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2019 Title 24 CASE Topics

Measure Overview

The primary focus of this report is to reinstate the minimum skylight area requirement to section 140.3(c) while maintaining the overall simplicity of showing compliance.

During the 2013 standards development there was an agreement that the calculation of effective aperture as an alternative method of showing compliance with this section was overly complex.  Calculating effective aperture was complex for two reasons:

  • It required calculating the area of the skylit zone – which can be a convoluted geometry when one includes the presence of racks and other obstructions which truncate the skylight zone.
  • It required the calculation of well efficiency which requires the calculation of room cavity ratio and looking up this value on a nomograph for various light well surface reflectances.

When the effective aperture requirement in the 2013 standards was deleted, the minimum total skylight area was also deleted.  Though these changes still left a requirement for a minimum fraction of the floor area being in the daylit zone this did not place any threshold of how much daylight or skylight area had to serve this zone.  As will be shown later on in this report, the revised structure of the skylighting requirement does not assure that enough skylight area is required to save as much energy as was the case in the 2008 Title 24 standards.  An unintended consequence was that the performance approach as documented in the Alternative Compliance Method (ACM) Reference Manual, no longer had a stable baseline of how much skylight area was required for the baseline building.

For the large open enclosed spaces subject to Section 140.3(c), this proposal provides a method of assuring there is enough skylight area in to provide sufficient daylighting while maintaining the overall simplicity of the 2013 daylighting requirements.  This will also result in a well-defined baseline daylighting system for the performance approach.

Reinstating the minimum skylight area requirements from the 2008 Title 24 also more closely aligns the Title 24 minimum skylight area requirements with those in the 2010 and 2013 versions of ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings.  This is desirable as designers that are used to complying with the ASHRAE 90.1 daylighting requirements will have less of a learning curve to comply with the proposed 2016 California Title 24 daylighting requirements.  In addition, the minimum skylight area requirements in ASHRAE 90.1 are identical to those found in the 2012 version of the International Energy Conservation Code (IECC 2012).  The 2016 IECC is used in 16 states.[1]

The following documents will be modified by the proposed change:

  • The text of the Title 24 Standards
  • The Nonresidential Alternative Compliance Method (ACM) Reference Manual
  • Compliance Form 2013-NRCC-ENV-04-E Minimum Skylight Area Worksheet
  • The Nonresidential Title 24 Compliance Manual

[1] International Codes-Adoption by State (September 2014)

Stakeholder Meeting

Several stakeholder meetings and CEC staff workshops were held regarding the Minimum Skylight Area building energy efficiency topic. The list of past meetings and hearings can be found here. If you would like to receive email updates on this CASE topic, click the link below.


  1. Final CASE Report