Proper Daylighting Techniques
Embrace Light, Yet Beware of Heat 
by John Lewis

Daylighting is receiving much attention as a green building alternative to energy-consuming electric lighting and as a means to improve the well-being and performance of building occupants from hospital patients to school children. But achieving optimal daylighting is not as simple as just maximizing the amount of glazed area contributed by windows and skylights. As in all things engineering, there are trade-offs.

A key consideration is the energy efficiency of the aggregate daylighting system. This usually is thought of in terms of the thermal transmittance of the glazing (U-factor) and how the energy used to counteract the winter heat loss, or to cool a structure against the summer heat gain, balances against the savings from reduced artificial lighting. But the considerations go deeper.

Reviewing the Basics
Minimizing the U-value of the glazing system is only part of the thermal performance story. Something must be done about solar (radiation) heat gain—keeping in mind that, in colder climates, solar gain can be quite beneficial. This is recognized within the U.S. Department of Energy’s latest proposed Energy Star^® windows criteria, now being evaluated with a target effective date of January 1, 2009. In addition to ratcheting down U-factor requirements, particularly in northern climate zones,  Energy Star sets criteria for both solar heat gain and the visual transmittance of the glazing. While the practicality of target numbers can be debated, the concept is on track.

The solar gain versus light transmission tradeoff pivots around two basic metrics:

• Solar Heat Gain Coefficient (SHGC). This measures the amount of the sun’s energy (visible and infrared wavelengths) coming through a fenestration unit to enter a building as heat. SHGC is the fraction of incident solar radiation that is transmitted through the glass and frame as well as that which is absorbed and subsequently released inward. The higher the SHGC, the more solar heat is transmitted.
• Visible Light Transmittance (VLT). This is the percentage of normally incident visible light passing directly through the glazing. A higher VLT indicates better light transmission. 

Per the proposed new Energy Star criteria, SHGC is limited in Southern zones to a maximum of 0.30; in the North, a minimum SHGC is recommended to capture winter solar heat gain. SHGC and U-factor can be traded off in both Northern and Southern zones. In addition, VLT criteria are set forth to ensure adequate light. 

It’s All in the Glazing
It is clear that regulating SHGC while maximizing VLT can be at odds. The key to resolving the conflict lies in the choice of glazing.

In warmer climates, where reducing SHGC is the objective, the reflectivity and color properties of glass can be altered using integral tints or reflective surface coatings. However, both have a tendency to reduce the visible light transmission. A better choice is often low-emissivity (low-E) glass. Low-E glass has a metal or metallic-oxide coating that is nearly invisible to the eye and reflects the infrared (IR) portion of the heat spectrum. Simply stated, low-E coated glass redirects (reflects) radiant heat back toward the source, thereby reducing winter heating costs and summer cooling costs.

Spectrally selective low-E glass combines the best qualities of low-E, tinted and reflective glass. As with typical low-emissivity coatings, standard low-E glass reflects back into the room the long wave radiation emitted by interior surfaces heated by solar gain, thus reducing heat loss from the building—a plus in colder climates. But spectrally selective low-E glass also selectively transmits visible light and reflects infrared heat. Thus, the SHGC is low (which is good for reducing solar heat gain in warm climates), the U-factor is low (which is good for reducing heat loss), and yet the VLT is high.

The green building movement is teaching us that all segments of the building components industry are challenged to expand our product-oriented viewpoints and embrace the idea that all components must work together. 

John Lewis serves as technical director for the American Architectural Manufacturers Association in Schaumburg, Ill. He may be reached at jlewis@aamanet.org. Mr. Lewis’ opinions are solely his own and do not necessarily reflect those of this magazine.

DWM
© Copyright 2008 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.