Residential Attic Ventilation

In the 2007 CBC Sect 1203.2 the attic ventilation was 1/150 with 1/300 allowed if 50% of the ventilation is in the upper half and a vapor barrier is installed.

In the 2010 CBC and the 2011 Residential Code Sect R-806.2 the requirement has been lessened to 1/300 and the vapor barrier is not required if 50% of the venting is in the upper portion of the attic. (There is al alternative that if 50% of venting is not in the upper portion, that is acceptable with a vapor barrier).

I would like to understand the code change that lessened this requirement. We have a house built under the 2007 code which meets the later requirement, but not the 1/150 requirement. It appears, based on later codes, that it is adequately vented.
Original Post
My 2010 CRC R806.2 says 1/150 with reduction to 1/300 with vapor barrier.

Most buildings that require attic ventilation are residential. The "regular" CBC would be applicable to typically commercial buildings and with the current energy code requirement for commercial building vented attics, most are done with the insulation at the roof plane to allow less cost for the ceiling.
Thanks Jim,

I was told by the HCD tech rep that the CRC said it needs 1/300 with a vapor barrier ONLY if there is not 50% of the venting in the upper portion of the attic. If there is 50% in the upper portion, then no barrier is required.

I don't have this code and I guess I better check this online to be sure of this wording.
R806.2 Minimum area. The total net free ventilating area shall not be less than 1/150 of the area of the space ventilated except that reduction of the total area to 1/300 is permitted provided that at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above the eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1/300 when a Class I or II vapor barrier is installed on the warm-in-winter side of the ceiling.

This is the Residential code off the ICC website. Jim, is this the same as what you are looking at?

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