance, allowing light to penetrate high into a space through the use of clerestories, light shelves or vertical baffles lets it travel deeper into a room, as does sloping the ceilings away from the windows to reflect the light. Daylighting also can be enhanced by integrating reflective surfaces, furnishings and finishes in lighter colors, and through the use of high reflectance paint on the ceilings.

There are multiple paths to greenness. The idea is to take a holistic look at the interplay of all factors affecting the structure and arrive at an optimized solution based on geometry and engineered features.

*Source: Greenwala Blog (the green social network), “Daylighting —The Benefit of Natural Light” by Greg Richardson, 9/29/08

Ken Brenden serves as technical standards manager for the American Architectural Manufacturers Association in Schaumburg, Ill. He may be reached at kbrenden@aamanet.org. His opinions are solely his own and do not necessarily reflect those of this magazine.


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