Skylighting Design Guidelines (Advanced Simulation Guidebook, Volume II)

Overview

This is the second edition (February 2014) of a guidebook first written in 1998 by the Heschong Mahone Group for Southern California Edison's Energy Design Resources program. It answers one question in depth: how do you design a skylight system for a commercial or industrial building so that it delivers uniform, comfortable daylight and actually saves energy? The scope is deliberately narrow, covering manufactured unit skylights in single-story buildings, not atriums or custom glazing, and it pairs the text with SkyCalc, a free Excel tool that sizes the skylight area for a given climate and building. Six chapters and an appendix run to 140 pages.

Key Findings

• The economic case is stated up front. Using 2013 Los Angeles-area prices, the authors estimate a grocery store could save about $24,000 a year ($0.48 per square foot), an elementary school about $11,250 ($0.35/sf), and a distribution center about $0.18/sf, with a plausible range across building types of $0.08 to $1.13 per square foot.
• The opportunity is largely untapped: the 2003 CBECS survey found fewer than 1 percent of U.S. commercial buildings had any skylights, even though warehouses spend about 50 percent of their electricity on lighting, retail about 43 percent, and schools about 39 percent.
• Chapter 2 (Designing With Skylights) covers daylight physics, skylight shapes, glazing, light wells and integration with lighting and HVAC. Its core rule of thumb: space skylights 1.0 to 1.5 times the ceiling height apart, center to center, assuming diffusing glazing and shallow wells.
• Chapter 3 (Specification Choices) introduces "skylight efficacy," the ratio of visible transmittance to solar heat gain adjusted for well losses, as the number that should drive glazing selection. It also summarizes 2013 Title 24: a 5 percent prescriptive cap on skylight area, a 90 percent minimum haze rating, and a requirement that spaces over 5,000 square feet with ceilings above 15 feet put 75 percent of their floor area in a daylit zone.
• Chapter 4 (Daylighting Controls) is blunt: without photosensor controls a skylit building saves nothing and may use more energy; with them, lighting energy can fall by half or more. Failure to commission is named as the single largest cause of control systems not performing.
• Chapter 5 (Optimizing the Design) uses SkyCalc runs to show how climate, building type, glazing and control strategy shift the optimum; a San Bernardino office peaks at roughly 3 percent aperture, and simple on/off switching can beat dimming where daylight saturation is consistent. It ends with a two-page list of design rules of thumb.
• Chapter 6 is the SkyCalc 3.0 user's guide, and the appendix supplies a glossary and reference list.

Why It Matters for Daylighting

Almost every argument made for prismatic skylights in warehouses and manufacturing plants traces back to this document or to the Heschong Mahone research behind it. Its emphasis on high visible transmittance with low solar gain, highly diffusing glazing, shallow white-painted wells, and closely spaced smaller units instead of a few large ones is a near-perfect description of a prismatic-skylight layout. Just as important, the guide is honest about the failure modes: oversized apertures that overheat the space, undersized ones that never justify the controls, and photosensors that get taped over because nobody explained them to the tenant. Owners planning a toplighting retrofit will find the control-zone, commissioning and Title 24 material directly usable, and SkyCalc remains a quick way to sanity-check a vendor's savings claim.

About the Source

Energy Design Resources was a design-assistance program funded by California utility customers under the California Public Utilities Commission and administered by Southern California Edison, PG&E, SDG&E, SMUD and LADWP. The first edition was written by the Heschong Mahone Group, whose skylight and daylighting studies underpin much of Title 24; TRC updated the technical passages and GeoPraxis handled editing for the 2014 edition. The program's website (energydesignresources.com) is no longer online, so the PDF in this library is currently the only accessible copy.