Attic Ventilation exception

[TRox]

Hi there,

I wanted to see if I can get your take on the 1/300 nfva exception for attics. When it comes to calculating vents for small attic spaces I am getting hung up specifying a vent that fits within the upper vents 40-50% limit.

R806.2 Minimum Vent Area. The minimum net free ventilation area shall be 1/150 of the vented space.
Exception: The minimum net free ventilation area shall he 1/300 of the vented space provided one or more of the following conditions are met:
...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.

For example:

• Attic over a porch is 123 s.f.
• At 1/300 the required total venting area is .41 s.f.
• The upper vents would need a maximum of .205 s.f. nfva.

Are there really any upper vents that small, and my question is, are we actually limited to that? Is it wrong to think that providing more total ventilation than is calculated to be required is ok/better, as long as the ratio is correct (with upper vents 40-50% of total provided ventilation)?

Thanks in advance for any comments!

 

[my250r11]

R806.2 Minimum Vent Area. Minimum being the magic word of all codes. Going above and beyond what is needed is not a problem. Providing adequate air flow is the important thing.

 

[TRox]

Ahh, Minimum vent area...Thanks for removing the wool from my eyes, I guess I glazed over that word. =)
So, just thinking out loud, it sounds like as long as the total nfva provided is more than the 1/300, and the uppers are between 40-50% of the total provided, then its compliant.

 

[my250r11]

Correct.

 

[TRox]

Thanks so much for your help!

 

[cda]

Welcome