Updated Code Requirements for Attic Ventilation and Radiant Barriers

Two important code updates have come out in recent months that have impacted the radiant barrier and attic ventilation requirements under the California Building Standards Code (Title 24). On December 29, 2010, the California Energy Commission released issue #98 of their Blueprint publication*, which included much-needed clarifications of the radiant barrier and attic ventilation requirements under the California Energy Code (Title 24, Part 6). Then on January 1, 2011, the 2010 California Building Standards Code went into effect. With it came simplified attic ventilation requirements under the California Building Code (Title 24, Part 2 or CBC)** along with new attic ventilation requirements under the new California Residential Code (Title 24, Part 2.5 or CRC)***. The impacts of these events are summarized below:

Impacts of revised requirements from the 2010 California Building Standards Code
1.) Under the 2010 California Residential Code: The mandatory minimum attic ventilation area is 1/150 of the area of the space ventilated for all enclosed attics or rafter spaces. This minimum can be decreased to 1/300 if either (i) at least 50% and not more than 80% of the required ventilation is at least 3 feet above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents or (ii) a Class I or II vapor barrier is installed on the warm-in-winter side of the ceiling.

NOTE #1: The 2010 CRC applies to all “detached one- and two-family dwellings and townhouses [dwellings that are constructed in a group of three or more, open on at least two sides] not more than three stories”*¹. The 2010 CBC applies to all other buildings.

2.) Under the 2010 California Building Code: The mandatory minimum attic ventilation area is now 1/300 of the area of the space ventilated for all enclosed attics or rafter spaces. At least 50% of the required ventilation must be at least 3 feet above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents.

Impacts of CEC interpretations from Blueprint #98
1.) Under the 2008 Performance Approach: There are no minimum attic ventilation requirements specified by the performance method, even when using a radiant barrier. However, the builder must still comply with the relevant attic ventilation requirements from the 2010 CRC or CBC.

NOTE #1: Under the 2008 performance method, a minimum attic ventilation area and a minimum percentage of “upper vents” (i.e., within 2 vertical feet of the roof ridge) must be specified as part of the compliance documentation. These ventilation design specifications can impact the compliance margin of the proposed design. The builder must then comply with these specifications in addition to the relevant attic ventilation requirements from the 2010 CRC or CBC. Take note of the difference in definitions between “upper vents” in the Energy Code and vent location requirements in the CRC and CBC. As long as the relevant requirements of the CBC or CRC are met and the building demonstrates compliance with the Energy Code, a radiant barrier can be installed with an attic ventilation area down to 1/300.

NOTE #2: The radiant barrier credit is not available for unvented attics or rafter/vaulted roof assemblies.

2.) Under the 2008 Prescriptive Approach: For homes using the prescriptive method of compliance where a radiant barrier is required for compliance, the minimum attic ventilation area is 1/150 with at least 30% of this ventilation from “upper vents”. Otherwise, there are no minimum attic ventilation requirements specified by the prescriptive approach. The builder must also comply with the relevant attic ventilation requirements from the 2010 CRC or CBC.

NOTE #1: Again, take note of the difference in definitions between “upper vents” in the Energy Code and vent location requirements in the CRC and CBC. Builders must comply with both requirements.

 

* Blueprint can be found on the California Energy Commission website at: http://www.energy.ca.gov/efficiency/blueprint/  
** Specifically Section 1203.2 of the 2010 CBC, which can be found at: http://publicecodes.citation.com/st/ca/st/b200v10/st_ca_st_b200v10_12_sec003_par001.htm  
*** Specifically Section R806.2 of the 2010 CRC, which can be found at: http://publicecodes.citation.com/st/ca/st/b400v10/st_ca_st_b400v10_8_sec006_par001.htm  
*¹ See Section R101.2, which can be found at: http://publicecodes.citation.com/st/ca/st/b400v10/st_ca_st_b400v10_1_sec027_par001.htm