These Must Recognize Daylighting to Save Energy
by Ken Brenden
Electrical lighting in large commercial buildings uses energy to produce light, as well as adding to the heat load that air conditioning systems must remove. Herein lies the key to balancing heat loss through relatively high U-factor skylights with energy savings due to natural daylighting, and the rationale behind a proposed change to the 2006 International Energy Conservation Code (IECC). These code changes were submitted to the ICC on March 14, 2006, by the American Architectural Manufacturers Association
“The [prescriptive SHGC] requirements now in the 2006 version of the energy code will do a lot to hurt the plastic glazed commercial skylight industry,” observes Randy Heather, standards product manager for the Vistawall Group and member of the AAMA codes working group. “To meet the requirements [SHGC of 0.35 in climate zones 1, 2, and 3] will cause the products to increase in cost and/or loose significant light transmission, to a point where they will no longer be economical to place on a building.”
The solution: link daylighting with automatic control of electrical lighting to preserve required task and ambient lighting levels while minimizing total energy use.
The AAMA proposal for the current IECC change cycle would revise IECC Section 502 and Table 502.3.1 [formerly 802.2(2)] to permit an increase in the maximum skylight coverage from 3 to 6 percent of the roof area in Mercantile buildings (Use Group M), such as big-box retailers and grocery stores, and in Warehouse buildings (Use Groups S-1 and S-2)–structures that commonly feature large open areas with high ceilings–which are equipped with multilevel daylighting controls. The skylights would have to meet certain specified criteria for U-factor, SHGC and haze:
1. The area weighted average U-factor and SHGC of the skylights cannot exceed the values given in the following table:
2. In order to ensure adequate distribution of useful daylighting within the building, the haze of the combined glazing material in the skylight assembly (ratio of the amount of diffusely transmitted light to the total light transmitted, as determined by test method ASTM D 1003-00) must be 90 percent or greater.
3. All ambient lighting in the daylit areas under the skylight is controlled by two step (On/50%/Off) or greater (multi-step or continuous) lighting controls.
The proposal is backed by a study conducted by Carli Inc. of Amherst, Mass., based on performance data of 24 different skylight types installed in buildings in 21 U.S. cities as obtained by the Heschong Mahone Group Inc. (HMG), a Fair Oaks, Calif., energy consulting firm. Published this past February, the study quantifies the tremendous savings in both energy consumption and energy cost that can be achieved by combining increased skylight area with daylighting controls.
The energy cost savings achieved in buildings equipped with glass or plastic-glazed skylights at 6 percent of roof area and lighting controls, compared with skylights that meet current code criteria at 3 percent of roof area and no lighting controls, is shown in the table on page 8.
Note that in many cases, energy cost savings exceed 20, or even 30 percent. These savings were seen in all climate zones for all skylights that met the criteria, except for one skylight in one of the studied cities in Climate Zone 4.
Even though many code jurisdictions have neglected the value of skylights, “if the proposal is approved at the September ICC hearings [September 20-30, 2006 in Lake Buena Vista, Fla.], it will be coming down the road for the remainder of the United States, as the respective states start adopting the IECC,” predicts Charlie Curcija of Carli. For example, the State of California–often a bellwether for building code trends– has already made it mandatory as part of its Title 24 Energy Code for single-story buildings over 25,000 square feet to have skylights and daylight control systems.
“If the code organizations are truly serious about saving energy, then we should have no trouble getting this proposal to pass,” adds Heather.
Note: (The Carli study can be downloaded from www.fenestration.com/Codes/Skylights/Skylight-Energy-Analysis_Daylighting_rev5.pdf.)
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