IRC 806.2 exception

[Sifu]

2108 IRC 806.2 requires attic ventilation at a rate of 1/150 as a base. Exc. #1 allows a reduction in rate to 1/300 if in CZ 6,7,or 8 with a class I or II vapor retarder. I have chewed on this for a while and can't definitively decide on whether this means the exception only applies to CZ 6,7 or 8, and if in a CZ 5 then there is no exception, OR if exc. #1 means that if you are in CZ 5 then you don't need the VR to satisfy that exception.

Nutshell: Given a ventilated attic, in CZ 5, with a 50/50 split to satisfy exc. #2, is the required rate 1/150 or 1/300?

 

[wwhitney]

2018 IRC R806.2 is excerpted below.

Condition 1 of the Exception only applies in CZ 6, 7, and 8. In other CZs, it is vacuously satisfied. Condition 2 applies in all cases and must always be complied with to use the exception.

Cheers, Wayne


R806.2 Minimum vent area.

The minimum net free ventilating area shall be 1/150 of the area of the vented space.

Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided both of the following conditions are met:

1.In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

2.Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

 

[Sifu]

Maybe I'm just being too thick, maybe my OCD is triggered. Still not sure if condition #1 is met in CZ5 or if it isn't applicable. If met, so be it. If not applicable then the "BOTH" verbiage trips me up. Or, unless you are in CZ 6,7, or 8, you can't possible satisfy BOTH conditions.

'21 IRC commentary:
The exception permits a reduction in the required venting area if both of the conditions are met. Condition 1 recognizes that in cold climates, a vapor retarder is required and permits a reduction based on the vapor retarder being installed on the warm-in-winter side of the ceiling.

If I consider the intent using the intent implied by the commentary I think the condition is met or doesn't apply:
Moisture in the attic space condenses on the cold surface of the roof deck, so we need 1/150 ventilation ratio to mitigate this. If I am not in a cold climate then there is less "cold surface" to condense on (and maybe less likelihood that the saturation point will be exceeded). Install a barrier to lessen the moisture from the conditioned spaces, and I have less moisture, so in a cold climate I have slowed down/lessened the moisture, therefore I am allowed less ventilation to mitigate the condensation. However, if I am not in a "cold" climate, then I can reduce the ventilation but I don't need the barrier to lessen the amount of moisture because there will be less "cold" surface to condense on in a climate that is not "cold".

 

[wwhitney]

Condition 1 may be equivalently worded as:

1) If the CZ is 6, 7, or 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

The other interpretation you are imagining would be written as:

1) The CZ is 6, 7, or 8
2) a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
3) Condition 2 in the current text

But that is not at all what the text actually says. The interpretation in post #2 is correct.

Cheers, Wayne