Requirements for Attic Ventialtion?

[Nick Ostrowski]

Are there any specific requirements/codes in regard to attic ventilation? I looked at a new construction home in a 55+ community. Some areas of the soffit bays in the attic appeared to have no space for air circulation. Also, a large area of the roof above the 2nd floor bedroom and garage had only two high-hat vents for ventilation on the upper end of the roof. The ridge of the roof is shared with an attached house so I am assuming a ridge vent is not doable as it might create a breach in the firerating of the attic wall. There are mesh ridge vents in the dormers that run perpendicular to the ridge but there is still a large enough area that will allow heat and humidity to stagnate.

[David Bell]

Are there any specific requirements/codes in regard to attic ventilation? I looked at a new construction home in a 55+ community. Some areas of the soffit bays in the attic appeared to have no space for air circulation. Also, a large area of the roof above the 2nd floor bedroom and garage had only two high-hat vents for ventilation on the upper end of the roof. The ridge of the roof is shared with an attached house so I assuming a ridge vent is no doable as it might create a breach in the firerating of the attic wall. There are mesh ridge vents in the dormers that run perpendicular to the ridge but there is still a large enough area that will allow heat and humidity to stagnate.

I believe that most states require one half of space air change per minute whether naturally or mechanical depending on mean temps during summer.

[Jerry Peck]

If all the ventilation is soffit vents, then 1 sq ft of net free vent area per 150 sq ft of attic space.

Net free vent area is the sum of the openings in the vent, not the size of the vent itself. This is one example: Take a 3" x 20" vent which is screened over, slide all the threads of the screen to one end and to one side, the remaining opening would be the net free vent opening, or ...

... Because it is not possible to do what I just described, but that does work for giving a visualization of what is done here - measure the threads (let's say 0.03125" each - 1/32" diameter threads) and there are 16 threads per inch, so the threads would take up 0.5" per inch, leaving 0.5" inch net free vent area per inch. Or, a 1" x 1" square would be 0.5 x 0.5 = 0.25 sq inches net free vent area per square inch.

Based on the above made-up example, that 3" x 20" vent would be 60 sq inches x 0.25 = 15 sq inches net free vent area. Thus it would take 10 of those vents to make 1 sq foot (144 sq inches).

Apply that to an attic which, including the overhang, is 3,000 sf and you would need 20 sq feet of net free vent area, or 200 of those vents. AND the vents should be located on opposite sites of each other to provide for cross-ventilation.

Now, if approximately 1/2 of the required venting is in the upper 1/3 height of the attic, and the other 1/2 is at the overhang, the ventilation is so much better than you only need 1 sq ft net free vent area for 300 sq ft of attic space - the amount of net free vent area just dropped to 10 sq feet, i.e., you only need 5 sq feet at the overhang and 5 sq feet near the top, 5 sq feet is only 50 vents at the overhang instead of 200 vents at the overhang.

I'm typing this as I am going through it, so if I made a mathematical error I am sure that it will soon be pointed out. :cool: