04/16/2007

Building the Case for Light Shelves

Energy-efficient daylighting requires the correct application of light shelves on building façades and curtainwall

 

Properties of the Model Space
In comparing the no-light-shelf model with the light-shelf model, the following properties applied to both spaces (described as typical perimeter offices):

  • Room size was 12-feet wide by 15-feet deep by 9-feet high.
  • Ceiling reflectance was 80 percent, assuming typical acoustical ceiling tile.
  • Wall reflectance was 50 percent, assuming medium-colored, matte paint.
  • Floor reflectance was 30 percent, assuming medium-colored carpet.
  • Glazing was south-facing; 70-percent transmittance starts at 2-feet, 6-inches above the finished floor and terminates at 8-feet, 6-inches above the floor.
  • Light shelf model was 3-foot, 6-inches deep light shelf (arbitrary value): 2-foot, 6-inches projects outside, 1-foot projects inside; light shelf mounted at 7-feet above finished floor.

By Michael A. Barber

While light shelves help redistribute the sun's direct visible energy deep into a building, they also reduce the amount of radiant energy at the building's perimeter (which, in turn, reduces the building's cooling load and associated energy consumption). The integration of light shelves to create high-performance building design requires a team approach since, not only are the building aesthetics a consideration, the visual and thermal environments are major design factors as well.

The design and application of light shelves requires some common sense and careful analysis. The following rules of thumb are relevant for projects located in the Northern Hemisphere:

  • Light shelves are most effective with south-facing glass.
  • Light shelves are not very effective with east- and west-facing glass. Limiting the size of windows and using blinds or shades are more effective ways to minimize thermal
  • heat gain and control glare from the low sun angles on these exposures.
  • Light shelves are not effective with north-facing glass. North-facing skies provide diffuse daylight rather than direct sunlight for much of the year.
  • Light shelves are not very effective on any building where the number of overcast days is significantly higher than the number of clear, sunny days at the project location.
  • Matte is the recommended finish for the top of the light shelf to diffuse the direct sunlight, which will minimize glare and provide uniform distribution of light inside the room. (Highly specular finishes are currently being studied to maximize daylight penetration; however, diffuse finishes are recommended until more information and design guidelines are defined.)

Now, to the careful analysis: To illustrate the light shelf's effectiveness, two computer models were prepared - one with a light shelf, the other without. Properties of the model spaces are defined in Properties of the Model Space (see sidebar and graphic).

 
Max. Horizontal Illuminance (footcandles)
Min. Horizontal Illuminance (footcandles)
Uniformity
No Light Shelf Model
6,546
92
71:1
Light Shelf Model
470
53
9:1
SOURCE: The Lighting Practice Inc.

A number of daylight calculations were performed at different times of the year under varying sky conditions within the room at 2-feet, 6-inches above the finished floor. The calculation was performed for Philadelphia on April 21 at noon under clear-sky conditions. The calculations were performed using Lumen Designer lighting software. The first observation should be the extremely high light levels closest to the window in the office with no light shelf. This begins to tell an even greater story about the light quality and characteristics within the space - the most telling being the uniformity.

It is critical to balance the uniformity of illumination at the work plane and in the immediate visual environment. Uniformity allows us to focus on visual tasks and reduces eye strain. Recent studies (Heschong Mahone Group Inc., 1999, 2001, 2003) have shown that student performance on test scores increased significantly in classrooms with a properly designed daylighting system - one that balances illumination levels and uniformity, and reduces direct glare.

Michael A. Barber, LEED AP, is a principal at The Lighting Practice Inc. (www.thelightingpractice.com), Philadelphia, which provides specialized design services in the application of lighting for architecture.

 


Visit our website today to learn about the design flexibility of a Morton building and the endless possibilities of partnering with our designBUILD team.


Wood construction is both cost and energy efficient. Check out Morton Buildings and our designBUILD team online today to discover all the benefits of post-frame construction.


When choosing a metal-clad building for your next construction project, consider Morton Buildings, Inc., and their designBUILD team, we’ll make your dream a reality.

We Can Help You Reduce Energy by 30%

Our mission is to help our customers manage their buildings' energy costs, improve reliability, and enhance performance while having a positive impact on the environment.
CLICK HERE to find out how.

 

Add highly responsive multi-zone comfort to any building project, in any climate. Our CITY MULTI H2i R2- and Y-Series VRF systems give you flexibility to fit the needs of any building. Enjoy 100% heating capacity at 0°F outdoor ambient, and 85% heating capacity at -13°F outdoor ambient.  For more information, log on to www.mitsubishipro.com


Tennant Company is a recognized leader in designing, manufacturing and marketing solutions that help create a cleaner, safer, healthier world. With thousands of satisfied customers already using award winning ec-H2O technology, why not see what you're missing? Test ec-H2O on your soils in your facility. Get a free demo.


Tennant Company is a recognized leader in designing, manufacturing and marketing solutions that help create a cleaner, safer, healthier world. With thousands of satisfied customers already using award winning ec-H2O technology, why not see what you're missing? Test ec-H2O on your soils in your facility. Get a free demo.

 

Replace core drilling with Connectrac In-Carpet and On-Floor Wireways, the proven connectivity solution in open interior spaces of all kinds. These elegant low-profile wireways are great for AV / telecom connectivity wherever it’s needed.

 


Visit our website today to learn about the design flexibility of a Morton building and the endless possibilities of partnering with our designBUILD team.


Wood construction is both cost and energy efficient. Check out Morton Buildings and our designBUILD team online today to discover all the benefits of post-frame construction.


When choosing a metal-clad building for your next construction project, consider Morton Buildings, Inc., and their designBUILD team, we’ll make your dream a reality.

We Can Help You Reduce Energy by 30%

Our mission is to help our customers manage their buildings' energy costs, improve reliability, and enhance performance while having a positive impact on the environment.
CLICK HERE to find out how.

 

Add highly responsive multi-zone comfort to any building project, in any climate. Our CITY MULTI H2i R2- and Y-Series VRF systems give you flexibility to fit the needs of any building. Enjoy 100% heating capacity at 0°F outdoor ambient, and 85% heating capacity at -13°F outdoor ambient.  For more information, log on to www.mitsubishipro.com


Tennant Company is a recognized leader in designing, manufacturing and marketing solutions that help create a cleaner, safer, healthier world. With thousands of satisfied customers already using award winning ec-H2O technology, why not see what you're missing? Test ec-H2O on your soils in your facility. Get a free demo.


Tennant Company is a recognized leader in designing, manufacturing and marketing solutions that help create a cleaner, safer, healthier world. With thousands of satisfied customers already using award winning ec-H2O technology, why not see what you're missing? Test ec-H2O on your soils in your facility. Get a free demo.

 

Replace core drilling with Connectrac In-Carpet and On-Floor Wireways, the proven connectivity solution in open interior spaces of all kinds. These elegant low-profile wireways are great for AV / telecom connectivity wherever it’s needed.

 


 
comments powered by Disqus

Related Products

Sponsored Links