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Improvement Forum ( -   Roofing/Siding ( -   -   Eave vents, how
much NFA per 2" hole? (  RM575 10-07-2012 07:49 PM  Eave vents,
how much NFA per 2" hole? Hello folks, I'm trying to calculate
how many holes to drill in the 2"x6" blocks of my eaves in a
11'x15' detached workshop that has a 4:12 hip roof. It will be
insulated and drywalled.  I'm trying to balance the attic
ventilation and will be using the 1/300 calculation instead of
the 1/150.  It looks like the NFA (net free area) of the attic
requires 40.32 sq inches for the exhaust and the same for the
intake.  I'm looking at installing a static roof vent that
provides 41 sq inches of NFA.  Then I plan to drill 2" intake
vents at the eave blocks but need to know how many to drill.  If
I've done the math correctly, a 2" hole is 3.14 sq inches in
area, which would mean 13 2" holes for intake.  Can someone
verify this? Is there a reduction in the area when screened with
1/4" wire mesh?